Marine Biology 45, 39-52 (1978)
MARINE BIOLOGY
9 by Springer-Verlag 1978
Relationship between Phytoplankton Cell Size and the Rate of
Orthophosphate Uptake: in situ Observations of an Estuarine
Population
E.S. Friebele, D.L. Correll and M.A. Faust
Smithsonian Institution, Chesapeake Bay Center for Environmental Studies; Edgewater, Maryland, USA
Abstract
Orthophosphate uptake by a natural estuarine phytoplankton population was estimated
using two methods: (I) 32p uptake experiments in which filters of different pore
sizes were used to separate plankton size-fractions; (2) 33p autoradiography of phytoplankton cells. Results of the first method showed that plankton cells larger
than 5 ~um were responsible for 2% of the total orthophosphate uptake rate. 98% of
the total uptake rate occurred in plankton composed mostly of bacteria, which
passed the 5 ~m screen and were retained by the 0.45 Lum pore-size filter. There was
no orthophosphate absorption by particulates in a biologically inhibited control
containing iodoacetic acid. Orthophosphate uptake rates of individual phytoplankton
species were obtained using 33p autoradiography. The sum of these individual rates
was very close to the estimated rate of uptake by particulates larger than 5 H/n in
the 32p labelling experiment. Generally, smaller cells were found to have a faster
uptake rate per bm3 biomass than larger cells. Although the nannoplankton constituted only about 21% of the total algal biomass, the rate of phosphate uptake by
the nannoplankton was 75% of the total phytoplankton uptake rate. Results of the
plankton autoradiography showed that the phosphate uptake rate per unit biomass is
a power function of the surface:volume ratio of a cell; the relationship is expressed by the equation Y = 2x70-11 xi.7, where Y is ~gP ~m-3 h-1 and x is the
surface:volume ratio. These results lend support to the hypothesis that smaller
cells have a competitive advantage by having faster nutrient uptake rates.
Introdu~io.
plankton has varied between cells with a
diameter of 65 bm to those less than 3bm.
The phytoplankton is normally an assemHowever, it is clear from these studies
blage of one-celled or multi-celled auto- and from the work of Watt (1971), w h o
trophs with diverse shapes and sizes,
used autoradiography to measure primary
Hutchinson (1961) was puzzled that such
production rates of individual phytodiversity could exist in an isotropic en- plankton species, that the smaller cells
vironment where competition for reprovide a large fraction of the total
sources is supposedly occurring.
primary production in the euphotic zone.
Phytoplankton species not only disAlthough nannoplankton were the most implay diversity in shapes and sizes, but
portant producers in all environments
they differ in their contribution to the
studied by Malone (1971a), the netplanktotal algal biomass and metabolic activton:nannoplankton productivity and chloity. Nannoplankton (very small algal
rophyll ratios were higher in coastal wacells) which, in the past, were overters than in oceanic waters. Nannoplanklooked with net phytoplankton sampling,
ton populations varied within a narrow
have recently been recognized as very im- margin, but generally netplankton standportant autotrophs in the plankton coming crop increased in areas of upwelling
munity,
along the California Current (Malone,
Many studies have documented the domi- 1971b). Yentsch and Ryther (1959) also
nance of "nannoplankton" in algal standreported a stable nannoplankton populaing crop, chlorophyll content, and protion accompanied by fluctuations in netductivity (Yentsch and Ryther, 1959;
plankton standing crop.
Malone, 1971a, b; McCarthy et al., 1974;
These observations suggest that perBerman, 1975). The definition of nannohaps the nannoplankton are the best
0025/3162/78/0045/0039/S02.8O
40
E.S. Friebele
et
al.:
a d a p t e d s p e c i e s of the p h y t o p l a n k t o n comm u n i t y in m a n y a q u a t i c e n v i r o n m e n t s . Possibly they r e p r e s e n t the m o r e o p p o r t u n i s t i c species, since they h a v e f a s t e r
g r o w t h and m e t a b o l i c rates than larger
s p e c i e s (Fogg, 1965). H o w e v e r , t h e i r
a b i l i t y to m u l t i p l y r a p i d l y is d e p e n d e n t
upon r a p i d and e f f i c i e n t u t i l i z a t i o n of
a v a i l a b l e r e s o u r c e s . Do the n a n n o p l a n k ton h a v e a c o m p e t i t i v e a d v a n t a g e in obt a i n i n g the r e q u i r e d n u t r i e n t s ?
M u n k and Riley (1952), who d e v e l o p e d
a n u t r i e n t a b s o r p t i o n m o d e l b a s e d on
fluid d y n a m i c s , p r e d i c t e d that s m a l l
cells h a v e a f a s t e r r e l a t i v e n u t r i e n t abs o r p t i o n rate than large cells. T h e y
s t u d i e d m o r p h o l o g i c a l and size v a r i a tions of p h y t o p l a n k t o n cells and conc l u d e d that n e i t h e r the r a t e of s i n k i n g
nor n u t r i e n t a b s o r p t i o n i m p o s e s s e v e r e
limits upon cells of less than 20 H/n in
size.
D u g d a l e (1967) p o i n t e d out the p o s s i b i l i t y for c o m p e t i t i v e a d v a n t a g e of one
p h y t o p l a n k t o n s p e c i e s over a n o t h e r b a s e d
upon M i c h a e l i s - M e n t e n n u t r i e n t u p t a k e
k i n e t i c s . E p p l e y e t al. (1969) o b t a i n e d
k i n e t i c c o n s t a n t s for a n u m b e r of species t a k i n g up a m m o n i a and n i t r a t e .
Their results showed several general
trends: (I) large s p e c i e s had h i g h K s
values; (2) s m a l l - c e l l e d s p e c i e s f r o m
the o c e a n s h o w e d low K s v a l u e s ; (3) fastg r o w i n g s p e c i e s s e e m e d to have lower K s
v a l u e s t h a n s l o w g r o w e r s . H o w e v e r , the
effect of n u t r i e n t u p t a k e rates on specific g r o w t h rate is m o d i f i e d by e f f i c i e n c y of light u t i l i z a t i o n of the p h y t o p l a n k t o n species. P a r s o n s and T a k a h a s h i
(1973) have d e v e l o p e d a m o d e l w h i c h predicts that small cells w i l l o u t g r o w larger cells in all e n v i r o n m e n t s e x c e p t
t h o s e w h e r e n u t r i e n t levels and light int e n s i t i e s are high.
The w o r k c i t e d a b o v e has i n c l u d e d
e i t h e r l a b o r a t o r y s t u d i e s of n u t r i e n t uptake k i n e t i c s w i t h a l g a l c u l t u r e s u n d e r
p r e s u m a b l y ideal c o n d i t i o n s , or m o d e l s
using k i n e t i c d a t a to p r e d i c t w h i c h
cells w i l l h a v e c o m p e t i t i v e a d v a n t a g e in
c e r t a i n e n v i r o n m e n t s . T h e p u r p o s e of the
f o l l o w i n g study w a s to o b s e r v e t o t a l and
i n d i v i d u a l n u t r i e n t a b s o r p t i o n rates of
a d i v e r s e g r o u p of p h y t o p l a n k t o n cells
u n d e r n a t u r a l c o n d i t i o n s . This w a s a c c o m p l i s h e d by m e a n s of 32p u p t a k e e x p e r i m e n t s e m p l o y i n g f i l t e r s to s e p a r a t e
p l a n k t o n sizes and by 33p a u t o r a d i o g r a phy of p l a n k t o n o r g a n i s m s . The latter
t e c h n i q u e e n a b l e s the r e s e a r c h e r to m e a sure the rate of a m e t a b o l i c f u n c t i o n in
an i n d i v i d u a l o r g a n i s m or species. Thus,
we can d e t e r m i n e w h e t h e r small cells do
h a v e a c o m p e t i t i v e a d v a n t a g e in n u t r i e n t
u p t a k e w h e n they are c o e x i s t i n g and per-
Phytoplankton Cell Size and Phosphorus Uptake
haps c o m p e t i n g w i t h other cells
natural aquatic environment.
in a
Materials and Methods
This s t u d y was c a r r i e d out on the Rhode
River, a s u b e s t u a r y on the w e s t e r n shore
of the C h e s a p e a k e Bay near A n n a p o l i s ,
M a r y l a n d , USA. The Rhode R i v e r is a shallow e s t u a r y w i t h a m e a n depth of 2 m.
F r e s h w a t e r e n t e r s the r i v e r from the
M u d d y C r e e k w a t e r s h e d , and saline w a t e r
enters from the Bay. The e x p e r i m e n t s
w e r e p e r f o r m e d in the fall, w h e n w a t e r
t e m p e r a t u r e s w e r e dropping, s o l u b l e rea c t i v e p h o s p h o r u s levels w e r e d e c l i n i n g
after a late s u m m e r peak, and p h y t o p l a n k ton and b a c t e r i a l n u m b e r s w e r e d e c r e a s ing.
On O c t o b e r 31, 1974, a 24 1 w a t e r sample was taken w i t h a p e r i s t a l t i c p u m p
from I m d e p t h at a Rhode River s t a t i o n
(Latitude 3 8 o 5 3 ' O O " N; L o n g i t u d e 76032 '
00" W), w h e r e the d e p t h is 4 m. The water was p l a c e d in glass carboys and
t r a n s p o r t e d to the S m i t h s o n i a n I n s t i t u tion C h e s a p e a k e Bay C e n t e r dock w h e r e
the e x p e r i m e n t s w e r e p e r f o r m e d . The water was d i v i d e d into e x p e r i m e n t a l bottles w h i c h w e r e s u s p e n d e d from the dock
at I m w a t e r depth.
The e x p e r i m e n t a l flask for a u t o r a d i o g r a p h y c o n s i s t e d of a 12 1 s p h e r i c a l
flask w h i c h had b e e n a c i d - w a s h e d and
e q u i p p e d w i t h a s t o p p e r and a w i r e harness, w i t h float and w e i g h t attached.
The flask was s u s p e n d e d in the w a t e r on
the s o u t h e r n side of the dock at I m
depth. The s t o p p e r was p i e r c e d by 3
tubes: I from a p e r i s t a l t i c pump, I from
a small air pump, and I for the exit for
air. For the d u r a t i o n of the e x p e r i m e n t ,
air was b u b b l e d g e n t l y t h r o u g h the f l a s k
in order to p r e v e n t the p l a n k t o n from
settling. 200 ~Ci of 33p as H3P04 was inj e c t e d into the flask t h r o u g h a large
s y r i n g e needle. At t i m e d intervals, 1 1
s a m p l e s w e r e p u m p e d f r o m the flask into
bottles containing buffered fixative
(final c o n c e n t r a t i o n : 0.5% g l u t e r a l d e hyde and O . 0 1 M
potassium phosphate buffer at pH 7.0). One ml of each s a m p l e
was also p l a c e d in a s c i n t i l l a t i o n vial
for c o u n t i n g at the same time. A f t e r
each s a m p l e was taken, the p u m p was rev e r s e d in o r d e r to clear the p u m p lines
of w a t e r u n t i l the n e x t s a m p l e was taken.
One ml of w a t e r was t a k e n at the b e g i n n i n g and at the end of the e x p e r i m e n t
for c o u n t i n g to check for leaks in the
apparatus.
T o t a l p h o s p h o r u s u p t a k e by the p l a n k ton was m e a s u r e d a c c o r d i n g to the m e t h o d
of F a u s t and C o r r e l l (1976), in t h r e e
250 ml bottles: light, dark, and one con-
E.S. Friebele et al. : Phytoplankton Cell Size and Phosphorus Uptake
t a i n i n g the b i o l o g i c a l i n h i b i t o r , iodoacetic acid. In each e x p e r i m e n t , 2 ~Ci
of c a r r i e r - f r e e 32p as o r t h o p h o s p h a t e
was added at t i m e zero. The b o t t l e s w e r e
s u b m e r g e d at I m d e p t h and, at t i m e d intervals, the b o t t l e s w e r e r e t r i e v e d and
a s u b s a m p l e was removed. P a r t i c u l a t e s in
this s u b s a m p l e w e r e then s e p a r a t e d acc o r d i n g to size w i t h a 5 ~um p o r e - s i z e d
N i t e x s c r e e n and a 0.45 bun M i l l i p o r e membrane. The f i l t e r s and I ml of f i l t r a t e
w h i c h had p a s s e d the 0.45 Lum m e m b r a n e
w e r e p l a c e d in s c i n t i l l a t i o n v i a l s for
counting.
A u t o r a di o g r a p h y
Fixed plankton samples were centrifuged
in the l a b o r a t o r y for 30 m i n at 5000 x g
at 5oc. The p e l l e t was w a s h e d t h r e e
times w i t h 15 ml of 0.002 N HCI at 3000
x g and r e s u s p e n d e d in I ml of d i s t i l l e d
water. In order to s e p a r a t e c l u m p e d
cells, each cell s u s p e n s i o n was sonic a t e d for 10 sec (Model W 140 C, Heat
S y s t e m Co., at 60 W output) and f i l t e r e d
via g r a v i t y t h r o u g h 8 ~m p o r e - s i z e
N u c l e p o r e filters. The cells w e r e r i n s e d
off the f i l t e r s and r e s u s p e n d e d in 2.2 ml
of d i s t i l l e d water,
Liquid-emulsion autoradiograms were
p r e p a r e d a c c o r d i n g to the m e t h o d of
B o g o r o c h (1972), s o m e w h a t m o d i f i e d as des c r i b e d by C o r r e l l et ai.(1975). E a c h
cell s u s p e n s i o n was s o n i c a t e d for 15 sec
and 3 drops w e r e s p r e a d on a g e l a t i n c o a t e d glass slide. S l i d e s of the 8 ~um
f i l t r a t e w e r e also made. The slides w e r e
a i r - d r i e d for 24 to 48 h.
In a dark room, the p h o t o g r a p h i c N T B - 2
e m u l s i o n was d i l u t e d 1:1 w i t h d i s t i l l e d
w a t e r and b r o u g h t to 44oc in a w a t e r
bath. T w o drops of e m u l s i o n w e r e p l a c e d
on each s l i d e and t h e n s p r e a d to an even
t h i c k n e s s w i t h a s t a i n l e s s steel r a c l e t t e
(Bogoroch, 1972). The slides w e r e allowed to dry for I h at r o o m t e m p e r a t u r e
(ca. 23oc), and w e r e then p l a c e d in
b l a c k l i g h t - t i g h t , d e s i c c a t e d slide boxes and i n c u b a t e d at 4oc for a 3-day exp o s u r e period. The s l i d e s w e r e d e v e l o p e d
in D e k t o l at 15oc for 2 min, r i n s e d for
10 sec, and f i x e d in a c i d fixer for 5 to
10 min, then r i n s e d in r u n n i n g tap w a t e r
for 30 min. T h e s l i d e s w e r e c l e a r e d for
I h in m e t h y l s a l i c y l a t e , and c o v e r s l i p s
were mounted with Permount.
G r a i n counts w e r e m a d e u s i n g a Zeiss
m i c r o s c o p e at 1OOO X m a g n i f i c a t i o n u n d e r
p h a s e - c o n t r a s t optics. The m i c r o s c o p e
was f o c u s e d on the top of a cell and the
g r a i n s in focus w e r e counted. Then the
m i c r o s c o p e was f o c u s e d d o w n u n t i l the
next set of g r a i n s was in focus and
41
t h e s e w e r e counted. This p r o c e d u r e w a s
r e p e a t e d u n t i l all g r a i n s a b o v e the cell
w e r e out of focus. S i n c e 33p is a m o d e r ately h i g h e n e r g y e m i t t e r w h i c h is likely to p r o d u c e g r a i n s at p o i n t s w h i c h do
not lie d i r e c t l y over the p o i n t of emission, some i m a g e - s p r e a d i n g due to "crossfire" is to be e x p e c t e d (Perry, 1964).
This was t a k e n into a c c o u n t v e r y r o u g h l y
by c o u n t i n g g r a i n s w i t h i n 5 ~m of the
cell m a r g i n s . G r a i n counts w e r e m a d e for
at least 20 cells of each s p e c i e s on
each a u t o r a d i o g r a m ,
i.e., at each time
i n t e r v a l of the e x p e r i m e n t . B a c k g r o u n d
grains w e r e c o u n t e d for areas w h i c h h a d
no cells, u n t i l the s t a n d a r d d e v i a t i o n
of g r a i n s per u n i t area was w i t h i n ~ 30%
of the m e a n for e a c h a u t o r a d i o g r a m . For
each time point, the n u m b e r of b a c k g r o u n d g r a i n s in the a r e a o c c u p i e d by
each type of p h y t o p l a n k t o n cell was calc u l a t e d and s u b t r a c t e d from the m e a n number of grains per cell.
A n a l y s e s for d i s s o l v e d p h o s p h o r u s
w e r e p e r f o r m e d on w a t e r s a m p l e s w h i c h
had b e e n f i l t e r e d t h r o u g h a 0 . 4 5 ~un p o r e sized M i l l i p o r e filter. I n o r g a n i c p h o s phorus was determined colorimetrically
on w h o l e and f i l t e r e d s a m p l e s by reaction w i t h a m m o n i u m m o l y b d a t e and r e d u c tion w i t h s t a n n o u s c h l o r i d e (American
Public Health Association,
1971). T o t a l
p h o s p h o r u s was a n a l y z e d by the same m e t h od after d i g e s t i o n w i t h p e r c h l o r i c acid
and n e u t r a l i z a t i o n
(King, 1932). M e t h o d s
for r a d i o i s o t o p e c o u n t i n g w e r e as des c r i b e d by F a u s t and C o r r e l l (1976).
P h y s i c a l p a r a m e t e r s w e r e m o n i t o r e d during the e x p e r i m e n t s w i t h r e c o r d i n g ins t r u m e n t s at the C h e s a p e a k e B a y C e n t e r
dock.
The p o p u l a t i o n of a l g a l s p e c i e s w a s
e s t i m a t e d in a I 1 s a m p l e of w a t e r by
the d i r e c t - c o u n t p r o c e d u r e of C a m p b e l l
(1973). The f i x e d cells w e r e c o n c e n t r a t e d
to I ml by c e n t r i f u g a t i o n at 5000 x g for
30 min, s o n i c a t e d for 10 sec, and 0.03 ml
was p l a c e d on a m i c r o s c o p e slide. A
2.5 cm s q u a r e c o v e r s l i p was p l a c e d over
the l i q u i d and the n u m b e r of cells of
each s p e c i e s in a n u m b e r of m i c r o s c o p e
fields was c o u n t e d u n d e r 1OOO X m a g n i f i cation. E x c e p t for the r a r e r species,
1OO cells of e a c h s p e c i e s w e r e c o u n t e d
in o r d e r to e s t i m a t e cell n u m b e r s w i t h i n
20% w i t h a c o n f i d e n c e c o e f f i c i e n t of
95% (Lund et al., 1958). V o l u m e and surface area of p h y t o p l a n k t o n cells w e r e
c a l c u l a t e d f r o m l e n g t h and w i d t h m e a s u r e m e n t s u s i n g s t a n d a r d f o r m u l a s for e i t h e r
a rod, a sphere, or o b l a t e spheroid.
M o s t of the d i n o f l a g e l l a t e s w e r e a s s u m e d
to be o b l a t e s p h e r o i d , w i t h an e c c e n t r i c ity of 0.9.
42
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
Calculations
The t o t a l rate of p h o s p h a t e u p t a k e by
the p l a n k t o n was c a l c u l a t e d from the
rate of 32p d i s a p p e a r a n c e from the filtrate. A c c o r d i n g to the e q u a t i o n for a
f i r s t - o r d e r r e a c t i o n , the rate c o n s t a n t ,
k, was o b t a i n e d f r o m t r a c e r m e a s u r e m e n t s :
log C o / C
-kt
- 2.303
'
c o n c e n t r a t i o n . The s p e c i f i c a c t i v i t y of
33p04 d u r i n g the time c o u r s e was calculated f r o m s p e c i f i c a c t i v i t y of 32pO4
using a r a t i o of the counts at the b e g i n ning of the 33p and 32p e x p e r i m e n t s .
(The d i s s o l v e d o r t h o p h o s p h a t e c o n c e n t r a tion r e m a i n e d constant.)
The rate of P u p t a k e d u r i n g each time
i n t e r v a l was then c a l c u l a t e d for each
algal species as follows:
w h e r e c is the d i s s o l v e d 32p r a d i o a c t i v A cpm
iAt
ity, c O is the a c t i v i t y at time zero,
- ~g P/iAt or ~g P i-i min-land t is time. The slope of a linear recpm/~g P
g r e s s i o n t h r o u g h the d a t a p l o t t e d as
The P u p t a k e rates w e r e then a v e r a g e d
log c v e r s u s t was u s e d to o b t a i n k. The
over all time i n t e r v a l s for w h i c h t h e r e
s t e a d y s t a t e rate of d i s a p p e a r a n c e of
was a slope, and a m e a n rate of p h o s p h o o r t h o p h o s p h a t e f r o m s o l u t i o n was then obrus u p t a k e for each species of p h y t o t a i n e d as follows:
p l a n k t o n was obtained.
P disappearance rate (~gPl -I h-l)
k Co
S
w h e r e s is the s p e c i f i c a c t i v i t y of the
p h o s p h a t e ion (CPM ~g p-l) at t = O.
Rates of p h o s p h o r u s u p t a k e by the two
p a r t i c u l a t e sizes w e r e c a l c u l a t e d from
the slopes of linear r e g r e s s i o n equations of the first p o r t i o n s of the two
a p p e a r a n c e curves as follows:
P uptake rate (#g P I-[ h -I) =
&c/At
S
'
w h e r e & C / A t is the slope r e p r e s e n t i n g
the c h a n g e in the 32p a c t i v i t y on each
of the f i l t e r s w i t h time.
O b t a i n i n g p h o s p h o r u s u p t a k e rates of
i n d i v i d u a l p h y t o p l a n k t o n s p e c i e s from
a u t o r a d i o g r a p h y r e q u i r e s a k n o w n relat i o n s h i p b e t w e e n g r a i n counts and a specific a m o u n t of r a d i o a c t i v i t y . No attempt was m a d e to q u a n t i f y the g r a i n
y i e l d of 33p on K o d a k N T B - 2 emulsion. Instead, a y i e l d of 1.8 g r a i n s per e m i t t e d
B p a r t i c l e was c h o s e n f r o m l i t e r a t u r e
v a l u e s for i s o t o p e s w i t h s i m i l a r e n e r g y
levels and on films of s i m i l a r s e n s i t i v ity (Herz, 1959; Perry, 1964; Pelc, 1972;
P r e s c o t t , 1964). G r a i n counts w e r e first
c o r r e c t e d for r a d i o a c t i v e d e c a y to the
day of the e x p e r i m e n t . The m e a n n u m b e r
of g r a i n s per cell was c o r r e c t e d for
the loss of a c i d - s o l u b l e 33p d u r i n g
acid w a s h i n g . An a c i d - s o l u b l e f r a c t i o n
of 17% of t o t a l cell P, as found in
Chlorella pyrenoidosa by B a k e r and S c h m i d t
(1964), was used. The (corrected) m e a n
n u m b e r of g r a i n s per cell was t h e n conv e r t e d to counts per m i n u t e (cpm) per
liter for each s p e c i e s as follows:
Results
V a l u e s for p h y s i c a l and c h e m i c a l p a r a m eters d u r i n g the e x p e r i m e n t s are pres e n t e d in Table I. The p l a n k t o n labelling e x p e r i m e n t s w e r e c a r r i e d out from
a p p r o x i m a t e l y 10.00 to 16.OO hrs, and
the a u t o r a d i o g r a p h y e x p e r i m e n t was from
0 9 . 3 0 to 15.30 hrs. The l a r g e s t c h a n g e s
in solar r a d i a t i o n and t e m p e r a t u r e took
p l a c e in the later stages of the e x p e r i ments. The s o l u b l e r e a c t i v e p h o s p h o r u s
c o n c e n t r a t i o n was low at 4 ~g 1-I, and
a p p r o x i m a t e l y 50% of the t o t a l d i s s o l v e d
P was organic. Most p h o s p h o r u s f r a c t i o n s
r e m a i n e d c o n s t a n t t h r o u g h o u t the e x p e r i ments, e x c e p t for an i n c r e a s e in total
reactive phosphorus. Turbidity remained
at 18 J a c k s o n units t h r o u g h o u t the day.
32p Tracer Experiments
32p u p t a k e was e s t i m a t e d by m e a s u r i n g
(I) the rate of 32p d i s a p p e a r a n c e from
the d i s s o l v e d o r t h o p h o s p h a t e pool in the
filtrate, and (2) the rate of 32p a p p e a r ance in the p a r t i c u l a t e fraction. The results of these e x p e r i m e n t s are s h o w n in
Figs. I and 2. V e r y little u p t a k e occ u r r e d in the b o t t l e c o n t a i n i n g iodoacetic acid (IAA), w h e r e b i o l o g i c a l uptake of p h o s p h o r u s was inhibited. C h e c k s
of total r a d i o a c t i v i t y at the b e g i n n i n g
and at the end of each b o t t l e e x p e r i m e n t
c o n f i r m e d that no 32p was lost to the
sides of the b o t t l e or by leakage. 32po 4
d i s a p p e a r e d r a p i d l y f r o m the w a t e r in
Mean grains/cell x no. of cells/l
the dark bottle. S i n c e this u p t a k e was
cpm/l = 1.8 grains/count x 4320 rain exposure time"
low w i t h IAA p r e s e n t , it was a s s u m e d to
be due to b i o l o g i c a l activity. The light
S p e c i f i c a c t i v i t y of d i s s o l v e d o r t h o p h o s p h a t e at e a c h time p o i n t in the
b o t t l e was lost d u r i n g the e x p e r i m e n t .
first 2 h was c a l c u l a t e d f r o m r a d i o a c t i v - However, the u p t a k e in light and dark
ity in the f i l t r a t e of the 32p u p t a k e ex- b o t t l e s has r a r e l y d i f f e r e d s i g n i f i c a n t p e r i m e n t and the d i s s o l v e d o r t h o p h o s p h a t e ly in e x p e r i m e n t s of this kind p e r f o r m e d
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
43
Table i. Physical and chemical parameters during 32p plankton labelling
and 33p autoradiography experiments, October 31, 1974
Time
(hrs)
Solar
radiation
(g-cal
cm-2 min-1)
Water
temperature
(oc)
8.OO
9.OO
iO.O0
ii.00
12.00
13.00
14.O0
15.00
16.00
0.24
0.50
0.70
0.78
0.80
0.76
0.64
0.40
0.20
15.2
15.2
15.3
15.5
15.7
16.O
16.4
17.0
17.2
Reactive phosphorus
(~g p i-i)
Total
Dissolved a
Total phosphorus
(~g P 1-1 )
Total
Dissolved
6
4
49
10
16
4
45
7
aAlthough much has been written about other phosphorus compounds which
might react with molybdate if they are present in natural waters, we
assume for our purposes that dissolved reactive phosphorus and dissolved orthophosphate are synonymous.
4.4
0.45.u m, dark
O0
bJ
I-
4.3
hl
(9
7
--I
<4.2
~~zk-'~ a
IK
I..-
m
A
0
6
E)
l-
n-
~4.1
O.
Z
U..
z__4.o
--
U)
0
_J
n
m 3.9_
Ig
n
@
3
2
13.
r
.o 3 . 7 _
5 @ m, dark
I
0
3.6_
i
0
4
--I
Q
E3.a
.
E)
I
I
I O0
I
200
i
I
I
300
0
T I M E (MIN)
I
i
,
I O0
TI ME
I
200
,
L
300
(MIN)
Fig. i. Disappearance of 32p from the filtrate
in dark bottle (circles) and bottle containing
the metabolic inhibitor iodoacetic acid, IAA
(triangles). The bottles were incubated in the
Rhode River at I m depth for 4 h
Fig. 2. Appearance of 32p in particulates. Circles: 32p radioactivity on 5 and 0.45 ~m poresized filters in dark bottle; triangles: 32p
radioactivity on 0.45 ~m pore-sized filter in
IAA bottle
in the Rhode River
( C o r r e l l et al., 1 9 7 5 ;
Faust and Correll,
1976).
The appearance
of 32p i n s i z e f r a c t i o n s of p a r t i c u l a t e s
is s h o w n in F i g . 2.
In t h e I A A b o t t l e , t h e r e w a s n o P u p t a k e
by either size fraction.
The majority
of
dark-bottle
uptake was by organisms
passing the 5 ~m screen and retained
on
the 0.45 ~m membrane.
Disappearance
of 3 2 p f r o m t h e w a t e r
appeared to follow a pseudo first-order
reaction.
I n F i g . I, it c a n b e s e e n t h a t
44
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
Table 2. Estimates of population numbers and biomass of phytoplankton
species from Rhode River station at i m depth on October 31, 1974
Species
Size
(~m)
Volume
(~m 3)
Peridineum
trochoideum
25 x 28
10300
4.9
O.O5[
2.5
Gymnodinium sp. 2
25 x 18
5890
[40.0
0.825
41.8
Gymnodinium
gracilentum
20 x iO
2090
72.3
O.151
7.6
Gymnodinium sp. 1
15 x 11
1300
68.3
0.0~9
4.5
Gyrodinium
mundulum
15 x iO
1180
22.3
0.026
1.3
Gymnodinium
galesianum
~5 x 15
i170 a
42.7
0.050
2.5
Gymnodinium
subroseum
11 x
8
546
33.4
O.018
0.9
Flagellate
IO x IO
523
50.0
0.050
2.5
Euglena ascus
20 x
392
117
0.046
2.3
19oa
],300
0.247
12.5
34
12,400
0.422
21.4
14,3OO
1.98
5
Katodinium
rotundatum
10-15 x 3-10
Nannoplankton
<5
Total
avalues from Campbell
Cells 1-I
(x iO3)
Biomass
(mm 3 1-1)
% total
biomass
(1973).
t h e log 32p in s o l u t i o n d e c r e a s e s
linearly f o r a p p r o x i m a t e l y
t h e f i r s t 2 h. It
t h e n l e v e l s off t o an e q u i l i b r i u m
value
w h e n 32p r e l e a s e i n t o t h e w a t e r b y
planktonic
organisms has begun.
As in a n y f i r s t - o r d e r
reaction, the
appearance
of t h e " p r o d u c t "
(i.e., 32p
in p a r t i c u l a t e s )
is a c u r v e d line. A l t h o u g h t h e r a t e of p h o s p h a t e
u p t a k e is
c o n s t a n t , t h e r a t e of a p p e a r a n c e
of 32p
in p a r t i c u l a t e s
decreases
as t h e s p e c i f ic a c t i v i t y of o r t h o p h o s p h a t e
decreases.
T h e f i r s t p o r t i o n of t h e c u r v e is f i t t e d
to a s t r a i g h t line.
The total P uptake rate calculated
from the disappearance
of 3 2 P 0 4 f r o m sol u t i o n is 1.07 bg P 1 -I h-1. U p t a k e
rates calculated
f r o m t h e r a t e of 32p
appearance
in p a r t i c u l a t e s
on t h e 5 a n d
0 . 4 5 Lum f i l t e r s a r e 2% (0.024 u g p 1-I h -I)
a n d 98% (1.O6 ~ g P 1 -I h - l ) , r e s p e c t i v e ly, of t h e t o t a l P u p t a k e rate.
Phytoplankton Population
The phytoplankton
in t h e R h o d e R i v e r o n
t h e d a y of t h e e x p e r i m e n t s
w a s n o t as
a b u n d a n t as d u r i n g t h e w a r m e r m o n t h s of
the year, but the species composition
w a s q u i t e d i v e r s e . T e n m a j o r s p e c i e s of
a l g a e w e r e o b s e r v e d , of w h i c h m o s t w e r e
dinoflagellates.
Phytoplankton
populat i o n n u m b e r s a n d b i o m a s s are g i v e n in
T a b l e 2. O n l y o n e s p e c i e s , Katodinium rotundatum, n u m b e r e d o v e r o n e m i l l i o n c e l l s
p e r liter. T h e n a n n o p l a n k t o n ,
which was
c o m p o s e d of m a n y s m a l l f l a g e l l a t e s ,
chrysophytes,
and green algae, was most
abundant. Small diatoms were quite numerous, b u t m o s t of t h e c e l l s a p p e a r e d t o
be senescent.
It c a n b e s e e n f r o m T a b l e 2
t h a t Gymnodinium s p e c i e s 2, K. rotundatum,
and the n a n n o p l a n k t o n
m a d e up t h e l a r g e s t p o r t i o n s of t h e a l g a l b i o m a s s :
41.8,
21.4, a n d 12.5%, r e s p e c t i v e l y .
33p Autoradiography
Fig. 3 s h o w s the a p p e a r a n c e
of 33p in
fixed, a c i d - w a s h e d
cells (concentrated
f r o m I i of w a t e r b y c e n t r i f u g a t i o n
and
l a t e r u s e d for a u t o r a d i o g r a p h y )
with
time. T h e r a t e of p h o s p h o r u s
incorporation into acid-insoluble
c o m p o u n d s inside plankton cells (including bacteria)
was calculated
f r o m Fig. 3 as 0 . 7 6 8 ~g
P I-I h - l , w h i c h is a p p r o x i m a t e l y
75% of
t h e t o t a l u p t a k e r a t e o b t a i n e d in t h e
32p u p t a k e e x p e r i m e n t .
We observed that autoradiograms
of
the Rhode River samples contained a
g r e a t e r n u m b e r of b a c k g r o u n d
grains than
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
45
w a s f o u n d in p l a n k t o n p r e p a r a t i o n s
by
B r o c k a n d B r o c k (1968) 9 T h i s p r o b l e m w a s
a t t r i b u t e d to t h e l a r g e n u m b e r of b a c t e r i a p r e s e n t in e s t u a r i n e w a t e r s 9 A u t o 4
radiograms
of p l a n k t o n at t h e f i r s t t i m e 9
points were relatively
f r e e of b a c k g r o u n d
c o u n t s , b u t t h e n u m b e r of b a c k g r o u n d
I ~9
9
f
grains increased steadily with time during the f i r s t 2 h; t h e n u m b e r t h e n re:3
m a i n e d c o n s t a n t d u r i n g t h e r e m a i n d e r of
the experiment.
Therefore,
it w a s n e c e s O
s a r y to s u b t r a c t b a c k g r o u n d
g r a i n s at
K
each time-point
as d e s c r i b e d
in " M a t e r i als a n d M e t h o d s "
2
Fig
4 illustrates
t h e i n c r e a s e in
t h e m e a n n u m b e r of g r a i n s p e r c e l l w i t h
O
t i m e for o n e d i n 9
species.
Generally,
the s t a n d a r d d e v i a t i o n of
g r a i n c o u n t s for a s p e c i e s at o n e t i m e
p o i n t w a s w i t h i n ~ 30% of the m e a n . T h e
i n c r e a s e in g r a i n s w a s r a p i d at f i r s t
and then leveled off when the specific
a c t i v i t y of the o r t h o p h o s p h a t e
p o o l in
the water had decreased 9 Similar curves
w e r e o b t a i n e d for a l l 10 s p e c i e s of p h y 0 (
toplankton.
All curves had shapes simiI
I
I
I
I
I
lar to t h e o n e s h o w n , w i t h t h e a b s o l u t e
6 0 120 180 2 4 0 3 0 0 3 6 0
0
n u m b e r of g r a i n s a n d t h e s l o p e d i f f e r i n g
for each species.
TIME (MIN)
R a t e s of p h o s p h o r u s
u p t a k e for all
phytoplankton
s p e c i e s w e r e c o m p u t e d , as
Fig. 3. Autoradiography experiment. Appearance
described
in t h e " M a t e r i a l s a n d M e t h o d s "
of 33p in acid-washed plankton cells and partis e c t i o n , f r o m t h e i n c r e a s e in g r a i n
culates with time. Rhode River water was inc o u n t s p e r c e l l o v e r time. T h e s e u p t a k e
cubated with 33p in 12 1 bottle in the light at
rates are presented
in T a b l e 3, w i t h
i m depth. Each I 1 subsample was fixed, centriseveral parameters
of c e l l d i m e n s i o n s 9
fuged, and washed with dilute HCI. Solid line
U p t a k e r a t e p e r c e l l is l a r g e s t for
represents a least-squares regression equation
l a r g e c e l l s ; h o w e v e r , it is o b v i o u s f r o m
giving best fit of first 7 data points; broken
t h e d a t a in t h e f i f t h c o l u m n of T a b l e 3
line is arbitrarily drawn
that the phosphorus
uptake rate per ~m 3
c e l l v o l u m e is h i g h e s t for s m a l l e r c e l l s .
The relationship
between size and uptake
r a t e is i l l u s t r a t e d m o r e c l e a r l y in
Fig. 5. A p l o t of log P u p t a k e p e r c e l l
m a s s a g a i n s t log s u r f a c e : v o l u m e
ratio
~150
o
_
_ @
/
for t h e v a r i o u s p h y t o p l a n k t o n
s p e c i e s is
/
/
linear. The line represents
the b e s t f i t
/
/
(r 2 = 0.93) of t h e l i n e a r r e g r e s s i o n
(D
equation,
log Y = (-10 7) + (I 7 log x)
~I00
w h e r e Y is u p t a k e in u g P p e r ~un3 c e l l
v o l u m e p e r h o u r for a s p e c i e s a n d x is
z
the s u r f a c e : v o l u m e
ratio 9 Taking the
~ 50
antilogarithm,
the equation becomes Y =
(2 x 10-11) x 1.7, a n d w e c a n see t h a t P
u p t a k e of t h e s e a l g a l c e l l s is a p o w e r
0
f u n c t i o n of t h e i r s u r f a c e : v o l u m e
ratio 9
I
I
I
I
t
I
To t e s t w h e t h e r t h e o b s e r v e d r e l a t i o n 0
60
120
180
240
500
560
ship between uptake and surface:volume
TIME (MIN)
r a t i o is t r u e f o r o t h e r R h o d e R i v e r p h y toplankton populations
rather than just
Fig. 4. Gymnodinium species i. Mean number of
for the one, w e a n a l y z e d t h e 33p a u t o grains per cell with time. Whole water samples
radiography
d a t a of J u l y 9, 1973 f r o m
containing this species were incubated with
C o r r e l l et al. (1975) in t h e s a m e w a y .
33PO4 in the Rhode River at i m depth for 6 h.
Their data shows a similar correlation
Solid line gives best-fit linear regression of
b e t w e e n g r a i n d e n s i t y p e r bm3 c e l l v o l first 5 data points; broken line is arbitrarily
ume p e r h o u r a n d s u r f a c e : v o l u m e
ratio
drawn
9
_
_
Table 3. Surface areas, surface:volume ratios, and phosphorus uptake rates for Rhode River phytoplankton species, October 31, ~974. Uptake rates were computed from grain counts of autoradiograms.
For phytoplankton cell counts and biomass estimates, see Table 2
Species
Surface area
(~m2 cell-l)
Surface:
volume
ratio
uptake rate Uptake rate Uptake rate
(~g P cell -I (~g P ~m -3
(~g P 1-I
h-i x 10-9)
h-i x 10-12 ) h-i x 10 -3 )
1927
O. 188
1 i. 84
1 . 15
0.058
O. 15
981
O. 167
7.74
i .31
1.084
2.84
Gymnodinium
gracilentum
739
O. 353
8.52
4.O7
0.616
1.62
Gymnodinium sp. i
490
O. 378
3.63
2 .80
O.248
0.65
Gyrodinium
mundulum
464
O. 394
2.60
2.21
0.058
O. 15
Gymnodinium
galesianum
707
0.604
6.28
5 .37
0.268
0.69
Gymnodinium
subroseum
261
O.512
4.43
8.76
0.148
0.38
Flagellate
314
0.600
3.48
6.65
O. 174
O. 45
Euglena ascus
314
O. 801
5.92
15.1
0.692
I .81
Katodinium
rotundatum
266
1 .400
5.02
26.4
6.539
17.14
50
i .480
2.30
67.7
Peri dineum
trochoi deum
Gymnodinium sp. 2
Nannoplankton
28.52
% Total phytoplankton uptake
rate per liter
74.75
38.41 = 0.038 ~ g P l - l h -I
Total uptake rate
(uptake rates were not available).
The
regression
equation
is Y = O . 1 7 x 2 . 4
(r 2 = 0 . 9 9 ) . T h e c o n v e r s i o n
from grains
-i0
t o u p t a k e in ~ g P i n v o l v e s
several constants which account for the difference
between the coefficients
of x i n t h e
jequations
for the 1973 and 1974 data.
However,
the similarity
between expoICJ
n e n t s of x (1.7 f o r o u r d a t a , a n d 2 . 4
E
f o r t h e 1 9 7 3 d a t a ) is i n t e r e s t i n g .
O.
T h e d a t a in t h e s i x t h a n d s e v e n t h c o l umns in Table 3 shows that nannoplankton
were responsible
for the largest part of
,.-Ii
the phosphorus
uptake by the phytoplankU.I
t o n i n t h e R h o d e R i v e r at t h i s t i m e of
year. Other species which showed signifiI-9
Q.
c a n t P u p t a k e a r e Katodinium rotundatum,
Gymnodinium graciientum, Gymnodinium s p e c i e s 2,
Q.
a n d Euglena ascus. T h e s e s p e c i e s , w h i c h
comprise
the largest portions
of t h e p h y L9
toplankton
b i o m a s s b e c a u s e of e i t h e r
0
..I
l a r g e n u m b e r s of c e l l s o r l a r g e c e l l
size, are also responsible
for the greate s t p a r t of t h e t o t a l p h o s p h o r u s
uptake
-t2
per liter. However,
no clear-cut
relationship between the total biomass
and
I
J
I
I
I
I
I
I
I
I
the rate of phosphate
uptake by each spe0
+Q3
- 1.0
- 0.5
c i e s is i n d i c a t e d .
With the correlation
in F i g . 5 in
LOG (SURF/VOL)
m i n d , an e x a m i n a t i o n
of t h e r e l a t i o n s h i p
between total cell surface area for a
Fig. 5. Relationship between phosphorus uptake
per ~m 3 cell volume per hour for various phytos p e c i e s a n d i t s P u p t a k e p e r l i t e r is
plankton species and their surface:volume ratios, m o r e e n l i g h t e n i n g .
Nannoplankton
a n d KatoLine drawn is linear regression of data: log Y =
dinium rotundatum h a d t h e h i g h e s t s u r f a c e :
(-1o.7) + 1.7 log x (r2 = 0.93)
volume ratios
( 1 . 4 8 a n d 1.4) a n d h i g h e s t
%
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
cell n u m b e r s ; these o r g a n i s m s w e r e res p o n s i b l e for 75 and 17% r e s p e c t i v e l y ,
of the t o t a l p h y t o p l a n k t o n P uptake rate.
In contrast, species w i t h low surface:
volume ratios, such as Gyrodinium mundulum
(0.394) and Peridineumtrochoideum (O.188)
and r e l a t i v e l y low cell numbers, had P
u p t a k e rates w h i c h w e r e O.15% of the total. Thus, we found that in the w a t e r
column, the P u p t a k e rate by a species
is c o r r e l a t e d w i t h its t o t a l s u r f a c e
area. The e q u a t i o n Y = (-1.4 x 10-3) +
(4.15 x 10-5 x), w h e r e x = s u r f a c e area
in m m -2 1-I for a species and Y
P uptake in ~g P 1-I h-1 for that species,
is the linear r e g r e s s i o n of the data,
w i t h r 2 = 0.90.
The sum of i n d i v i d u a l p h y t o p l a n k t o n P
uptake rates as given in T a b l e 3 is
0.038 ~g P 1-I h-1. The rates were calc u l a t e d a s s u m i n g that a c i d - s o l u b l e P was
17% of t o t a l cell P. If the a c i d - s o l u b l e
P f r a c t i o n w e r e a s s u m e d to be 50% as it
is in some diatoms, the t o t a l u p t a k e
rate w o u l d be 0.063 ~g P 1-I h-1. The
sum of the rates of i n d i v i d u a l species
o b t a i n e d from a u t o r a d i o g r a p h y is in v e r y
good a g r e e m e n t w i t h the rate of 0 . 0 2 4 ~g
P 1 -I h -I for p l a n k t o n cells "larger than
5 %~m" o b t a i n e d in the 32p u p t a k e e x p e r i ments.
47
f r a c t i o n a t i o n by this m e t h o d is not perfect by any means, however. S h e l d o n and
S u t c l i f f e (1969) have s t u d i e d the accuracy of size s e p a r a t i o n of p l a s t i c particles w i t h d i f f e r e n t types and pore
sizes of filters. Their r e s u l t s show
that m a n y p a r t i c l e s w i t h a d i a m e t e r less
than the s t a t e d p o r e size of the f i l t e r
are r e t a i n e d due to c l o g g i n g of pores
and o b s t r u c t i o n of p a r t i c l e p a t h t h r o u g h
the filters. The type of f i l t e r used, as
w e l l as its pore size, d e t e r m i n e d the
q u a l i t y of s e p a r a t i o n obtained; s c r e e n s
rather than m e m b r a n e filters g a v e the
best separation. In our e x p e r i m e n t , particles s m a l l e r than 5 ~m (including the
n a n n o p l a n k t o n ) w e r e p r o b a b l y r e t a i n e d by
the 5 ~m screen. B a c t e r i a are n o t o r i o u s
for a t t a c h i n g to surfaces, i n c l u d i n g
s u s p e n d e d s e d i m e n t s and other p l a n k t o n
cells. This p h e n o m e n o n m i g h t lead to
o v e r e s t i m a t i o n of p h o s p h a t e u p t a k e by
p l a n k t o n "larger than 5 H/n" in size.
A n d e r s o n (1965) r e p o r t e d e x p e r i m e n t s
w i t h size f r a c t i o n a t i o n of p l a n k t o n off
the O r e g o n Coast. He found that large
f r a c t i o n s of p h y t o p l a n k t o n p a s s e d t h r o u g h
a 5 Lum filter, w h i l e v e r y little p a s s e d
t h r o u g h a 0.8 ~m filter. B e r m a n (1975),
w o r k i n g in the Gulf of C a l i f o r n i a , found
13 to 83% of total carbon f i x a t i o n of
n a t u r a l p h y t o p l a n k t o n p o p u l a t i o n s was
a s s o c i a t e d w i t h o r g a n i s m s p a s s i n g a 3 ~m
Discussionand ~nclusions
N u c l e p o r e filter. The r e s u l t s of these
size f r a c t i o n a t i o n e x p e r i m e n t s depend,
The e x p e r i m e n t s p r e s e n t e d here w e r e deof course, upon the size d i s t r i b u t i o n of
s i g n e d to m e a s u r e the p h o s p h o r u s u p t a k e
natural phytoplankton populations. While
of a n a t u r a l e s t u a r i n e p h y t o p l a n k t o n com- these two s t u d i e s w e r e c a r r i e d out in
m u n i t y u n d e r n a t u r a l c o n d i t i o n s . E n v i r o n - open m a r i n e waters, size f r a c t i o n a t i o n
m e n t a l v a r i a b l e s in t h e s e e x p e r i m e n t s
e x p e r i m e n t s w i t h p l a n k t o n from a s h a l l o w
such as solar r a d i a t i o n , w a t e r t e m p e r a e a s t - c o a s t e s t u a r y showed m o s t a l g a l
ture, salinity, and n u t r i e n t levels,
cells to be r e t a i n e d by a 5 ~m N i t e x
w e r e t h o s e o c c u r r i n g n a t u r a l l y in the
screen (Faust and Correll, 1976). Also,
Rhode River. T h e r e f o r e , the n u t r i e n t upa u t o r a d i o g r a m s of the f i l t r a t e of our
take rates o b s e r v e d in this study are
cell s u s p e n s i o n s f i l t e r e d via g r a v i t y
likely to be t h o s e w h i c h occur w h e n a
t h r o u g h an 8 ~m N u c l e p o r e f i l t e r cond i v e r s e g r o u p of a l g a e is p r e s e n t and ex- t a i n e d few p h y t o p l a n k t o n cells. A l t h o u g h
cess n u t r i e n t s are not available.
m i c r o s c o p i c e x a m i n a t i o n of the f i l t e r s
The rate of t r a n s f e r of p h o s p h o r u s
was not m a d e in this study, it is asfrom the d i s s o l v e d o r t h o p h o s p h a t e pool
sumed that the m a j o r i t y of p h y t o p l a n k t o n
into p l a n k t o n o r g a n i s m s was m e a s u r e d by
was r e t a i n e d by the 5 ~m screen as shown
(I) t r a c i n g the m o v e m e n t of 32p04 into
by p r e v i o u s studies in the same e s t u a r y
size f r a c t i o n s of p a r t i c u l a t e s , and (2)
(Faust and Correll, 1976).
by f o l l o w i n g the m o v e m e n t of 33po 4 into
A l t h o u g h t h e r e are no r e s u l t s f r o m
i n d i v i d u a l s p e c i e s of p h y t o p l a n k t o n .
the light bottle, we h a v e f o u n d that toUsing the first method, the total phostal P u p t a k e and u p t a k e by d i f f e r e n t parp h o r u s u p t a k e rate of the p l a n k t o n was
t i c u l a t e sizes g e n e r a l l y do not vary sigd e t e r m i n e d to be a p p r o x i m a t e l y I ~g P
n i f i c a n t l y b e t w e e n light and dark bot1-I h-1. A c o n t r o l e x p e r i m e n t w i t h biotles in 32p t r a c e r e x p e r i m e n t s on the
logical i n h i b i t o r s h o w e d very little adRhode River (Correll et al., 1975; F a u s t
s o r p t i o n of the r a d i o a c t i v e t r a c e r to
and Correll, 1976). T a f t et al. (1975)
n o n - l i v i n g p a r t i c l e s . P l a n k t o n cells realso o b t a i n e d i d e n t i c a l r e s u l t s w i t h
t a i n e d on the 5 ~m screen w e r e r e s p o n s i light and dark b o t t l e s in 32p t r a c e r exble for about 2% of the u p t a k e (0.024 ~g
p e r i m e n t s w i t h p l a n k t o n of C h e s a p e a k e
P 1-I h-l), w h i l e those p a s s i n g the 5 ~um Bay. Thus, it a p p e a r s that the o b s e r v e d
screen d i s p l a y e d 98% of the uptake. Size
P u p t a k e does not r e q u i r e light and is
=
48
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
not d i r e c t l y d r i v e n by p h o t o s y n t h e s i s ~
this u p t a k e also is not i n h i b i t e d n o t i c e ably by light.
The m a j o r i t y of u p t a k e is by small
o r g a n i s m s w h i c h i n c l u d e few algae; light
or the a b s e n c e of light has l i t t l e effect u p o n t h e i r P uptake. T h e r e f o r e ,
m o s t of the p h o s p h a t e u p t a k e m u s t be att r i b u t e d to b a c t e r i a . M a n y l a b o r a t o r y
s t u d i e s have e s t a b l i s h e d the r a p i d uptake of i n o r g a n i c p h o s p h e r u s by b a c t e r i a
(Rigler, 1956; Harris, 1957; J o h a n n e s ,
1964; Rhee, 1972; B e u c h l e r and Dillon,
1974). We h a v e less k n o w l e d g e of this
p r o c e s s o c c u r r i n g in nature. R i g l e r ' s exp e r i m e n t s (1956) w i t h lake p l a n k t o n ind i c a t e d a h i g h p h o s p h a t e u p t a k e by b a c teria. Taft (1974) and T a f t et el. (1975)
argue that b a c t e r i a are not i m p o r t a n t
c o n s u m e r s of i n o r g a n i c p h o s p h a t e in est u a r i n e waters. T h e y n o t e that p a r t i c u lates a b s o r b i n g p h o s p h a t e in their studies had N and P to c h l o r o p h y l l a r a t i o s
w h i c h are close to t h o s e of m o s t a l g a l
cells. In c o n t r a s t , F a u s t and C o r r e l l
(1976) h a v e shown a h i g h c o r r e l a t i o n bet w e e n p h o s p h a t e u p t a k e rates by p a r t i c u lates s m a l l e r than 5 Lun and b a c t e r i a l
b i o m a s s in e s t u a r i n e w a t e r s at v a r i o u s
times of the year.
The s e c o n d c o n s i d e r a t i o n was the effect of cell size and s h a p e upon g r a i n
density. For a p o i n t source of r a d i o a c tivity l o c a t e d at the c e n t e r of a disc,
the n u m b e r of g r a i n s r e s u l t i n g f r o m the
e m i t t e d ~ p a r t i c l e w h i c h are c o u n t e d
over a large disc w i l l be h i g h e r than
those c o u n t e d over a small disc. H o w e v e r ,
this e f f e c t is g r e a t l y r e d u c e d by counting grains at the m a r g i n s and s e v e r a l
m i c r o m e t e r s beyond. The data p r e s e n t e d
by K n o e c h e l and K a l f f (1976) shows that
the g r a i n c o u n t i n g e f f i c i e n c y over a
cell 10 Lum in d i a m e t e r , w h e n c o u n t i n g is
e x t e n d e d 5 Lum b e y o n d cell m a r g i n s , is
75%. Thus, since g r a i n c o u n t i n g was ext e n d e d that far in this study, the effic i e n c y was r e d u c e d v e r y little. Also,
since our cells w e r e w i t h i n a n a r r o w
size range, any r e d u c t i o n in e f f i c i e n c y
w o u l d be n e a r l y c o n s t a n t for all the
cells. The a u t h o r s also note that selfa b s o r p t i o n of r a d i a t i o n by the cells m a y
cause error. However, s e l f - a b s o r p t i o n is
g r e a t e r in large cells; since m o s t of
our cells w e r e less than 20 Lum in diameter, this e f f e c t w o u l d r e d u c e a c t u a l
g r a i n counts by 15% at most, a c c o r d i n g
to e s t i m a t e s by K n o e c h e l and K a l f f (1976).
For the s m a l l e s t cells, s e l f - a b s o r p t i o n
w o u l d r e s u l t in a r e d u c t i o n in g r a i n
Only 2% of the p h o s p h a t e u p t a k e was
counts of only a few percent.
by o r g a n i s m s r e t a i n e d on the 5 %um screen,
F i n a l l y , error is i n t r o d u c e d in estip r e s u m a b l y algae. A l t h o u g h the algal abm a t i n g the g r a i n y i e l d of a r a d i o a c t i v e
s o r p t i o n rate seems a r e m a r k a b l y small
i s o t o p e on a p a r t i c u l a r e m u l s i o n or film.
part of the t o t a l uptake, it is conIdeally, an i n t e r n a l s t a n d a r d i z a t i o n bef i r m e d by the sum of i n d i v i d u a l s p e c i e s
t w e e n g r a i n s and d i s i n t e g r a t i o n s h o u l d
rates o b t a i n e d f r o m a u t o r a d i o g r a p h y . The
be m a d e using cells w i t h a k n o w n radiop h y t o p l a n k t o n r a t e f r o m the f i l t r a t i o n
activity. As s t a t e d before, no a t t e m p t
t e c h n i q u e is 0.024 ~g P 1-I h-l, as comat this was m a d e in this study. However,
p a r e d w i t h 0 . 0 3 8 to 0 . 0 6 3 ~g P 1-I h-1
the c o m p a r i s o n b e t w e e n algal species is
from a u t o r a d i o g r a p h y , d e p e n d i n g u p o n the
not a f f e c t e d by any e r r o r in the g r a i n
v a l u e u s e d for the a c i d - s o l u b l e P conyield, as it is m e r e l y a c o n s t a n t used
tent of the p h y t o p l a n k t o n cells.
in c a l c u l a t i o n of all u p t a k e rates. The
The t e c h n i q u e of a u t o r a d i o g r a p h y was
sum of i n d i v i d u a l p h o s p h a t e u p t a k e rates,
u s e d to c o m p a r e the p h o s p h o r u s u p t a k e
i.e., the total p h y t o p l a n k t o n u p t a k e
rates of i n d i v i d u a l s p e c i e s of p h y t o rate, m i g h t r e f l e c t an error in the
p l a n k t o n . K n o e c h e l and K a l f f (1976) h a v e
g r a i n yield, but the g r a i n y i e l d used
c r i t i c a l l y r e v i e w e d the use of g r a i n (1.8) w o u l d have to be u n d e r e s t i m a t e d by
d e n s i t y a u t o r a d i o g r a p h y for c o m p a r i s o n
an o r d e r of m a g n i t u d e b e f o r e a s i g n i f i of m e t a b o l i c a c t i v i t y in p l a n k t o n specant d i f f e r e n c e in the total p h y t o p l a n k cies. T h e y s u b m i t that errors r e s u l t
ton u p t a k e rate w o u l d be observed. The
from s e v e r a l sources: (I) i n a d e q u a t e
s o u r c e s of error w h i c h K n o e c h e l and
cell p r e p a r a t i o n ; (2) f a i l u r e to conK a l f f list are s i g n i f i c a n t only if autosider the e f f e c t of cell size and geomr a d i o g r a p h y is used for s t u d y i n g large
etry on g r a i n d e n s i t y ; (3) i n a d e q u a t e
p h y t o p l a n k t o n cells w i t h a high e n e r g y
m e t h o d s of d e t e r m i n i n g g r a i n yield.
isotope, or if one is c o m p a r i n g the results of s e v e r a l d i f f e r e n t a u t o r a d i o g In this study, the m e t h o d of cell
r a p h y e x p e r i m e n t s in w h i c h there are difp r e p a r a t i o n was not d e t r i m e n t a l to the
f e r e n c e s in t o t a l r a d i o a c t i v i t y .
cells, as no a c e t o n e , h a r s h f i x a t i v e s ,
As n o t e d in the results, the autoheat, or h i g h c e n t r i f u g e speeds w e r e
r a d i o g r a m s c o n t a i n e d a n u m b e r of backused. F i x a t i o n was complete: F r o t a r g o l
g r o u n d g r a i n s w h i c h i n c r e a s e d w i t h time.
s t a i n i n g of the g l u t e r a l d e h y d e - f i x e d
A u t o r a d i o g r a m s of the f i l t r a t e o b t a i n e d
cells s e v e r a l m o n t h s later s h o w e d e x c e l by f i l t e r i n g 3 3 p - l a b e l l e d cell s u s p e n lent f i x a t i o n and no d e t e r i o r a t i o n of
sions t h r o u g h an 8 H/n p o r e - s i z e f i l t e r
cell o r g a n e l l e s .
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
also c o n t a i n e d grains w i t h o u t any v i s i ble cells u n d e r them, and t h e s e g r a i n s
also i n c r e a s e d w i t h time. A r e a s o n a b l e
source for t h e s e g r a i n s is b a c t e r i a ,
w h i c h are not r e s o l v e d in a u t o r a d i o g r a m s
at 1000 X m a g n i f i c a t i o n w i t h o u t staining,
C e r t a i n l y , a d s o r p t i o n of 33p to sediments or i n c o m p l e t e d w a s h i n g of s o l u b l e
33p is r u l e d out b e c a u s e the n u m b e r of
b a c k g r o u n d grains i n c r e a s e d w i t h time.
Fuhs and C a n e l l i (1970) o b s e r v e d s i m i l a r
c l u s t e r s of b a c k g r o u n d grains in 33p
a u t o r a d i o g r a m s . They s u s p e c t e d that
clumps of b a c t e r i a w e r e the cause, but
they could not p r o v e that b a c t e r i a l
cells w e r e present. F a u s t and C o r r e l l
(1977) w e r e able to o b s e r v e g r a i n s over
b a c t e r i a l cells in 33p a u t o r a d i o g r a m s by
s t a i n i n g their cell p r e p a r a t i o n s . As exp l a i n e d in " M a t e r i a l s and M e t h o d s " , a
c o r r e c t i o n for b a c k g r o u n d counts was
m a d e for each a u t o r a d i o g r a m . However, no
c o r r e c t i o n for the a d h e r i n g and c l u m p i n g
of b a c t e r i a on p h y t o p l a n k t o n cells could
be made. This could r e s u l t in o v e r e s t i m a tion of p h o s p h o r u s u p t a k e by p l a n k t o n
cells.
R e s u l t s from a u t o r a d i o g r a p h y show
that u p t a k e of p h o s p h o r u s by the nannop l a n k t o n (algal cells less than 5 ~m in
size) a c c o u n t s for a l m o s t 75% of the total u p t a k e by p h y t o p l a n k t o n . The importance of n a n n o p l a n k t o n in the t o t a l
p r i m a r y p r o d u c t i o n of m a n y p l a n k t o n i c
c o m m u n i t i e s has b e e n d i s c u s s e d in the "Int r o d u c t i o n " . Taft (1974) s h o w e d that
there was no d i f f e r e n c e in i n o r g a n i c
p h o s p h a t e u p t a k e rates of p l a n k t o n passing a 35 bm and a 10 V/n filter. He also
c o n c l u d e d that s m a l l e r p l a n k t o n cells
(which c o u l d i n c l u d e bacteria) w e r e res p o n s i b l e for the m a j o r i t y of the p h o s p h o r u s u p t a k e in C h e s a p e a k e Bay.
The f r a c t i o n of the t o t a l rate of uptake by the n a n n o p l a n k t o n exceeds their
c o n t r i b u t i o n to the t o t a l p h y t o p l a n k t o n
biomass. W a t t (1971) found this to be
true for n a n n o p l a n k t o n p r o d u c t i v i t y in
14C a u t o r a d i o g r a p h y e x p e r i m e n t s . E a r l i e r
33p a u t o r a d i o g r a p h y of Rhode River p l a n k ton was p e r f o r m e d d u r i n g a b l o o m of
large d i n o f l a g e l l a t e s (Correll et al.,
1975). A l t h o u g h the d i n o f l a g e l l a t e s m a d e
up 90% of the p h y t o p l a n k t o n b i o m a s s , the
rate of p h o s p h o r u s u p t a k e per <un3 of
cell v o l u m e by the n a n n o p l a n k t o n was 300
to 500 times the rate of the d o m i n a n t
larger p h y t o p l a n k t o n cells. Thus, the total p h o s p h a t e u p t a k e rate of the nannop l a n k t o n was still h i g h e r than that of
the n u m e r o u s large d i n o f l a g e l l a t e s .
Fuhs et al. (1972) o b s e r v e d the e f f e c t
of cell s u r f a c e : v o l u m e ratio on n u t r i e n t
u p t a k e and m a x i m u m g r o w t h rate. They
found that m a x i m u m g r o w t h rates of two
d i a t o m s p e c i e s o c c u r r e d at the same p h o s -
49
phate c o n c e n t r a t i o n b e c a u s e the lower
a f f i n i t y for p h o s p h a t e (larger K s ) of the
s m a l l e r s p e c i e s was o f f s e t by its g r e a t er s u r f a c e : v o l u m e ratio. Our r e s u l t s of
the a u t o r a d i o g r a p h y e x p e r i m e n t i n d i c a t e
that the algal p h o s p h o r u s u p t a k e r a t e
per b i o m a s s unit is r o u g h l y a f u n c t i o n
of the s q u a r e of cell s u r f a c e : v o l u m e
ratio:
u/v ~ K' (s/v) 2.
We m a y ask why this s h o u l d be so. Is
there a simple r e l a t i o n s h i p b e t w e e n cell
d i m e n s i o n or g e o m e t r y and n u t r i e n t uptake rate u n d e r l y i n g these r e s u l t s ?
Let us a s s u m e that the P u p t a k e rate
per cell is linear w i t h cell radius for
s p h e r i c a l cells. (There was no i n d i c a tion from our data that u p t a k e rate was
a linear f u n c t i o n of cell s u r f a c e area.)
U=Kr
then for a sphere,
Kr
u/v = ~ / 3 ~ r 7
and,
4~r2
4/3zr-~ = 3/r
S/V-
r
--
-
3
(s/v)
-
!
therefore,
u/v =
K
= K ' (S/V) 2
T a k i n g an a v e r a g e radius for each
cell in this study, we e x a m i n e d the ass u m p t i o n of l i n e a r i t y b e t w e e n r a d i u s and
P u p t a k e per cell. L i n e a r r e g r e s s i o n of
the data gave u = -0.7 + 0.8 r w i t h r 2 =
O.76. The s c a t t e r in the d a t a is p r o b a bly due to the fact that m o s t of t h e s e
cells are not true spheres. The r e l a t i o n ship b e t w e e n axial d i m e n s i o n and v o l u m e
is, of course, m o r e c o m p l e x for dinof l a g e l l a t e s than for p e r f e c t spheres.
The m o d e l of M u n k and Riley (1952)
p r e d i c t s that the time r e q u i r e d for an
algal cell to a b s o r b as m a n y grams of nut r i e n t as are a l r e a d y c o n t a i n e d i n c r e a s e s
w i t h i n c r e a s i n g size. A c c o r d i n g to their
c a l c u l a t i o n s , n u t r i e n t a b s o r p t i o n rate
per unit mass is d o u b l e d to q u a d r u p l e d
w h e n cell d i m e n s i o n is halved, d e p e n d i n g
upon cell shape. This is r o u g h l y confirmed for s p h e r e s by our results. If
1
U/r ~K, and r/V = 4 / 3 ~ r ,then U/V a ~
K
=
K'
-~ ,
and it is o b v i o u s that w h e n cell dimension is halved, n u t r i e n t u p t a k e per cell
mass is q u a d r u p l e d .
The h i g h rate of p h o s p h a t e u p t a k e by
b a c t e r i a l o r g a n i s m s w o u l d be e x p e c t e d
from the o b s e r v e d r e l a t i o n s h i p b e t w e e n
u p t a k e and s u r f a c e : v o l u m e ratio. Fuhs et
al. (1972) s h o w e d that b a c t e r i a h a v i n g
50
E.S. Friebele et al. : Phytoplankton Cell Size and Phosphorus Uptake
lower a f f i n i t y for p h o s p h a t e than a l g a e
(higher Ks and lower Vma x per unit surface area) w e r e able to o u t g r o w the algae b e c a u s e of t h e i r f a v o r a b l e surface:
v o l u m e ratio. H o w e v e r , Lewis (1976) reports that b a c t e r i a l s : v r a t i o s can be
e q u a l e d or e x c e e d e d by algae. U n f o r t u nately, no data on b a c t e r i a l n u m b e r s and
s u r f a c e : v o l u m e r a t i o s w e r e o b t a i n e d in
our e x p e r i m e n t s , and the c o r r e l a t i o n
found for a l g a e c o u l d not be t e s t e d further.
W i t h t h e s e r e s u l t s in mind, the "paradox of the p l a n k t o n " remains. Why, if
n a n n o p l a n k t o n h a v e such a d i s t i n c t adv a n t a g e over large cells in a b s o r b i n g
n u t r i e n t s , do larger p h y t o p l a n k t o n cel~s
exist? One a n s w e r is that in some env i r o n m e n t s w h e r e all n u t r i t i o n a l res o u r c e s are p l e n t i f u l , a l g a l cells may
not be c o m p e t i n g for n u t r i e n t s , but this
c o n d i t i o n is not one that occurs cons t a n t l y in m a n y e n v i r o n m e n t s . Second, if
p h y t o p l a n k t o n p o p u l a t i o n n u m b e r s are low,
c o m p e t i t i o n for n u t r i e n t s m a y not occur,
H u r l b u r t (1970) f o l l o w e d f l u c t u a t i o n s in
a b u n d a n c e of m a r i n e p h y t o p l a n k t o n species and c a l c u l a t e d that the m a x i m u m
n u t r i e n t - d e p l e t e d zones a r o u n d algal
cells c o u l d not o v e r l a p at the cell densities found in the o p e n oceans; h o w e v e r ,
overlap, and thus c o m p e t i t i o n for n u t r i ents, o c c u r r e d at cell d e n s i t i e s a b o v e
3 x 108 cells 1-I, w h i c h are s o m e t i m e s
f o u n d in e s t u a r i e s and c o a s t a l w a t e r s .
He found that e s t u a r i n e p h y t o p l a n k t o n
forms w e r e s i g n i f i c a n t l y smaller, w i t h
less d i v e r s i t y and g r e a t e r numbers.
A n o t h e r l o g i c a l a n s w e r to the q u e s tion p o s e d is that other f a c t o r s as w e l l
as n u t r i e n t a b s o r p t i o n rate act as s e l e c tive forces on the shapes and sizes of
p h y t o p l a n k t o n cells. Lewis (1976) s h o w e d
that t h e r e is c o n s e r v a t i o n of s : v r a t i o s
for the g r e a t e s t axial d i m e n s i o n of phyt o p l a n k t o n cells. C e r t a i n l y a lower
limit of the s : v r a t i o is u n d e r s t a n d a b l e ,
but o b v i o u s l y some u n k n o w n s e l e c t i v e
p r e s s u r e s are p r o d u c i n g an u p p e r limit
on the s : v ratio,
S e m i n a (1972) d e s c r i b e s some i n t e r e s t ing m u t u a l e f f e c t s of p h y s i c a l f a c t o r s
on p h y t o p l a n k t o n cell size in the oceans,
In general, she f o u n d i n c r e a s i n g cell
size w i t h i n c r e a s i n g v e l o c i t y of a s c e n d ing w a t e r , and this e f f e c t was a m p l i f i e d
by i n c r e a s i n g d e n s i t y g r a d i e n t in the
p y c n o c l i n e and i n c r e a s i n g p h o s p h a t e conc e n t r a t i o n . M a l o n e (1971b) s u g g e s t s t h a t
i n c r e a s e s in n e t p l a n k t o n s t a n d i n g crop
in areas of u p w e l l i n g c o u l d r e s u l t f r o m
the v e r t i c a l w a t e r m o v e m e n t as w e l l as
from
i n c r e a s e s in n i t r a t e c o n c e n t r a t i o n .
Thus, c e r t a i n p h y s i c a l factors can select large cells h a v i n g low S : V ratios.
P a r s o n ' s and T a k a h a s h i ' s m o d e l (1973)
p r e d i c t s that small cells w i l l be dominant u n d e r m o s t light and n u t r i e n t regimes. H o w e v e r , the k i n e t i c c o n s t a n t s
used in t h e i r m o d e l w e r e o b t a i n e d w i t h
c u l t u r e s in the l a b o r a t o r y , and are dep e n d e n t upon m a n y factors, i n c l u d i n g env i r o n m e n t a l h i s t o r y and p h y s i o l o g i c a l
state of the cells. A c c o r d i n g to observ a t i o n s of C a r p e n t e r and G u i l l a r d (1971),
algal cells are a p p a r e n t l y able to a d a p t
n u t r i e n t u p t a k e rates to a g e n e r a l range
of n u t r i e n t c o n d i t i o n s e n c o u n t e r e d in
their e n v i r o n m e n t .
E x i s t i n g d a t a on the r e l a t i o n s h i p between p h y t o p l a n k t o n cell size and
M i c h a e l i s - M e n t e n k i n e t i c c o n s t a n t s is
c o n f u s i n g . E p p l e y et ai.(1969) note a
t r e n d of d e c r e a s i n g K s for n i t r o g e n compounds w i t h d e c r e a s i n g cell size.
M a l o n e ' s (1971b) o b s e r v a t i o n that nannop l a n k t o n p r o d u c t i o n i n c r e a s e s r a p i d l y at
very low n i t r a t e c o n c e n t r a t i o n s , w h i l e
netplankton productivity increases more
r a p i d l y at n i t r a t e levels g r e a t e r t h a n
1.5 V/n, lends s u p p o r t to this h y p o t h e s i s .
In contrast, Fuhs et al. (1972) found
that the larger of two d i a t o m species
had a lower Ks for p h o s p h a t e than the
smaller. The r e s p o n s e of n a t u r a l p h y t o p l a n k t o n p o p u l a t i o n s to n u t r i e n t levels
also a p p e a r s to be ambiguous. M a c I s a a c
and D u g d a l e (1969) s h o w e d that the uptake of n i t r o g e n c o m p o u n d s by n a t u r a l
p h y t o p l a n k t o n p o p u l a t i o n s is d e p e n d e n t
upon n u t r i e n t c o n c e n t r a t i o n at the lower
c o n c e n t r a t i o n s , but that m a x i m u m u p t a k e
v e l o c i t y is r e p r e s s e d at h i g h e r c o n c e n t r a t i o n s by o t h e r l i m i t i n g factors. Taft
et al. (1975) o b s e r v e d t y p i c a l M i c h a e l i s M e n t e n k i n e t i c s of p h o s p h a t e u p t a k e by
n a t u r a l p h y t o p l a n k t o n of C h e s a p e a k e Bay.
H o w e v e r , in their l o n g - t e r m study, rate
c o n s t a n t s for p h o s p h a t e u p t a k e by natural p h y t o p l a n k t o n p o p u l a t i o n s f l u c t u a t e d
r a n d o m l y t h r o u g h o u t the year, r e g a r d l e s s
of p h o s p h a t e c o n c e n t r a t i o n ; the only pattern o b s e r v e d was an i n v e r s e r e l a t i o n ship b e t w e e n s o l u b l e r e a c t i v e p h o s p h o r u s
c o n c e n t r a t i o n and p h o s p h o r u s u p t a k e rate
c o n s t a n t s in the w i n t e r months.
The r e l a t i o n s h i p b e t w e e n Ks for diss o l v e d n u t r i e n t s and cell size is thus
c o m p l e x and at this p o i n t unclear. Our
r e s u l t s do not r e s o l v e this problem, but
they do show that s m a l l cells u n d e r natural c o n d i t i o n s and at low n u t r i e n t levels do h a v e m u c h f a s t e r P u p t a k e rates
per cell m a s s than larger cells. It
w o u l d be i n t e r e s t i n g to use a u t o r a d i o g r a p h y to o b s e r v e the n u t r i e n t u p t a k e of
i n d i v i d u a l s p e c i e s at d i f f e r e n t (naturally occurring) n u t r i e n t levels; h o w e v e r ,
we w o u l d e x p e c t the r e l a t i o n s h i p b e t w e e n
u p t a k e rate and n u t r i e n t c o n c e n t r a t i o n
to be c o m p l i c a t e d by other factors, such
E.S. Friebele et al.: Phytoplankton Cell Size and Phosphorus Uptake
as p h y s i o l o g i c a l
s t a t e of t h e p h y t o p l a n k ton and limiting factors repressing maximum nutrient uptake.
F i n a l l y , G a v i s (1976) r e c a s t t h e M u n k
a n d R i l e y m o d e l in t h e l i g h t of r e c e n t
knowledge of algal nutrient uptake kinetics a n d t h e e f f e c t of n u t r i e n t d i f f u s i o n limited transport
(Pasciak and Gavis,
1 9 7 4 ) . In h i s m a t h e m a t i c a l m o d e l , t h e
d i m e n s i o n l e s s p a r a m e t e r P ( w h i c h inc l u d e s V m a x a n d K s) i n c r e a s e s w i t h d e c r e a s i n g c e l l size; as a r e s u l t , t h e inf l u e n c e of d i f f u s i o n t r a n s p o r t l i m i t a t i o n is d i m i n i s h e d . H e s u g g e s t s t h a t
t h i s is w h y o c e a n i c r e g i o n s w i t h l o w n u trient levels contain cells which are
s m a l l in s i z e a n d h a v e l o w K s v a l u e s .
A l t h o u g h t h e e u p h o t i c z o n e is a s o m e what homogenous environment, we can see
from the literature that regional differe n c e s in l i g h t , d e n s i t y g r a d i e n t s , v e r tical currents, and nutrient levels do
exist. However, each local environment
u s u a l l y s u p p o r t s a d i v e r s e g r o u p of p h y toplankton cell shapes and sizes. From
o u r r e s u l t s , t h e r e l a t i v e n u t r i e n t abs o r p t i o n r a t e s of t h e c e l l s m u s t c o n v e y
a competitive advantage on smaller cells,
but other selective forces must also be
at w o r k to b a l a n c e t h i s m e t a b o l i c a d v a n t a g e of s m a l l c e l l s . C e r t a i n l y z o o plankton grazing, which was not discussed, must play a large role. More research with natural phytoplankton populat i o n s in c o m p l e t e l y n a t u r a l c o n d i t i o n s
is n e e d e d in o r d e r to d e t e r m i n e t h e f a c tors most strongly influencing the types
of p h y t o p l a n k t o n w h i c h s u c c e s s f u l l y coexist.
Acknowledgements. This paper includes part of
an MA thesis submitted to the Johns Hopkins University by E. Friebele. We would like to thank
Dr. J.F. Ferguson for his careful reading of
that manuscript and his comments. We are also
grateful to Mr. G. Chirlin for his helpful
thoughts on the results. Mr. R.L. Cory of the
USGS maintains dock instruments which provided
physical estuarine data for these experiments.
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Elaine S. Friebele
Smithsonian Institution
Chesapeake Bay Center for
Environmental Studies
Route 4, Box 622
Edgewater, Maryland 21037
USA
Date of final manuscript acceptance: July 22, 1977. Communicated by M.R. Tripp, Newark