Oceanogr., 36(4), 1991, 657-669
0 199 1, by the American Society of Limnology and Oceanography, Inc.
Limnol.
Density-dependent effects of protozoans on specific growth rates in
pica- and nanoplanktonic assemblages
Christine
Universite
France
Ferrier and Fereidoun
Rassoulzadegan
Pierre et Marie Curie (Paris 6), URA 7 16, Station Zoologique,
B.P. 28, 06230 Villefranche-sur-Mer,
Abstract
We experimentally
assess nutrient feedback of oligotrichous ciliates and phagotrophic flagellates
on growth in naturally occurring Mediterranean
autotrophic microbial populations as well as on
the microphytoplankton.
Dialysis bags were used to separate cultured protozoa from the <7-pm
plankton filtrates yet allow nutrient exchange. Growth rates in microbial autotrophic populations
increased with protozoan densities added to bags. Microbial
growth half-saturation
constants,
expressed in terms of protozoan densities added, varied from 11 to 17 ciliates ml-’ and 2-5 x 1O3
flagellates ml-l. The “transdialysis
contact” of protozoa with microbial planktonic populations led
to growth rates 2 to > 10 times higher than those observed without added protozoa. These findings
suggest that in situ, autotrophic pica- and nanoplanktonic
growth is limited by protozoan standing
stocks. Lower generation times and half-saturation
constants were obtained for autotrophic nanoplankton (~3 h, K, = 10 ciliates ml-’ and 1.7 x 1O3 phagotrophic flagellates ml-l, respectively).
Microphytoplankton
(e.g. diatoms and dinoflagellates) showed no significant functional responses
to the addition of microprotozoa,
suggesting a possible dependence of the latter on new rather than
on recycled nutrients.
Johannes (1965) first showed the importance of protozoa as remineralizers
in
marine environments. Recent studies show
that in the oligotrophic
ocean up to 90%
of the nutrients in the euphotic zone are
recycled (Eppley and Peterson 1979; Harrison 1980) by plankton
< 100 pm (and
sometimes < 10 pm, Glibert 1982). Within microbial food webs (Azam et al. 1983),
aplastidic flagellates together with the ciliated protozoa are believed to be major
components in maintaining
the NH4+ pool
in surface waters (Sherr et al. 1983; Goldman and Caron 198 5; Caron and Goldman 1990).
Most of these studies used 15N isotope
dilution
(Harrison
1983; Glibert
1988;
Selmer 1988). More information
is needed, however, on the relative importance
of the major functional groups, as well as
on possible competition
between pica-,
nano-, and microplanktonic
algae for utilization of recycled nutrients.
Our aim here is to assess experimentally
the feedback from ciliates or phagotrophic
flagellates to naturally occurring populations of autotrophic pica-, nano-, and microplankton.
The induced growth of the
latter is then related to the microprotozoan potential in nutrient recycling.
Material
and methods
The first experiment was carried out in
the laboratory in spring and summer 1989
with samples taken from point B, a standard oceanographic station, located at the
mouth of the bay of Villefranche-sur-Mer
(43”41’10”N, 7”19’O”E).
As the nutrient recycler, stock populations of an oligotrichous
ciliate, Strombidium sulcatum Clap. & Lachm., were maintained on a wheat-grain bacterioplanktonic
culture (Rivier et al. 1985). To transfer ciliates with a minimal concentration of bacteria, we kept them, prior to subsampling,
in a medium containing low concentrations
of bacteria for 24 h. Subsamples from this
culture-were then transferred into sterile dialysis bags containing 0.22-pm Milliporefiltered seawater. After being tightly closed
they were transferred into series of 2-liter
bottles filled with seawater from point B.
This water was first passed through nylon
mesh screen (100 pm, to remove larger zooplankton) and then through a reverse-flow
filter (10 pm) to remove naturally occurring
ciliates. The concentration of ciliates in the
dialysis bags was adjusted to give final concentrations in the bottles of 0 (control bottle), 10, 50, 75, 100, and 150 ciliates ml-l.
657
658
Ferrier and Rassoulzadegan
We used dialysis bags in order to separate
prey and predators, hence avoiding grazing
on the natural phytoplanktonic
populations
by the ciliates or the flagellates added.
Bottles were then incubated in the shade
in running seawater at in situ temperature.
The average level of illumination
in the incubator was - 10% of the level of direct
sunlight.
The second experiment was carried out
only during spring and was similar to the
ciliate experiment, but with a natural population of phagotrophic
flagellates. The
phagotrophic flagellate (Pseudobodo-like, see
Rivier et al. 1985) cultures were obtained
by adding a few drops of seawater, screened
on a 5-pm Nuclepore membrane (to obtain
populations of flagellate-sized cells), to a
bacterial culture containing lo7 cells ml-‘.
The culture was kept in the dark at 12°C
until the flagellates reached maximal density. Five different concentration ranges of
phagotrophic flagellates were then prepared
in separate dialysis bags. They were prepared in the same way as for ciliates (above)
to get final concentrations of “added” flagellates of 0 (control bottle), 1 x 103, 5 x
103, 1 x 104, and 2 x lo4 cells ml-‘.
The concentration change in the principal
autotrophs such as the cyanobacteria, pica-,
and nanophytoflagellates
was then studied
through 8-h incubation (ts). Samples were
counted by epifluorescence microscopy following the method of Rassoulzadegan and
Sheldon (1986). Ciliates and microphytoplankters were examined by the Utermiihl
-method.
These counts were used to get hourly specific growth rates (p) and doubling times
(DT) :for principal phytoplankters at different concentrations
of added ciliate or
phagotrophic flagellates:
p = (In N1 - In NJl(t,
DT = ln,/p
- to)
(1)
(2)
where No and N, are the phytoplankter
numbers at the beginning (to) and the end
(ts) of the experiment.
The relationships between p and the concentrations of added ciliates or flagellates
were established and the plots obtained fit-
ted to a Michaelis-Menten
model to give
the kinetic parameters of growth:
CL= ~ma,(S- W[W, f (s - @I. (3)
Here pmax is the maximal ciliate- or flagellate-delpendent specific growth rate, S the
concentration
of added ciliates or flagellates, bsthe concentration of added ciliates
or flagellates below which there is no algal
growth, and Kp the half-saturation constant
(ciliate or flagellate concentrations producing half the maximal algal growth).
Before these experiments, we ensured (by
prelimjinary experiments) that this membrane lhad no effect either on prey or on
predators, that the ammonia produced by
protozoan excretion passed through the dialysis membrane and that, since varied
amounts of protozoan culture were used in
the dialysis bags, there was no significant
carryover of inorganic nutrients from cultures via protozoan transfer to these bags.
Results
Ciliate experiments-Heterotrophic
bacteria and autotrophic pica- and nanoflagellates incubated outside the dialysis bags
containing ciliates increased in number (Table 1). Cyanobacteria exhibited growth only
in the spring experiment.. The spring experiment revealed an asymptotic growth pattern for all the autotrophic
cells (Fig. 1).
Although standard deviations are quite large,
the fits to Eq. 3 are highly significant (Ftest, a! = 0.03, 0.01, 0.05, and 0.01 for
bacteria, cyanobacteria, pica-, and nanoflagellates). From these fits, the growth halfsaturation constants are (mean + 1 SD)
57.742 30 ciliates ml-’ for heterotrophic
bacteria (maximum ciliate-dependent
specific growth rate, prnax = 2.06IfIO.33 d-l),
16.531L5.45 ciliates ml-l for cyanobacteria
(Pmax =’ 1.88+ 1.27 d-l), 14+9 ciliates ml-’
for plastidic
picoflagellates
(p,,,,, =
ciliates
3.38 HI.43 d-l), and 11.04k2.29
ml-l for plastidic nanoflagellates (pmax =
4.16kO.19 d-l). Values for specific growth
rates without ciliates added (control bottles)
are highly variable: 0.50+_0.06 d-l for heterotrophic bacteria, 0.2010.21 d-’ for cyanobacteria, and 0.14+_0.19 d-l for plastidic nanoflagellates. Plastidic picoflagellates
Protozoan-microalgal
did not exhibit any significant growth in the
2
0
‘FI
control bottles.
For the summer experiment, only auto8
trophic pica- and nanoflagellate growth is . 2
enhanced by ciliates and shows an increase
8
with ciliate concentrations (Fig. 2). The half- . 2
saturation constants are 77 + 122 ciliates . 13
9
ml-’ (P,,, = 3.620.4 d-l) for the picoflagellates and 72 + 78 ciliates ml-l (p,,.,,, =
3
3.7kO.2 d-l) for the nanoflagellates. As for
*s
the spring experiment, specific growth rates
3
without ciliates added (control bottles) are g
highly variable: 0.4 1 +O. 10 d-’ for heterotrophic bacteria, 1.47kO.06 d-l for cyano2
bacteria, 0.1 O&O. 13 d-l for plastidic pica4
flagellates, and 0.29kO.41 d-l for plastidic
{
nanoflagellates.
ii
In terms of equivalent C biomass, for a z
change in added ciliate number from 0 to
2
150 cells ml-l, the microbial biomass (pg .s
C liter-l)
varied from 9.23kO.02
to
3
8
12.96kO.42 in spring and 12.7710.54 to
15.2 l&O.82 in summer for heterotrophic
bacteria, from 3.22kO.26 to 5.36kO.37 in
1
spring for cyanobacteria, from 0.18&0.02
1
to 0.53kO.08 in spring and 0.0410.01 to
9
0.17-t-0.03 in summer for autotrophic pi3
1
coflagellates, and from 5.26kO.42 to 10.38
(unique
observation)
in spring
and
1.86kO.68 to 7.56kO.77 in summer for
.i
rcl
autotrophic nanoflagellates.
0
%
There is no marked trend for the func*s
tional response of the microphytoplankton
g
cell numbers to the introduced ciliate con8g
centrations (Table 2). For dinoflagellates in
3
spring, however, an increase of cell number
g
occurs when the ciliate number increases
2
from 0 to 10 cells ml-‘. The same pattern
‘I=
is observed for pennates (Nitzschia and others) in the spring and summer experiments.
it
g
In neither experiment did growth functional response of microphytoplankton
:S
2
(Figs. 3, 4) in the presence of added ciliates
8 9
present any significant trend such as those
observed for pica- and nanoplankton.
For
35
2
four of the groups studied (Nitzschia, pen1 e
nate diatoms, and small and large dinofla2 2
gellates), there was a slight increase between
0 and 10 ciliates ml-l added (avg p 0.31g i
1.04 d-l for Nitzschia, 1.61-3.58 d-l for
other pennates, 1.36-2.8 1 d-l for large dinoflagellates, 1.37-2.94 d-l for small dinoflagellates) only for the spring experi-
feedback
659
660
Ferrier and Rassoulzadegan
. 035
n
i
I
a=
.192
. 19
.142
. 1 4,
I
I
I
r
0
$
. 091
. 042
. 04,
I
I
I
I
-.Ol
S. sulcatum added
.- S
I
I
40
85
I
130
I
17s
(cell ml -l)
specific growth rates as a function of ciliate concentrations
in
Fig. I. Kinetics of pica- and nanoplanktonic
dialysis bags after 8 h of incubation. Spring experiment. (A-bacteria;
B-cyanobacteria;
C-autotrophic
picoflagellates; D-autotrophic
nanoflagellates).
Specific growth rates were fitted to a Michaelis-Menten
model
(see methods section).
ment. With respect to the naturally occurring
oligotrichous ciliates, neither concentration
nor growth seems to be affected by the ciliates added in the dialysis bags.
Flagellate
experiments -Counts
show
(Table 3) an increase for all hetero- and
autotrophic flagellates incu bated outside the
dialysis bags containing flagellates. There are
no significant changes in bacterial popula-
tions (auto- and heterotrophic) in the same
bottles’ (Table 3). These changes reveal an
asymptotic growth pattern for the autotrophic flagellates. As fitted to a MichaelisMenten model (Eq. 3) the relationships
between growth rates (p) and the concentrations of introduced flagellates are highly significant (F-test, a! = 0.0 1; Fig. 5). From these
fits, K,, values are 4,850+2,120
flagellates
Protozoan- microalgal
-
.24
A
. 19
-
-.05-
-5
I
I
40
85
S. sulcatum added
I
I
130
17s
(cell ml -l)
Fig. 2. As Fig. 1, but for summer experiment (Aautotrophic
nanoflagellates;
B-autotrophic
picoflagellates).
661
feedback
ml-l for autotrophic picoflagellates (maxima1 flagellate-dependent
specific growth
= 2.92kO.52 d-l) and 1,900+200
trophic nanoflagellates
(p,,, =
2.7k0.8 d-l). Despite the increase of the
aplastidic flagellates, specific growth with
the concentrations of those added in the dialysis bag, the points cannot be fitted significantly to Eq. 3; the values show high
standard deviations.
Nevertheless,
the
maximal specific growth rates are 2.50k0.24
and 3.16&O. 15 d-l for heterotrophic picoand nanoflagellates (Fig. 6). Specific growth
rates without aplastidic flagellates added
(control bottles) are 0.37kO.02 d-l for heterotrophic bacteria, 0.57kO.25 d-l for cyanobacteria, 0.18 +O. 16 d-l for plastidic picoflagellates, and 0.23 4 0.10 d-l for plastidic
nanoflagellates.
In terms of equivalent C biomass (pg C
liter-‘) for a change in added flagellates from
0 to 2 x lo4 flagellates ml-‘, the biomasses
of flagellates change from 0.21 kO.015 to
0.5 3 + 0.09 for autotrophic picoflagellates,
7.67kO.75 to 19.9k5.1
for autotrophic
nanoflagellates, 0.11 kO.03 to 0.25 kO.02 for
heterotrophic picoflagellates, and 2.89 k 0.52
to 8.1 kO.4 for heterotrophic
nanoflagellates. There is evidently no change of equivalent C biomass for heterotrophic and autotrophic bacteria.
Table 2. Ciliate addition experiment. Concentrations
of microphytoplankton
of incubation vs. ciliate concentrations added to bags at to.
populations
at
to and after 8 h
Concentrations* (cells ml-l)
Final
Groups
Spring
Nitzschia sp.
Large peridinians
Small peridinians
Thalassiothrix
sp.
Thallassionema sp.
Leptocylindrus sp.
Pennates
Ciliates
Summer
Nitzschia sp.
Large peridinians
Small peridinians
Ciliates
Leptocylindrus sp.
Rhizosolenia sp.
* Same as Table 1.
Initial
1
2.53
0.16
0.16
0.28
0.12
0.08
0.04
0.80
2.53kO.84
1.44f 1.41
0.27-tO.10
0.29kO.21
0.44kO.36
0.03-to.05
0.09+0.09
1.32kO.45
11.57
0.16
0.52
0.68
7.50
11.22
10.51 k3.90
0.37kO.34
0.78kO.55
0.74kO.30
8.39k0.95
15.08-1-3.06
2
3.54kO.24
0.32kO.11
0.5 l&O.34
0.29kO.18
0.35kO.23
0.08+0.14
0.15-tO.08
1.33kO.31
12.52& 12.14
0.39kO.17
0.5 120.09
0.73-t-0.37
6.6lk5.24
12.24k2.20
3
3.03f0.64
0.16kO.04
0.15,0.02
0.33+0.10
0.16-tO.19
0.04f0.07
0.05 40.02
1.36kO.46
12.15k3.51
0.46kO.34
0.44kO.14
0.57kO.20
9.66k3.95
13.56k3.75
4
3.29kO.15
0.09Iko. 10
0.08 ??I.04
0.37kO.15
0.19*0.02
0.07 +0.08
0.04~0.07
0.48kO.31
12.23k2.86
0.30*0.08
0.39kO.22
0.45 f0.27
2.66k3.77
9.17k6.11
5
2.73f0.06
O.l3t-0.02
0.3310.06
0.3210.04
0.1 lfO.O1
0.09+0.06
0.07 +0.06
0.78kO.02
11.80* 1.23
0.1410.07
0.33-tO.27
0.35kO.16
7.28-t0.83
9.23kl.42
662
Ferrier and Rassoulzadegan?
0.25
Small dinoflagellates
0.15
0.10
j
0.10
3
0.05
Large dinoflagellates
0.15
0.20
_
0.05
0
y_,,,
0.00
_
0
0.00
0
50
100
150
I
0
200
50
100
Thallassothrix
Thallassionema sp
’
200
150
sp
0.05
0.00
50
100
150
Leptocylindrus
0
200
50
100
'1 50
200
0.06
sp,
Nitzschia sp.
0.04
0.02
0.00
1
0
0.20
0.10
I
50
100
;Iy
150
Pennate diatoms
0
50
100
50
100
200
150
Naturally occurring ciliates
0.08
0.06
0.04
0.02
;t
I
0
50
0.00
150
Strombidium
Fig. 3. Microplanktonic
Spring experiment.
0
0.10
,:,I,
0.00
0.00
200
200
sulcatum
I
added
specific growth rates vs. ciliate concentrations
=
I
’
100
8
I
150
’
200
(cells ml-l)
in dialysis bags after 8 h of incubation.
Protozoan- microalgal
0.20
0.06
Large dinoflagellates
0.15 /_(I
0.10
0.00 -
0
;E
-
663
feedback
0.08
1
I
50
1
1
I
100
0.06
'
I
150
'
200
Rhizosolcnia sp,
Lcptocylirlchus SR
32
T
0.06
0.04
0.02
i3
,
”
,
,
,
y
,
0.00
1.4
Nitzschia sp.
Naturally occurring ciliates
1.2
1.0
0.8
0.6
0.4
0.2 i
0
50
100
150
Strombidium
Fig. 4.
200
0
sulcatum
added
50
100
150
200
(cells ml-l)
As Fig. 3, but for summer experiment.
The results obtained for the microphytoplankton are similar to those obtained in
the ciliate experiments (Table 4). As observed for the ciliate experiment, there is no
clear response of microphytoplankton
to the
added concentrations of phagotrophic flagellates. One notices an increase of the ciliates, small dinoflagellates, and three diatoms (Thallassionema, pennates group, and
Leptocylindrus),
however, when the number of phagotrophic flagellates increases from
0 to 1,000 cells ml-l in the dialysis bags.
There is also an increase of growth rates for
these groups when 1 x lo3 flagellates ml-’
are added (avg h = 1.441.2, 1.44kO.96,
4.32kO.19, and 2.88f2.64
d-l for ciliates,
small dinoflagellates,
the pennates, and
Leptocylindrus, Fig. 7).
Discussion
Dialysis membranes have been used previously to study phytoplankton growth rates
Ferrier and Rassoulzadegan
-.006
-1
Phagotrophic
3
I
7
flagellates
11
added
15
19
( x 10 3 cells ml -1)
Fig. 5. As Fig. 2, but of phagotrophic
late concentrations.
microflagel-
(Jensen et al. 1972). Furnas (1982) studied
comparative growth rates in natural assemblages of phytoplankton,
however, and obtained higher rates for incubations in l-pm
pore-size Nuclepore diffusion chambers than
in dialysis bags. He related this diff’erence
to greater permeability
of the Nuclepore
membranes (exchange in - 1 h) than of the
dialysis bags (exchange in -3 h). Because
of the inclusion of 5 1-pm microbial assemblages in our study, we chose dialysis membranes to separate populations. The dialysis
exchange times (5 3 h) were shorter than our
incubation times (8 h).
Recent studies show the dominance of
< 1OO-pm zooplankton in remineralization
pathways (Glibert
1982; Wheeler et al.
1989). These studies are based on the chemical quantification
of standing stocks or circulation pathways of mineral nutrients by
means of microcosms or size fractionation.
Few workers have studied this, question
through manipulations
of the multispecies
assemblages (Goldman et al. 1985; Berman
et al. 1987; Caron et al. 1988). Our approach
is complementary.
It ignores, for the moment, nutrient exchanges, but estimates the
Protozoan- microalgal feedback
Aplastidic picoflagcllatcs
0.12 -
0
10
0.20
20
30
Aplastidic nanoflagellates
0.15
I
0.10
0.05
0.00
1,
0
I
I
10
20
30
Phagotrophic flagellates added
(X 103 cells ml-9
Fig. 6. Specific growth rates in naturally occurring
aplastidic pica- and nanoflagellates vs. phagotrophic
microflagellate
concentrations
in dialysis bags after 8
h of incubation.
magnitude of the microprotozoan
per-cell
effect on the enhancement of specific growth
in microbial autotrophic assemblages, which
are considered the major group responsible
for primary production in the open ocean
(Platt et al. 1983; Hagstrdm et al. 1988).
Indeed, primary production
in the open
665
ocean is-believed to be due (up to 90%) to
nutrient recycling within the euphotic zone
(Eppley and Peterson 1979).
Eight hours of incubation in the dialysisseparated presence of phagotrophic microprotozoans were sufficient to get growth rates
for all the naturally occurring assemblages
of microbial populations. For the experiments done in spring, ciliates induced growth
in all microbial groups.
Heterotrophic
bacteria also seem to be
influenced by the excretions of ciliates. This
result agrees with previous studies showing
competition between bacteria and algae for
limiting nutrients in natural systems (Bratbak and Thingstad 1985; Glide 1985). Caron et al. (1988) recently showed that bacteria can compete successfully with algae for
NH,’ when bacterial substrates have a high
C : N ratio. In a N-limiting oligotrophic oceanic environment,
these substrates can be
constituted partly of phytoplankton
exudation that may represent an overflow of
photoassimilated
carbon (high C : N) exceeding biomass production
(Myklestad
1977).
The values for growth rates induced by
transdialysis contact of protozoa are in the
same range as those calculated elsewhere for
cyanobacteria (Fahnenstiel et al. 1986; Craig
1984), whereas maximal growth rates measured with flagellates (3.6-4 d-l) are higher
than those obtained in other studies (Goldman et al. 1985; Parslow et al. 1986). The
growth rates of bacteria in the ciliate experiment (0.06 h-l after 8 h of incubation)
are in agreement with those given in the
literature (Wheeler and Kirchman 1986).
Table 4. Phagotrophic flagellate addition experiment. Concentrations of microphytoplankton
populations at
t,, and after 8 h of incubation vs. phagotrophic flagellate (pf) concentrations added to bags at to. (NG-no
growth.)
Concentrations* (cells ml ‘)
Final
Groups
Spring
Nitzschia sp.
Large peridinians
Small peridinians
Ciliates
Leptocylindrus sp.
Thallassionema sp.
* Same as Table 3.
Initial
0.80
0.04
0.12
0.40
0.04
0.24
1
1.56kO.14
0.05 kO.06
0.16*0.08
0.56-t-0.07
NG
0.19f0.13
2
3
4
5
1.3640.11
0.04-t-0.04
0.2 1 -t-o.07
0.69kO.3 1
0.12~0.10
0.24+0.00
0.33~0.11
o.oo+o.oo
0.08f0.14
0.72kO.14
0.05 kO.06
0.29kO.06
0.73kO.12
0.07+0.11
0.09+0.07
0.29&O. 17
NG
0.31~0.11
0.48kO.32
NG
0.13+0.06
0.2 1 kO.06
NG
0.25 +0.08
666
Ferrier and Rassoulzadegan
Specific growth half-saturation constants,
expressed in terms of ciliate densities added,
obtained for the spring period varied from
11 to 17 ciliates ml-’ across autotrophic
populations. An average concentration
of
13 ciliates ml-l seems to induce half the
maximal growth for these populations. This
ciliate concentration is about what we usually observe for the spring bloom period in
our waters (Rassoulzadegan 1977; Sherr et
al. 1989). These values are, however, higher
than the annual average density of ciliates,
which is - 1 ciliate ml-l.
In the summer experiment, the half-saturation constants obtained are higher than
those obtained in spring, and only flagellate
groups show a functional response to an addition of ciliate cells. For other groups, such
the nonsignificant
reas cyanobacteria,
sponse could be due to limitation of nutrient
uptake by other factors, such as light and
temperature (Probyn 19 8 8).
As far as the phagotrophic flagellates are
concerned, their addition seems to induce
growth only for naturally occurring flagellate populations (both auto- and heterotrophic). The specific growth half-saturation
constant, expressed in terms of phagotrophic flagellate densities added, varied for
autotrophic flagellates from 2 to 5 x 10”
flagellates ml-l. These values are also higher
than the in situ annual average concentration of phagotrophic flagellates for the studied area (1 x 102-1 x 1O3 flagellates ml-l,
Rassoulzadegan and Sheldon 1986).
The unexpected, marked functional response of heterotrophic
flagellates to the
concentration
of phagotrophic
flagellates
added in bags (Fig. 6) could be explained
by the fact that during the incubation time
for this experiment, we did not observe significant growth of heterotrophic
bacteria
(Fig. 2), which suggests that they have been
grazed by naturally occurring phagotrophic
flagellates. In general, we noticed that the
presence of - 15 ciliates ml-’ induced an
autotrophic growth rate comparable to those
obtained by adding 5 x lo3 phagotrophic
picoflagellates
or 2 x lo3 phagotrophic
nanoflagellates (-0.8 h-l).
These findings suggest that in marine environments
pico- and nanoplanktonic
growth :may be limited by microprotozoan
concentrations.
Moreover, we noticed the
lowest growth half-saturation
constants for
autotrophic nanoflagellates (KS - 10 ciliates
added ml-’ or 2 x lo3 phagotrophic flagellates adrded ml-‘). It suggests adaptation of
nanoflagellates to concentrations of microprotozoa that can occasionally occur in situ.
They appear therefore best adapted to survival in oligotrophic waters, taking up virtually all nutrients excreted by microprotozoan. Some in situ experiments agree with
our results, having shown that uptake of
NH3 by nanoplankton is generally the most
important (Furnas 1983; Probyn 1985).
For oligotrophic
waters, phytoplankton
doubling times have been reported to be - 5
d (Sharp et al. 1980; Perry and Eppley 198 1).
We found that, in terms of upper limits of
the intrinsic specific growth rates, pica- and
nanoplanktonic
autotrophs can reach values > 2-10 times higher than those observed either in control bottles or for added
concentrations
of microprotozoa
comparable to in situ levels (final concn = 1 ciliate
ml-’ and I 10’ phagotrophic flagellates added ml-l, see Figs. I, 4). This finding agrees
with some earlier studies showing that
nanoplankton can reach generation times as
short as 3 h in oligotrophic oceans (Sheldon
and Sutcliffe 1978).
Such short phytoplankton
generation
times might occur in nutrient patches like
those described by Lehman and Scavia
(1982). Indeed, studies have shown that
marine phytoplanktonic
field assemblages
living in nutrient-impoverished
waters may
be able to efficiently exploit high-nutrient
patches created by zooplankton excretion
through their occasional encounters and very
rapid utilization
of NH4+ during brief periods (McCarthy and Goldman 1979; Glibert and Goldman 198 1). Our bottles with
relatively high nutrient content diffusing
667
Protozoan- microalgal feedback
0.15
Small dinoflagellates
0.10
0.10
0.05
0.05
:(I:,:,
0.00
0.00
0
10
cI 0.08
5
30
20
10
20
30
0
10
20
30
0.25
Thallassionema sp.
0.20
0.06
T
I
0.15
T
0.02
0.10
4
0.05
0.00 L-u
0
0
0.00
I
10
20
30
0.2
0.10
Pennate diatoms
s
0.08
0.06
0.1 0.0
i
.
0
,
[
,
10
Naturally occurring ciliates
0.08
I
0
10
20
30
Phagotrophic flagellates added
(X 103 cells ml-l)
i
20
,
30
668
Ferrier and Rassoulzadegan
through dialysis bags can represent water
sampled within a patch or a microcosmboth with elevated protozoan concentrations.
Our results suggest that only with more
than 1Q ciliates or 2 X lo3 flagellates ml-’
do nutrients seem to afford rapid growth of
either autotrophic
pica- or nanoplankton
populations. If we consider the average excretion rates of 56 pg N ciliate-l h-l and
0.58 pg N (phagotrophic
flagellate)’
h-l
(Gast and Horstmann 1983; Goldman et al.
1985), concentrations of 10 ciliates or 2 x
1O3flagellates ml-’ would represent mineral
nutrient supplies of -560 and 1,200 ng N
liter-’ h-l when ciliates or phagotrophic flagellates are present.
As far as the microphytoplankton
(diatoms and dinoflagellates) is concerned, there
is no significant effect of adding microprotozoa. This result suggests dependence of
these groups on newly upwelled nutrients.
We may then assume that regeneration of
nutrients by microheterotrophs
is chiefly
important for the microbial groups (e.g. cyanobacteria, autotrophic pica-, and nanoplankton). We have shown that protozoa
such as phagotrophic flagellates and ciliates
enhance the growth of pica- and nanophytoplanktonic cells, not only by grazing (which
maintains the cells in exponential growth)
but also by their excretion products. As a
consequence, intrinsic growth rates of these
groups (generation times as short as 3-5 h)
are limited by microprotozoan
concentrations in situ.
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Submitted: 30 April 1990
Accepted: 26 February 1991
Revised: 8 April I991