Download Are planktonic naked amoebae predominately

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Are planktonic naked amoebae
predominately floc associated or free
in the water column?
ANDREW ROGERSON*, O. ROGER ANDERSON1 AND CATHERINE VOGEL
OCEANOGRAPHIC CENTER OF NOVA SOUTHEASTERN UNIVERSITY, 8000 N. OCEAN DRIVE, DANIA BEACH, FL
LAMONT–DOHERTY EARTH OBSERVATORY OF COLUMBIA UNIVERSITY, PALISADES, NY
*CORRESPONDING AUTHOR:
33004 AND 1BIOLOGICAL OCEANOGRAPHY,
10964, USA
[email protected]
The spatial distribution of planktonic naked amoebae in the waters of a mangrove and estuarine
habitat was investigated. Amoebae were grouped either as ‘attached’ (when on suspended flocs) or
‘free’ (when floating in the open water). Consistent with previous studies, amoebae were numerically
important in the water column. For example, in mangrove water from south Florida, they averaged
94 640 cells l 1. In the mangrove, 91.6% of planktonic amoebae were attached to suspended
flocs. Likewise, the majority of amoebae in Hudson waters were floc associated (86.7%). The
results using a novel capture protocol, employing suspended capillary tubes to catch floating
amoebae, suggested that free amoebae readily colonized available surfaces. Transmission electron
microscopy demonstrated that amoebae were capable of penetrating deep into the cracks and crevices
of floc particles. The implications of these results are far reaching. For example, in the mangrove
waters where the floc fraction in a liter of water accounted for 0.5 ml volume, the absolute density
of amoebae at these loci was 173 380 amoebae per milliliter of floc material. Such high local
abundance may have important trophic implications, particularly if amoebae, because of their close
association with surfaces, graze attached bacteria unavailable to other micrograzers. The results
presented here clearly show that future studies on the microecology of flocs need to include amoebae
as well as the more widely investigated ciliates and heterotrophic flagellates.
INTRODUCTION
A recent series of review articles on the microbial ecology
of the oceans (Kirchman, 2000) largely ignores the marine amoebae. This stand is justified because there is
insufficient information about how amoebae contribute
to energy flow within pelagic food webs (Caron and
Swanberg, 1990). Elsewhere in the text, Strom comments
that the typical low abundance of sarcodine and other
amoeboid consumers implies that their contribution to
total bacterivory is likely to be small (Strom, 2000). In
light of these statements, the naked amoebae are clearly a
group of protists deserving of more attention. What has
been overlooked in this text are the important clues in the
literature suggesting that amoebae may play a significant
role in marine systems. For example, their abundance
often far exceeds that of the well-studied ciliated protozoa. In the Clyde estuary, Scotland, amoebae can number up to 43 000 l1 (Rogerson and Laybourn-Parry,
1992a; Anderson and Rogerson, 1995) and, in the Black
Sea, densities up to 380 000 l1 have been reported
(Murzov and Caron, 1996). The other important key
point is their ability to associate with surfaces, where
they are often numerically important (Caron et al., 1982;
Rogerson, 1991; Rogerson and Laybourn-Parry, 1992b;
Rogerson and Gwaltney, 2000). Densities of up to 104 000
amoebae l1 have been reported in the floc-rich waters of
a mangrove (Rogerson and Gwaltney, 2000) and up to
237 120 amoebae l1 have been reported in the saline waters
of the Salton Sea, California (Rogerson and Hauer, 2002).
In both these latter studies, the number of amoebae was
positively correlated with the weight of suspended material
in the water column. By employing conversion factors
given in Børsheim and Bratbak (Børsheim and Bratbak,
1987) and Rogerson et al. (Rogerson et al., 1994), these
amoeba densities represent carbon (C) biomass values of
up to 18.9 and 43.2 mg C l1, respectively.
It has been speculated that naked amoebae may be
important consumers of attached bacteria by virtue of
doi: 10.1093/plankt/fbg102, available online at www.plankt.oupjournals.org
Journal of Plankton Research 25(11), Ó Oxford University Press; all rights reserved
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their close association with surfaces (Rogerson and
Laybourn-Parry, 1992a,b), and their flat shape, making
them ideal for removing firmly attached bacteria. Moreover, their small size (the majority are often <10 mm in
length), and plastic cell shape, may enable them to
invade crevices inaccessible to other micrograzers.
Understanding the biological transformations on particles, be they oceanic or inshore, is important since up
to 25% of bacteria on a floc particle can be attached
(Turley and Mackie, 1994) and sinking bacteria can
supply 90% of bacterial carbon demand in Atlantic
waters (Turley and Mackie, 1995). If amoebae are spatially segregated on suspended flocs, their relative
numerical importance increases markedly, as does their
potential importance as grazers of attached or loosely
associated floc-bacteria.
Amoebae can also become dislodged from surfaces,
either due to the action of waves and currents or without
any obvious external driving force (Capriulo, 1990). These
‘planktonic’ forms often have extended pseudopodia that
presumably keep them suspended until they find a new
surface on which to attach. Although it is tempting to
suggest that floating amoebae do not feed, this has not
been convincingly demonstrated. There is even an account
of at least one species (Boveella obscura) that has only been
observed in the floating condition (Sawyer, 1975). However, based on numerous observations over the years of
amoebae in laboratory culture, it is fair to conclude that
the vast majority of active amoebae are attached. Thus, in
commenting on the ecological role of amoebae in the
water column, two distinct groupings must be considered:
the attached amoebae and the freely suspended amoebae.
This study is the first to directly compare the numbers of
amoebae inhabiting floc particles relative to the open water.
In so doing, it highlights the importance of considering the
microspatial distribution of amoebae in the water column
and further emphasizes the need to consider these protozoa
as important grazers of attached bacteria in the plankton.
METHOD
Enumeration of amoebae
Samples were collected between April and December
2001 from mangrove waters in the John U. Lloyd State
Recreational Park, Broward County, Florida. Two samples were also collected from the Hudson River Estuary,
New York, in June 2001. In all cases, water was collected
by carefully drawing subsurface water into a plastic
culinary baster consisting of a rubber bulb attached to
a tapered plastic tube. Essentially, this sampler is akin to
a large Pasteur pipette with a volume of 100 ml. Care
was taken not to disrupt floc particles. Immediately upon
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return to the laboratory, the sample was dispensed into a
Petri dish and allowed to settle for 20 min. This short
settling time was chosen to ensure that ‘free’ amoebae
did not settle out and become firmly attached to the
Petri dish surface. Under a high-resolution dissecting
microscope with oblique light illumination, individual
floc particles and small aggregates that had settled to
the bottom of the dish were gathered using a 10 ml
micropipetter. The majority of particles were 10 mm,
although overall they ranged in length from 0.1 mm to
1 mm. An equivalent number of ‘floc-free’ water samples,
selected from areas devoid of particulates, was also collected. In short, we compared the numbers of amoebae
in a floc-rich 10 ml aliquot of water with the numbers in
a floc-free aliquot. Each aliquot was pipetted into the
wells of two tissue culture dishes (24-well plates), each
well containing 2 ml of filtered water (0.45 mm) either
from the mangrove site or the Hudson Estuary. For a
few samples, we used a single 24-well plate. Each well
also contained a small cube (8 mm3) of malt yeast (MY)
agar (Page, 1983) to nourish and stimulate the growth of
natural bacterial flora in the sample. The agar was not a
significant source of nutrients for the amoebae; rather,
the growing bacterial population was the food source
for any heterotrophic amoebae present in the aliquot.
Samples were incubated at 20 C for up to 2 weeks and
wells were tallied for the presence or absence of growing
amoebae. Each morphospecies that grew out in a well
was noted and tallied. All observations were carried out
using an inverted microscope with phase-contrast optics
at a magnification of 600. The high magnification was
necessary since many free-living amoebae are small
(<10 mm). The development of a population of amoebae
in individual wells implied that at least one individual of
a given species was present in the 10 ml inoculum. The
number of amoebae per liter (N ) in the original sample was
computed as N = (n 106)/V, where n is the number of
positive tallies of amoeba morphospecies in the 24 or 48
wells and V is the total volume (ml) of water processed (i.e.
240 or 480 ml). Since the distribution of cells in the wells
was Poisson based, and a statistical adjustment is recommended to correct for underestimation in wells with high
densities, a maximum likelihood estimate for the counts
was obtained by employing the method given in Anderson
et al. (Anderson et al., 2001). This culture-enrichment
enumeration method has been used previously to count
amoebae, and additional details are given in Anderson and
Rogerson (Anderson and Rogerson, 1995) and Rogerson
and Gwaltney (Rogerson and Gwaltney, 2000).
It is interesting to note that different authors have on
occasion used fragments of rice grains, rather than small
blocks of MY agar, to stimulate the growth of bacterial
prey and hence the growth of amoebae. Prior to this
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study, a comparison was made between these two culture options for mangrove amoebae. Water samples
(10 ml) were collected on August 21, 2001 and vortexed
on maximum setting for 1 min to evenly disperse amoebae
in the sample. Aliquots (10 ml) were dispensed into 48
wells containing sea water with either a block of MY
agar (8 mm3) or a similarly sized fragment of rice grain.
This trial was replicated twice and the results showed
that MY agar was superior to rice in promoting the
growth of amoebae (i.e. mean counts of 94 640 amoebae
l1 with MY agar versus 58 240 amoebae l1 with rice).
Consequently, for this study, MY agar was the preferred
source of nourishment in the culture wells. This density
estimate (94 640 cells l1) was used in subsequent
estimations of the number of amoebae likely to be associated with the floc material in the water column.
Do suspended amoebae in the water
column readily attach to floc particles?
To address the question of flux between open water and
flocs, artificial ‘surfaces’ were suspended in the water
column and the number of amoebae attaching was
counted after 1 h. After preliminary trials with different
surfaces (e.g. suspended agar-covered beads), the experimental set-up adopted was a string of suspended glass
capillary tubes with a bore of 1 mm (Figure 1). These
tubes could be filled with sea water or dilute MY solution
(0.1 g of malt extract, 0.1 g of yeast extract, 1 l of sea
water) to test whether amoebae were preferentially
attracted to nutrient-rich surfaces. The area of the opening was 0.8 mm2, and this was the only entry point to the
inside of the capillary from which amoebae were sampled
(the other end of the tube was capped). On each experimental run, 12 capillary tubes were suspended in 3 l of
freshly collected mangrove water. After 1 h, the outsides of
the tubes were wiped and the contents of each tube were
flushed into the wells of a tissue culture plate containing
sterile sea water and a block of MY agar. Care was taken to
expel only the internal contents of each tube to minimize
contamination of the inoculum with amoebae that may
have become attached to the surface either during the
course of the experiment or during retrieval from the
water. Development of a population of amoebae (presumably originating from a single amoeba from the capillary
tube) was determined by observing each of the wells with
an inverted microscope after 7 and 14 days incubation at
20 C. This experiment was repeated 11 times between
June and August 2001.
Transmission electron microscopy (TEM)
To confirm first hand that amoebae were often floc
associated, and approximately where they occurred in
the floc particle, intact particles were examined by
Fig. 1. Diagram of the capillary experiment used to capture freely
suspended amoebae from the water column.
TEM. Flocs from mangrove water were carefully transferred to 2% electron microscope grade glutaraldehyde
prepared in cacodylate-buffered amoeba saline (0.05 M,
pH 7.2, 4 C). After 20 min, the glutaraldehyde-fixed
flocs were post-fixed for 50 min on ice in 2% buffered
osmium tetroxide prepared in the 0.05 M cacodylatebuffered saline. Flocs were enrobed in 2% warm agar to
stabilize the floc-associated biota, washed with distilled
water, dehydrated through an acetone series and
embedded in Spurr’s low-viscosity resin. Semi-serial sections were cut on a Porter–Blum ultramicrotome with a
diamond knife, collected on uncoated copper grids,
stained with Reynold’s lead citrate and examined with a
Philips EM 201 electron microscope operated at 60 kV.
RESULTS
By selecting both floc and open-water aliquots, this
enumeration method clearly showed that considerably
more amoebae in mangrove water were floc associated.
Overall, 91.6% of these amoebae were attached to floc
particles (97 915 amoebae l1) compared with only 5493
amoebae l1 (8.4%) free in the water column (Table I).
Although only two samples were processed from the
Hudson Estuary, the same trend was observed. The
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Table I: Comparison of numbers of amoebae (l 1) in floc-rich samples and open-water samples
Sample no.
Amoebae on flocs
Amoebae in open water
Total
1M
27 080 (76.5)
8330 (23.5)
35 410
2M
68 750 (97.1)
2080 (2.9)
70 830
3M
52 080 (89.3)
6250 (10.7)
58 330
4M
60 420 (93.5)
4170 (6.5)
64 590
5M
25 000 (100.0)
b.d.a (0)
25 000
6M
33 330 (88.9)
4170 (11.1)
37 500
7M
20 830 (100.0)
b.d. (0)
20 830
8M
12 500 (75.0)
4170 (25)
16 670
9M
385 420 (96.9)
12 500 (3.1)
397 920
10M
277 080 (95.7)
12 500 (4.3)
289 580
11M
114 580 (94.8)
6250 (5.2)
120 830
12H
31 200 (79.0)
8300 (21.0)
39 500
13H
33 000 (94.3)
2000 (5.7)
35 000
Data were collected between April and December 2001. Samples are from mangrove water (M) and the Hudson Estuary (H). The percentages of the
total relative population residing in flocs and open water are given in parentheses.
a
Below detection.
densities were less; however, the majority of these amoebae were floc associated (86.7%).
The data on floc abundance can be examined to
estimate how many amoebae are present in 1 ml of
floc material as follows. Given that 1 l of water (after
vortexing a 10 ml sample) contains 94 640 amoebae, and
that 91.6% of the amoebae are floc associated, then
86 690 of these amoebae are on flocs. Since the flocs represent 0.5 ml volume in a liter, then the relative density of
amoebae on floc material is 2 86 690, or 173 380
amoebae per milliliter of floc.
Additional support for amoebae inhabiting flocs is
shown in the electron micrograph of an ultrathin section
through the interior of a floc particle (Figure 2). This
small amoeba was found after sectioning relatively
deeply into the floc and is associated with indurated,
apparently mineral, particles (arrow) suspended in the
floc. In addition to evidence of other amoebae, we also
noted the occurrence of cyanobacteria, pennate diatoms,
autotrophic dinoflagellates and other unicellular algae.
Since we examined only a limited sample of floc particles,
it is not possible to estimate how many of the floc-dwelling
amoebae are associated with the surface versus deeper
layers of the floc particle. However, our data confirm
that some amoebae, particularly small forms, are internal
to the floc particle.
The capturing of amoebae by the suspended capillaries gives some indication of the flux rate of freely
suspended amoebae onto particles. The small surface
area afforded by the one open end of the capillary
(0.8 mm2) provided a site for suspended amoebae to invade
the capillary space. While we cannot be sure that flocs,
with amoebae, were not moved against this opening by
currents, it is unlikely that amoebae would actively leave
the flocs even if sporadically pressed up against this
Fig. 2. Transmission electron microscope image of an ultrathin section
through a floc particle containing an in situ gymnamoeba surrounded by
remnants of indurated material (arrows). Scale bar = 1 mm.
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Table II: Number of amoebae captured in capillaries suspended for 1 h in mangrove water
Sample number
1
2
3
4
5
6
7
8
9
10
11
MY
2
2
0
0
0
1
2
2
7
0
6
SW
2
1
0
0
0
0
2
1
n.d.a
n.d.
n.d.
For each sample, 12 replicate capillaries were suspended. Some capillaries were filled with sterile sea water (SW), others with dilute malt/yeast
suspension (MY).
a
No data.
opening. Rather, it is likely that any colonization of the
capillaries was due to free amoebae encountering this opening by chance. In either instance, the data provide evidence
of the dynamics of transfer of amoebae from one locus in
the water column to another, including solid surfaces.
Table II summarizes the results of this experiment. There
was no obvious preference by amoebae for capillaries filled
with sea water versus sea water enriched with MY solution.
This suggests that colonization was passive, relying strictly
on chance encounter with the capillary end. A total of 228
capillaries were suspended in the water during the experimental period, and 28 of these were colonized by amoebae
that invaded from the water column within 1 h. This is a
high level of colonization given that there were only an
average of 5493 amoebae l1 free in the water column.
Given this density of suspended amoebae, only 0.006
amoebae would occupy the 1 mm3 space within the
water column in the vicinity of the small capillary opening.
Clearly, to attain a colonization rate of 0.1 amoebae per
capillary per run (the observed rate), then the system has to
be viewed as dynamic rather than static. Water currents
were undoubtedly moving amoebae around and amoebae
were readily colonizing the capillaries. This suggests that
the floating state is a transient condition and that the
dynamics of the water column may be moving dislodged
amoebae from floc to floc.
The two commonest types of amoebae captured were
vannellids (flattened fan-shaped amoebae, in 50% of the
replicate runs) and eruptive limax amoebae (vahlkampfiids,
in 44% of the replicate runs). There was no obvious difference in the types of amoebae found in floc samples versus
open-water samples. The vannellids and vahlkampfiids
simply reflect the commonest morphotypes found in plankton samples in general. Interestingly, amoebae captured in
the capillaries ranged in size from 10 to 30 mm with most in
the 20–30 mm size range.
DISCUSSION
An earlier paper (Rogerson and Gwaltney, 2000)
demonstrated the importance of vortexing mangrove
water samples before counting amoebae by the aliquot
method. Vortexing increased the count some three-fold,
presumably because floc particles, rich in amoebae, were
disrupted and cells were evenly distributed in the sample.
After vortexing, amoebae ranged between 50 000 and
104 000 cells l1 in mangrove water, which is close to
the mean calculated in the present study. Here, amoebae
averaged 94 640 cells l1 (vortexed sample). It is clear
that most amoebae in the mangrove water were floc
associated (91.6%). Although based on fewer replicates,
the same was true for the Hudson Estuary samples,
which indicated that some 86.7% of all amoebae were
on the flocs. It should be noted that the samples from the
Hudson Estuary were collected near the shore where
there is typically considerable turbulence due to minor
wave action, and some of this energy could have disrupted the floc system and accounted for more free
amoebae in the water column.
However, the above percentages are probably underestimates for three reasons. First, the ‘open-water’ aliquots may have contained small aggregates that were
inadvertently picked up in the 10 ml aliquots. Secondly,
although care was taken to collect undisturbed samples,
it is likely that some fragile flocs were disrupted during
collection, releasing amoebae into the water column.
Finally, transparent exopolymer particles (TEP), thought
to be the precursors of aggregates (Passow and Wassmann,
1994), were not resolved by the low magnification used
when picking up floc and open-water samples. Since these
‘invisible’ aggregates are rich in bacteria and presumably
amoebae, their inclusion would increase the ‘free’ amoeba
count. Likewise, a bias may be introduced during the
20-min settling period, leading to changes in the number
of amoebae associated with floc either through attrition by
locomotion of the amoebae onto the surface of the Petri
dish, or by addition of amoebae that settled out of the
water column and may have attached to the floc particle.
However, our TEM evidence indicates that some amoebae are deeply situated in the floc particles and therefore it
is less likely that the bulk of amoebae detected in the flocs
are due to surface contamination effects. Moreover, the
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number of suspended amoebae detected in our studies is so
low that it is unlikely that settling out on particles could
produce a substantial bias in the data.
The importance of understanding the spatial distribution of grazing protists in the water column has been
recognized by Alldredge and Silver (Alldredge and
Silver, 1988). In their paper on marine open-water aggregates, it is concluded that marine snow acts as a center of
microbial activity in the water column, and attracts and
concentrates some classes of microorganisms more than
others. Pertinent literature is reviewed and all the various microorganisms associated with aggregates are
listed; however, no mention is specifically made of amoebae. In more recent work on aggregates from coastal
marine systems, the naked amoebae are still ignored.
Artolozaga et al. compared the numbers of protists in
the open water, on aggregates and in the 50 mm zone
around the aggregates (Artolozaga et al., 2000). But their
study only enumerated the heterotrophic flagellates and
ciliates, which were some three-fold more abundant on
and around aggregates when compared with open-water
samples. These authors did note the presence of two
amoebae (Vannella sp. and Amoeba radiosa, an invalid
taxon). However, the numerical importance of amoebae,
as indicated by the present study, was overlooked. Since
some 90% of all amoebae in the water samples were floc
associated, the absolute concentration of amoebae on
aggregates was estimated to be 173 380 amoebae per
milliliter of actual floc material (or 1.73 108 per liter of
floc material). This represents a 1160-fold concentration
on the flocs, similar to that found for other protists
(Artolozaga et al., 2000).
The capillary capture experiment trapped 28 amoebae within a total of 228 suspended capillary tubes. This
suggests that free amoebae will readily attach to a surface when it becomes available. If it is assumed that
tubes were colonized by floating amoebae, rather than
flocs, which would be unlikely to lodge in the small tubes
(1 mm diameter), the total surface area of 1.79 cm2 (sum
of all 228 tubes) captured 28 amoebae in 1 h. This is
surprisingly high since there were only on average 5.5
free amoebae per milliliter of water (equivalent to 9.9
amoebae in a volume extending 1 cm ahead of the total
capillary opening). This would suggest that amoebae
readily attach to a surface when it becomes available.
At any time, an average of 8.4% of the total population
were free in the water column and these were probably
very transient. Although it has never been demonstrated
experimentally, it is likely that floating amoebae are
unable to feed. Even if they can, bacteria free in the
water column are distributed randomly and therefore
not readily accessible to grazing amoebae. Attached
bacteria, on the other hand, which generally comprise
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10% of the bacterial population (Griffith et al., 1994),
represent localized concentrations that are amenable to
grazing protozoa. Moreover, attached bacteria are more
active and larger than suspended bacteria (Griffith et al.,
1994). Prey capture would thus be a major reason why it
is advantageous for planktonic amoebae to be surface
associated. The capture experiment also suggests that
the system is dynamic and that dislodged amoebae readily colonize new attachment sites.
The two most common types of amoebae encountered
in the capture experiments were fan-shaped vannellid
amoebae (probably of the genera Vannella and Platyamoeba) and eruptive, limax amoebae (vahlkampfiids).
Vannellids, particularly the genus Vannella, are thought
to be common in both marine and freshwater habitats
(Page, 1983), and this fact alone may explain its frequent
encounter in the capillary tubes. However, vannellids
are also characterized by their ability to extend long
radiating pseudopodia when floating and this may have
helped them rapidly find a new attachment site. It is also
worth noting that numerous vannellid amoebae have
been observed extended on the surface of mangrove
water (at the air/water boundary) (A. Rogerson, unpublished data). Having a flattened cellular form, they are
well adapted to inhabit this neustonic site, which is
characteristically rich in bacterial prey. However, they
may be prone to sinking from this zone during a disturbance (wave or wind induced), thus helping to explain
their abundance as planktonic amoebae. The other common group of amoebae captured in the water column
was the eruptive vahlkampfiids. Based on observations of
cultures in the laboratory, eruptive limax amoebae are
often easily dislodged from the base of culture dishes. It
follows that this type of amoeba might often be dislodged
from floc particles in the water column. Recent work has
shown that most amoebae in various habitats are
<10 mm [60% in a study by Butler and Rogerson (Butler
and Rogerson, 1995)]. The predominance of larger
amoebae in the tubes suggests that these amoebae are
more prone to being swept off particles and be free in the
water column. Smaller amoebae are presumable lodged
in crevices or inhabit the central portions of aggregates,
as is the case for the amoeba depicted in Figure 2.
This is the first study to show that naked amoebae can
be predominately floc associated in the plankton.
Although most of the data have been gathered from
mangrove waters, which are rich in suspended aggregates,
it is likely that planktonic amoebae in general are floc
associated and that the high numbers described in other
planktonic locations (Rogerson and Gwaltney, 2000;
Rogerson and Hauer, 2002) are located on suspended
solids. Certainly, the two samples processed from the
Hudson Estuary support this conclusion, as does previous
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work in the Sargasso Sea by Anderson (Anderson, 1977).
Here, amoebae were found in floating tufts of the cyanobacterium Trichodesmium (Oscillatoria sp.), along with
other small protists and microalgae. These tufts, with
their associated microbiota, somewhat parallel the
observations we report here for mangrove and estuarine
aquatic environments.
The high local densities of amoebae on suspended
aggregates (173 380 amoebae per milliliter of floc
material in the mangrove) have important implications.
For example, surfaces rich in trapped floc material will
probably have high densities of amoebae, as has been
shown in the case of the surface material on the submerged roots of mangrove trees (B. T. Maybruck and
A. Rogerson, unpublished results). Clearly, the ecological
role of naked amoebae in the plankton needs to be considered. Not only are they numerically important, but their
trophic role might be different from that of other micrograzers. As pointed out earlier (Rogerson and LaybournParry, 1992a,b), amoebae might be important grazers of
attached bacteria and those sequestered in inaccessible
regions on the aggregate. Furthermore, the TEM data
showing the presence of autotrophic protists within the
floc add further evidence that these particles may be
highly productive microcenters, and that a more continuous supply of carbon and energy may be available to
support bacteria, amoebae and other heterotrophic
planktonic protists than would be assumed based solely
on the constituent organic contents of the floc. The
unique ability to graze floc-associated bacteria by a significant population of amoebae could have important
consequences for the turnover of materials in the plankton and suggests that further more detailed studies on
the microecology of flocs are warranted.
ACKNOWLEDGEMENTS
We thank the staff at John U. Lloyd State Park for
granting permission to collect mangrove water samples.
This is Lamont–Doherty Earth Observatory contribution number 6502.
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Received on September 17, 2002; accepted on August 20, 2003
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