Limnol Oceanogr.,26(6), 1981,
1034-1044
Products of photosynthesis
in phytoplankton
off
the Orinoco River and in the Caribbean Seal
1. Morris,2 A. E. Smith,3 and H. E. Glover
Bigelow
Laboratory
for Ocean Sciences,
West Boothbay
Harbor,
Maine
0457Fi
Abstract
Incorporation
of [14C]bicarbonate
into the major end-products
of photosynthesis
(low molecular weight metabolites,
lipid, polysaccharide,
and protein) was measured at a number of
stations in tropical waters. Stations with highest chlorophyll
concentrations
showed almost
equal synthesis of low molecular weight metabolites,
lipid, polysaccharide,
and protein. The
“oligotrophic”
stations showed a greater incorporation
into polysaccharide
and protein and
proportionally
less into the other two fractions. Inshore stations also showed higher activities
of the photosynthetic
carboxylating
enzyme, ribulose-1,5-bisphosphate
carboxylase,
in relation to that of the anaplerotic
replenishment
enzyme, phospho(eno1) pyruvate carboxylase.
The inshore,
high chlorophyll
stations
were characterized
b> higher
particulate
carbon : chlorophyll
ratios. However, the polysaccharide
and protein contents (normalized to
particulate
carbon) were not higher in the offshore samples showing high incorporation
of
14C into those polymers. Carbon-specific
assimilation
rates were greater for total C incorporation than for incorporation
into the various polymers. This was most marked \vhen as\imilation was measured only over a period in the light.
The question of phytoplankton
growth
in the surface oligotrophic
waters of the
oceans is of fundamental
interest. In essence, the problem revolves around determining
whether
low phytoplankton
biomass and low nutrient concentrations
indicate low growth rates or result from
high turnover in which high growth rates
are paralleled by high loss rates.
In the absence of reliable direct estimates of the absolute growth rate of a
phytoplankton
population,
some emphasis has been placed on measurements of
their physiological
state. Such measurements include
pigment
ratios (Manny
1969), Chl:C ratios, assimilation numbers
(Thomas 1970), RNA: DNA ratio (Devulder 1969), ammonium stimulation
of dark
CO, fixation (Morris et al. 1971), energy
charge (Falkowski 1977), elemental composition (Redfield 1934; Fleming
1940;
Goldman et al. 1979), and other aspects
of chemical composition
(e.g. Sakshaug
and Holm-Hansen
1977). In all such
studies, the measured physiological
charl Thi\ work was supported by NSF grant OCE 7718722.
2 Present address: CEES, Box 775, Univ. XLlaryland, Cambridge 21613.
3 Present address: Marine Program Building, Uni\,ersitb of New Hampshire,
Durham 03824.
acteristic is related to that same characteristic measured in suspensions of algae
whose growth rate is limited by some environmental
factor. From this, it is hoped
to estimate the growth rate of a population (j.~) relative
to some maximum
growth rate (pmax) ~neasured in the laboratory. There is therefore a fundamental
difference
between
the physiological
state of a population (an indication of the
relatiw
growth rate) and the &olute
growth rate of such a population.
We have proposed (Morris et al. 1974)
that the conventional
14C Illcthod of‘measuring primary productivity
can be expanded relatively
conkreniently
so as to
measure the flow of‘carboil into the major
end-products
and suggested that such a
flow might indicate
the physiological
state of the algae. Such an approach has
been applied
to detecting
seasonal
changes in temperate waters (hlorris and
Skea 1978). It has also been related to the
excretion
of dissolved
organic
(‘onipounds by phytoplankton
from the Gulf
of Maine (Il/lague et al. 1980). LVe here
apply it to ~~ll~,toplarlktol~ populations
from tropical waters off the coast of Venezuela and in the Caribbean
Sea and
compare inshore waters showing
relativelJ- high phytol,lanktoil
biomass with
oceanic offshore waters having illuch
1034
Photosynthesis
lower biomass values. Most of this work
involved a transect leading away from the
mouth of the Orinoco River, Venezuela.
Also, other stations off the coast of Venezuela and in the open Caribbean
Sea
were included.
We thank J. C. Laird, E. Bettinen, C.
Garside, P. Luedee, W. K. W. Li, J. Rollins, and T. Buzzell.
Mnteriuls and methods
Data were collected on cruise EN-034
of RV Endeavor
(22 March-14
April
1979), in particular
from a section out
from the mouth of the Orinoco River (stations 17, 19, 22, 23, and 24) and from stations 7, 15, and 28 (Fig. 1).
Water sampling and light und hydrographic measurements-Water
samples
were collected with 30-liter Niskin bottles. Temperature was measured with reversing thermometers
and salinity with
a salinometer.
Nitrate, phosphate,
and
silicate were determined
in prefiltered
frozen samples (Strickland
and Parsons
197.2). Light intensities
throughout
the
water column and incident on the incubators were measured
with a Lamda
quantum
meter and are expressed
as
PEinst *me2 * s-l.
Incubations
with [14C]hicarhonate--In
all experiments, photosynthesis
was measurcd by incubating
l-2 liters of 183qm
Nitex-screened
seawater with 250-500
&i
of sodium [14C]bicarbonate.
Incubations were started just after dawn in 4liter bottles placed in Plexiglas
containers, subject to natural lighting
and
kept at ambient temperature with a continuous flow-through
water system. Light
intensities
were varied by wrapping the
4-liter bottles with layers of nylon screening. There were two types of experiments. First, time-course measurements,
in which water collected from 100, 30,
and 1% light regimes was incubated for
24 h at various light intensities.
At 2, 4,
8, 12, 16, 20, and 24 h after addition of
the label, 50 and 100 ml were filtered
through Gelman A/E glass-fiber filters.
Labeled
particulate
matter collected
from filtration of the 50 ml was assayed
for total 14C incorporation
after removal
products
1035
-.
Fig. 1. Location and dates of stations on cruise
034 of RV Endeavor, 29 March-12 April 1979. Sta.
7: 11”32.4’N,
65”14.6’W,
29 March;
Sta. 15:
10”42.O’N, 64”0.4’W,
1 April;
Sta. 17: 9”33.3’N,
60”42.7’W, 5 April; Sta. 19: 9”57.2’N, 60”30.7’W, 6
April; Sta. 22: 9”36,1’N, 60”41.4’W, 7 April; Sta. 23:
10”25,O’N, 6O”lO.O’W, 8 April; Sta. 24: 11”27.O’N,
60”0.2O’W, 9 April; Sta. 28: 14”15,9’N, 62”15.3’W,
12 April.
of residual [14C]bicarbonate
with acetic
acid : methanol
(5:95 vol/vol).
Filters
from 100 ml of filtration were stored in
1.2 ml of distilled water at - 10°C for later
extraction. Second, simulated in situ incubations, in which water collected from
100, 30, 12, and I% light regimes was
incubated
for 20 h at their respective
light intensity. Volumes of 50 and 100 ml
were filtered 8 and 20 h after addition of
the label (corresponding
to dusk and
dawn) and treated as above.
Fra’ctionation
of cell constituentsThe method is a modification
of that of
Smith and Morris (1980) as described by
Li et al. (1980). To the filters stored in 1.2
ml of distilled water, we added 1.5 ml of
chloroform and 2.0 ml of methanol. The
suspension was vortex-mixed
for 1 min
and then placed at 4°C for 10 min, followed by filtration through a glass-fiber
filter (Whatman GF/A). After the residue
was washed with 1.5 ml of chloroform,
1.5 ml of distilled water was added to the
combined filtrate. The filtrate was vortexmixed for 1 min then centrifuged at 2,000
rpm for 10 min to break the emulsion,
yielding a chloroform layer and a methanol-water
layer. The filtered
residue
was resuspended
in 4 ml of 5% trichloroacetic acid (TCA) at 95°C for 30 min.
After acid hydrolysis,
the contents were
filtered through another glass-fiber filter
(Whatman GF/A) and washed with 4 ml
of cold 5% TCA. Thus, the cellular fractionation separated cell material into four
fractions: chloroform-soluble
compounds
(lipid),
methanol
water-soluble
compounds (low molecular weight metabolites), hot TCA-insoluble
compounds
(protein),
and hot TCA-soluble
compounds
(polysaccharide).
(Thin-layer
chromatography
has been used to elucidate the major classes of compounds
present in each of the four fractions from
laboratory cultures. However, an extensive chromatographic
separation of the
individual
components
of each m;ljor
class of compounds has yet to be done.)
Measurement
of radioactitiity-The
assimilated 14C was measured in a Beckman LSlOO C liquid scintillation
spectrometer, with Handifluor
as the scintillant. The scintillation
system necessitated
evaporating
each of the liquid fractions
methanol-water,
and Fi%
(chloroform,
TCA) before addition of the scintillant
and subsequent counting of the incorporated label. The filters treated with acetic
acid : methanol had to be freed of the
quenching
liquid in the same manner.
The TCA-insoluble
fraction
could bt
counted directly
upon addition
of the
scintillant.
For all fractions, counting efficiencies ranged from 87 to 91% as tletermined from an external standard.
Clzc’micul meusuremerlts of purticulutc~
trzuttcr-Chlorophyll
u was measured by
the fluorometric
method of Yentsch and
Menzel (1963) with the modified extraction technique described by Glover and
hlorris (1979). Particulate carbon and nitrogen were measured with a HewlettPackard 185B carbon-hydrogen-nitrogen
analyzer by the method of Sharp (1974).
Lipicl, carbohydrate, and protein content
were determined calorimetrically
on the
fractionated
particulate
matter, total lipids by the method of Pancle et al. (1963)
and carbohydrates by the method of Kochert (1978). Values for protein, which
we (letermined
by the method of I,owqT
et al. (19Fsl), can clr’pend on the amino
acid composition. \t’e used 1)ovine seru111
albumin as the standard for protein assaq~
and it is possible that the absolute protein contents reported her<> are systenlatically in error.
Attempts to specify (1 assirllilatioll
rates
from incorporation
of ‘jC into various
polymers depend on deternlining
the C
content of each of‘ the polymers. 111 thv
absence of direct measurement, we ha\~tl
made assumptions based on the reported
amino acid composition of‘proteiii (Chilecas and Riley 1969), the assumption that
100% of the polysaccharide
is a p-( l-:3)
glucan and the reported conlposition
of‘
lipid (Beattie et al. 1961; Lee et al. 197 1)
of several marine diatoms. \li. K. \V. Li
developed this approach. These calcul;itions yielded
the following
estim:ite\:
protein C was Fj4% of total protein, pal! saccharicle C was 44% of total polq’sac*charicle, and lipid (1 \I’;~s 71% of total lipid.
CurJmxyluw
~~.~.~~~~/.~-Ril~~~los~~-l,Fj-l)isphosphate carbox>.lase (RuBPCase) anal
phosopho(erlo1)
pyruvate
carboxt,lase
(PEPCase) were assay-ed l)> the Inethoti
of Glo\rer and \lorr-is ( 19’79).
P/l!j.sicul, c*llcjr,licscll, clr~tl c~12loropll!~ll
c~larllc,tc~)-i.~ti~~.~c?f‘ tllcs ~-(1riolcs .sttrtiorls--The temper:iture,
salilllt! , arid lllltrieilt
colicentratioll,\Ltioilsfor s~irflice saniplc~s froilr
the various stati0115 ;Irc sllinmarizecl
ill
Tablt~ 1. The stations art‘ arrangc~cl to IX’fleet the major coil~parisons being iil:icle
throughout
this paper. Stations 17-2-i 1ic>
on the section le;lcliilg away from the>
mouth of the Orinoco River (sonic &pths
changiilg fl-011)abollt 25 III Lit station 17 to
2,000 iii ;it statioil 24). Station 1-5 \z’;ls l)catween thcl inainl:mcl of‘~‘~~i~r~zuel~~;~11cltllrl
Isla cle \Iurgarita
(soilic clc>pth al)out -U)
in) in ii regioil of’ Iip\\Tclliilg,
statioil 7 111
the eastern l,asin of the <1ariaco ‘l‘reilc~h
(depth hoiit
1,400 III), anal station 2X III
the opera Caribl)can
Scla (deptll ~~l~orit
2,700 in). Neither teiilperatlire
nor salilrity changed significantl>- 0~ er thy clcpth
range of‘ the cuphotic Y,OII<~(to the clcptll
correspoilcling
to 1(X>of‘ ~slii*tl~c~~
light ).
Photosynthesis
Decreasing
chlorophyll
concentration
in the section away from the mouth of the
Orinoco River was obvious (stations l724 in Table 1). Such changing
phytoplankton
biomass significantly
affected
the attenuation of light through the water
column: the depth of the euphotic zone
(to 1% surface light) increased from about
5 m at station 17 to 60 m at station 24.
Similarly,
the euphotic zone was about
25 m deep at station 15 and 70 m at station 28. Much of this paper focuses on
comparisons of the various stations using
water collected from a depth corresponding to 30% of surface light. In the section
out from the Orinoco River these depths
were 2-3 m (station 17), 5 m (station 22),
12 m (station 23), and 20 m (station 24).
Comparable depths for station 15 were 5
m, for station 7, 12 m, and for station 28,
20 m.
Kinetics of [14C]bicarbonate
assimilation-Time
courses of [14Clbicarbonate
incorporation
were emphasized
only in
the section out from the Orinoco River.
Water from a depth corresponding
to 30%
of surface light intensity was incubated
at 30% (Fig. 2), 100% (Fig. 3), and 1%
(Fig. 4) of surface light.
The kinetics of incorporation
into protein were different from those of the other end-products.
Incorporation
of 14C into
protein
continued
during
darkness
whereas the amount of 14C in low molecular weight metabolites, lipid, and polysaccharide either remained constant or
decreased during the dark period. Thus,
specifying the proportion of 14C incorporated into the various fractions depended
on the time at which the measurements
were made. However, for the purposes of
the comparisons to be emphasized here,
an important feature of the kinetics is the
initial period of 8 h of illumination,
In all
experiments,
incorporation
into all fractions was approximately
1inear throughout the day (the lipid fraction in station
22 of Fig. 2 is a possible
exception).
Thus, the more extensive analyses of the
14C incorporated
after 8 h to be emphasized below are valid ways of considering
the rates of 14C incorporation
into the
various photosynthetic
products.
It is
1037
products
Table 1. Surface values of
and nutrient concentrations.
Sta.
&
s
(%O)
Chl
(pg. liter-‘)
temperature, salinity,
NC&- SiOd4 -
Pcq-
(pg atom. liter-‘)
17
22
23
24
27.10
27.11
27.13
27.08
35.11
35.52
33.87
35.6s
5.69
0.22
0.11
0.04
0.5
0.4
0.1
0.2
4.1
3.8
2.7
5.3
0.6
1.2
0.1
0.2
15
7
28
19.40
20.18
26.55
36.62
36.71
35.73
2.4
0.28
0.06
0.3
0.0
1.2
2.0
0.3
0.0
possibly
interesting
that incorporation
appears linear over a period when irradiance levels did not remain constant.
This probably reflects the frequency
of
sampling. More frequent samples might
be needed to detect deviations from linearity.
Products of photosynthesis
as a function of station position and light intensity-Figures
2, 3, and 4 illustrate
the
way in which the pattern of photosynthctic [‘4C]bicarbonate
assimilation
varied with light intensity and with the type
of station occupied. The essential nature
of these observations
is summarized
in
Figs. 5 and 6. The data in Fig. 5 were
derived from experiments in which water
from a single depth (30% surface light)
was incubated at the range of light intensities. Figure 6 summarizes
data from
simulated in situ experiments
in which
water sampled from a particular
depth
was incubated at the light intensity corresponding to that depth. At all light intcnsities, the proportion
of 14C incorporated into the sum of polysaccharide
and
protein increased with a change from inshore “high chlorophyll”
areas to the offshore oligotrophic
regions. This increase
occurred at the expense of incorporation
into low molecular
weight metabolites
and lipid. For example, at 12% of surface
light, polysaccharide
and protein
accounted for about 48% of 14C fixed at station 17 and 64% at station 24. Comparable values for low molecular
weight
metabolites and lipid were 60% at station
17 and 35% at station 24. There seemed
to be no consistent change in polysaccha-
Morris
1038
0
4
8
12
I6
20
24
0
4
et al.
8
Hours
12
16
20
24
Hours
Fig. 2. Time courses of incorporation
of [r4C]bicarbonate
into low molecular weight metabolites
(0),
lipid (x), polysaccharide
(A), and protein (A) fractions. Total incorporation-O.
Water from indicated
stations collected from depth corresponding
to 30% of surface light intensity was incubated on deck under
neutral density screens yielding 30% of ambient light. Upper part of figure shows changing light intensity
during 24-h period.
ride : protein ratios from one station to
another.
Superimposed
on these changes from
one type of station to another was an effect of light intensity. At station 17, reduced light intensity caused an increased
proportion of 14C incorporation
into protein. This did not occur at station 24. Interestingly,
photoinhibition
at surface
8
12
Hours
Fig. 3.
16
20
24
0
4
8
light intensity was observed at station 24
but not at station 17 (cf. Figs. 5, 6) and,
at station 24, the effect of supraoptimal
light intensity
(cf. 100% with 30% light
levels) resembled that of suboptimal
intensities in showing the increased proportion of 14C incorporated
into protein.
No such effect was observed at the station (17) showing no photoinhibition.
12
16
20
24
Hours
As Fig. 2, but for samples from 30% light depth incubated
0
4
8
12
16
20
Hours
at surface light intensity.
24
I I
Photosynthesis
0
4
Fig. 4.
’
STA
6
12
Hours
I
I?
16
20
I
I
i STA. 22
24
0
4
6
16
12
20
24
As Fig. 2, but for samples from 30% light depth incubated
STA
s
LOW mol. wt.
LIpId
0
4
6
Hours
Carboxylase
activities-Another
indication of changing patterns of photosynthesis emerged from a comparison of the
activities of the two types of carboxylating enzymes responsible for the primary
reactions in CO2 fixation. The activity of
RuBPCase
(the essential
enzyme required for autotrophic growth) relative to
that of PEPCase (catalyzing a replenishment reaction required for continued operation of the tricarboxylic
acid cycle)
was greatest at the inshore stations (Table 2). Also, the ratio between measured
photosynthesis
and the potential
as indicated by the carboxylase activities was
greatest at the inshore station (Table 2).
I
products
17
POlysOcc.
Low mol.wt.
Lipid
16
20
at 1% of surface light intensity.
Comparison
of the chemical composition of particulate
matter and the products of photosynthetic
C assimilationData on the chemical composition
of the
particulate
material are summarized
in
Table 3. We find that the particulate
carbon : particulate nitrogen (PC : PN) ratio varied little from the one type of station to the other (it is not certain whether
significance
can be attached to the value
of “9.4” at station 7). The C:Chl a ratio
was highly
variable,
being highest at
those offshore stations showing lowest
biomass.
The measurements of particulate lipid,
polysaccharide,
and protein were con-
STA. 22
Protein
12
Hours
Polysacc.
STA
Protein
LOW mohwt,
Lipid
24
Polysacc.
Protein
Fig. 5. Effect of light intensity
on proportion
of [14C]bicarbonate
incorporated
into the major endproducts when water samples from 30% light depth were incubated for 8 h under various neutral density
screens. Values expressed as percentages of nteasur-ccl total 14C incorporated.
Morris
1040
et al.
50
ae-
STA 17
Low mol.wt.
Lipid
Pdysacc.
Protein
Low mol. wt.
STA
STA. 22
Lipid
Pdysacc.
Protein
LOW mot. wt.
Fig. 6. As Fig. 5, but for data taken from a simulated in situ experiment
depth was incubated at light intensity corresponding
to that depth.
verted to units of carbon (lipid C, polysaccharide C, and protein C) using the
assumptions
of the C content of these
compounds
in marine algae described
above. About 50% of particulate
C appeared to be protein, that in lipid varied
between about 5 and 30% and that in
polysaccharide
between
about 8 and
20%. The three polymers accounted for
69-95% of the total particulate C.
When 14C incorporation
was measured
over the 8-h light period, there was little
agreement
between
changing
proportions of [‘“Cl bicarbonate
incorporation
into the major polymers and changing
composition
of the particulate
fraction.
For example, the increase in 14C incorporation into the sum of polysaccharide
and protein (Figs. 5,6) was not paralleled
by similar increases in polysaccharide
plus protein content (Table 3). Similarly,
the decreased proportion of 14C incorporated into lipid with a change from station 17 to 24 was paralleled
by an increased
proportion
of lipid
C in
particulate C.
To a large extent, this discrepancy
is
caused by calculating
the 14C incorporation data from 8-h incubation
periods.
Clearly
for lipid and polysaccharide,
such data would be affected significantly
by considering their consumption
during
the night. Table 4 summarizes the changing proportion
of 14C incorporated
into
the major polymers when measured after
both 8 and 20 h. For comparison,
data
from Table 3 on the proportion of particulate C found in these types of compounds are included.
Differences
be-
Lipid
24
Polysacc
Protein
in which water from a particular
tween the patterns of 14C incorporation
and the chemical composition
were less
marked when the 14C incubation time included the dark period (Table 4).
Some estimates of speci$c assimilation
rates-Rates
of [14C]bicarbonate
assimilation into the various end-products
of
photosynthesis
were normalized
to the
chemical composition
of the particulate
material in an effort to estimate C-specific assimilation
rates [the reciprocal of replacement (turnover) times for the particulate matter].
That is, the rate of C
incorporation
(e.g. pg C incorporated into
protein. liter-’ *h-l) was normalized
to
protein content (pg protein C-liter-l)
to
yield specific
assimilation
rates with
units of “h-l.” This approach depends on
making assumptions
about the carbon
content of the three types of cell products-lipid,
polysaccharide,
and protein
(see methods). In addition, such calculations of specific assimilation
rates are always made uncertain by the presence of
Table 2. Comparison
of P,,, and activities
of
RuBPCase and PEPCase at various stations. All
data from depth corresponding
to 30% of surface
light.
P max
sta.
17
22
19
23
24
28
RuBPCase
PEPCase
(pg C.h-‘.liter-‘)
10.0
2.68
0;4
0.67
0.36
8.51
5.29
0.46
0.66
1.11
0.26
3.06
4.33
0.33
0.99
2.54
0.56
RU-
P mar :
Xase
BPCase :
PEPCase
0.86
0.28
2.78
1.22
1.42
0.66
0.44
0.46
059
0.18
0.43
Photosynthesis
Table 3. Chemical
surface light intensity.
composition
of particulate
matter
Li
PC
Sta.
sampled
id
E
PN
1041
products
PolyCsacc.
from depth
corresponding
to 30% of
Protein
C
Protein
C:PC
Po$rsa.
PC:Chl
PC:PN
(fig.liter-I)
(1
8:A:
(pg.liter-‘)
17
22
24
197.2
146.5
42.3
30.9
21.5
6.2
6.4
6.8
6.8
34.7
5,050
604
9.7
28.2
13.0
22.8
20.7
6.3
104.0
72.5
20.8
0.05
0.19
0.31
0.12
0.14
0.15
0.53
0.49
0.49
15
7
28
101.5
96.8
59.2
17.6
20.3
8.9
5.7
9.4
6.7
49.5
262
623
23.8
20.1
11.7
7.7
20.5
6.5
59.5
34.7
30.0
0.23
0.21
0.20
0.08
0.21
0.11
0.59
0.36
0.51
detritus in the particulate biomass measurement to which the assimilation
is
normalized;
the variable PC:Chl a ratios
in Table 3 suggest highly variable contributions
of detritus and other nonphytoplankton material to the measurements
of biomass. Another uncertainty
comes
from the differences in kinetics of 14C incorporation
into the various end-products, in particular the fate of 14C in different fractions during the dark period.
Table 5 summarizes specific assimilation rates for the various polymers into
which 14C incorporation
was measured.
Estimates from incorporation
into lipid
agreed most closely with those calculated
from total 14C incorporation.
For all fractions, agreement with total 14C incorporation was closest when 14C assimilation
was measured over a 20-h period. When
incorporation
was measured only over
the light period (8 h) incorporation
into
polysaccharide
and protein gave turnover
times considerably
different from those
calculated
from total 14C incorporation.
Moreover, specific incorporation
rates in-
to polysaccharide
and protein were greater at the offshore stations than at the inshore ones. The reverse was true when
total 14C incorporation
was measured.
This difference in the trend from inshore
to offshore was not apparent when the incubation period was 20 h.
Discussion
Most attempts to determine the physiological state of phytoplankton
from the
oligotrophic
regions of tropical oceans
h ave failed to identify any specific features suggesting that the growth of such
phytoplankton
populations
is less than
maximal because of limitation
by some
environmental
factor (generally assumed
to be nutrient concentration).
Most measurements on phytoplankton
from tropical oceanic regions suggest that the physiology of such algae resembles that of
cells growing at their maximum growth
rate. The failure of Morris et al. (1971) to
observe any indication
of N deficiency
(as measured by the effect of ammonium
Table 4. Comparisons of patterns of [‘“C]bicarbonatc
assimilation
and chemical composition
of particulate matter. All data are from samples taken from a depth corresponding
to 30% of surface irradiance and
14C data from incubations at 30% of surface irradiance for 8 and 20 h.
% laC incorporated
8h
20 h
% particulate
C
sta.
lipid
17
22
24
27.1
23.6
17.9
26.5
23.4
41.0
20.0
30.8
23.0
19.4
16.8
13.6
20.4
29.7
30.3
41.8
33.3
39.7
5.0
19.0
31.0
12.0
14.0
15.0
53.0
49.0
49.0
15
7
28
24.1
12.3
8.9
24.9
44.4
41.1
23.3
24.4
30.8
235
7.5
8.6
21.1
35.7
35.2
26.5
38.2
39.5
23.0
21.0
20.0
8.0
21.0
11.0
59
36
51
polysacc.
protein
lipid
polysacc.
protein
lipid
C
polysacc.
C
protein
C
1042
Morris
et al.
Table 5. C-specific assimilation
sites of particulate
matter as calculated from incorporation
of [“Clbicarbonate
into various fractions and normalized to C content of that fraction in particulate
fraction. All
data from water samples taken from a depth corresponding
to 30% of incident irradiance and incubated
at a comparable irradiance for 8 and 20 h. Data have units of h-l.
8h
sta.
z
lipid
17
22
24
0.051
0.018
0.015
0.028
0.022
0.009
15
28
0.021
0.006
0.027
0.003
20 h
polysacc.
protein
x
lipid
0.012
0.030
0.042
0.002
0.011
0.007
0.017
0.007
0.006
0.014
0.008
0.003
0.030
0.014
0.014
0.013
0.010
0.006
0.009
0.028
0.001
0.004
0.014
0.002
0.014
0.001
0.032
0.008
0.005
0.001
on dark CO, fixation) in phytoplankton
from the Straits of Florida is one example. Goldman et al. (1979) have documented the way in which the C:N:P ratios of seston in oceanic waters resemble
those of cultures growing at their maximum growth rate.
Our earlier
attempts
(Morris
et al.
1974; Morris and Skea 1978) to relate the
path of C assimilation
during photosynthesis to the physiological
state of phytoplankton are part of such studies. Using
such an approach, we now report differences in the physiology of phytoplankton
populations
between inshore waters off
the coast of Venezuela (notably off the
Orinoco
River) and that in the more
oceanic regions. The dominant
difference appears to be an enhanced proportion of 14C being incorporated
into the
sum of polysaccharide
and protein at the
expense
of lipid
and low molecular
weight metabolites.
Interestingly,
the enzyme activities described here illustrate the fact that the
sum of RuBPCase and PEPCase activities is greater than the measured P,,, values at the offshore stations and more or
less approximates
to P,,, at the most
productive
station off the Orinoco River.
This contrasts with earlier work with laboratory cultures and phytoplankton
populations in temperate waters where carboxylase activities were consistently
less
than the observed P,,, values.
Also accompanying the change from inshore to offshore stations is a decrease in
the ratio of RuBCase : PEPCase activities. This ratio is highest at those locations where productivity
is greatest, as in
polysacc.
protein
temperate
waters (Glover
and Morris
1979). Possibly, this phenomenon
may
reflect an increasing significance of storage product synthesis when phytoplankton photosynthesis
is greatest.
The significance of the changes in phytoplankton
physiology
reported here is not clear. In
other work (Morris et al. 1971; Morris and
Skea 1978) we have emphasized the way
in which the proportion
of 14C incorporated into protein
might indicate
the
physiological
state of phytoplankton.
In
that earlier work, changes in isotope incorporation
into protein were generally
associated with opposite changes in polysaccharide. Here, with changing station
location, we find no change in the ratio
of isotope incorporated
into protein to
that into polysaccharide.
It is difficult, of
course, to make precise comparisons with
the earlier work. The influence of species
composition
is unknown. The effects of
changing species composition
might be
superimposed
on any changes of cellular
physiology
from inshore
to oceanic
waters.
Although the observed changes in the
paths of carbon assimilation
cannot be
linked too precisely to the physiological
state of the phytoplankton
populations,
the fact of such changes is of interest. Interestingly,
too, the differences between
inshore and offshore stations revealed by
the dynamic measurements of the pathways of photosynthesis
are not paralleled
by differences
in the more static measurements of the chemical composition of
the particulate
material,
particularly
when the pathways of photosynthesis
are
measured
only over the light period.
Photosynthesis
When the incubation
time includes the
dark period, differences between the radiochemical
incorporation
data and the
chemical composition
are less distinct.
The chemical composition
is the endproduct of cell growth and metabolism
occurring
over both light and dark periods. It is not unexpected, therefore, that
the paths of 14C assimilation measured in
short term experiments should not reflect
the chemical composition.
Also, it is apparent from the data presented here that
the difference between the two types of
measurements depends on the nature of
photosynthetic
C assimilation.
Thus, a diversity in the paths of C assimilation may have a profound effect on
our attempts to extrapolate from C fixation to phytoplankton
growth, making
such an extrapolation
more valid under
one set of conditions than under another.
The estimates of specific assimilation
rates presented here illustrate this problem. Estimates based on 14C assimilation
into the selected polymers (particularly
polysaccharide
and protein)
differed
from those based on total C assimilation
when incorporation
was measured over
the light period. Differences
were less
marked when the incubation time included the dark period. Again, the extent of
the differences depends on the location.
It is difficult
to attach significance
to
the precise values of specific assimilation
rates calculated from incorporation
into
the various polymers. Assumptions about
the C content of the polymers, difficulties
of distinguishing
phytoplankton
protein
(for example) from other material, the
complications
of detritus, the specific activity of the precursors, etc. mean that
such estimates must be treated with caution. After measurements of 14C incorporation over the light period, replacement
(turnover)
times calculated from 14C incorporation
into protein were considerably longer than those estimated from total C assimilation,
particularly
at the
inshore stations. This may result from
failure of the precursor pools to saturate
with the isotope. However, during the 8
h of illumination,
there is no evidence of
an increasing
rate of 14C incorporation
products
1043
into protein caused by increasing specific
activity of the precursors. Similarly, it is
difficult to attach significance to the short
turnover times calculated from 8-h 14C incorporation
into polysaccharide
at the
oceanic locations. The rate of incorporation of 14C into such a storage product
over periods of hours in the light might
be much higher than any long term “net”
synthesis of the material over days of alternating light and dark periods. These
data therefore confirm the idea that extrapolation
from C assimilation
to phytoplankton growth is most valid when incubation times are sufficiently
long to
include both the light and dark periods,
although potential errors with such prolonged incubations arc well known.
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