FEMS Microbiology Ecology 85 (1991) 301-312
1991 Federation of European Microbiological Societies 0168-6496/91/$03.50
Published by Elsevier
ADONIS 016864969100083S
301
0
FEMSEC 00338
On the abundance of marine naked amoebae on the surfaces
of five species of macroalgae
Andrew Rogerson
Uniuersry Marine Biological Station Millpori, Isle of Cumbrae, Scoiland U.K.
Received 22 February 1991
Revision received and accepted IS March 1991
Key words: Gymnamoebae; Protozoa; Spatial distribution; Littoral algae: Bacteria
1. SUMMARY
Naked amoebae constituted a numerically significant component of the surface microbial community of five species of seaweeds. They were
most abundant in the summer months on the
brown macrophytes, Fucur and Laminaria, where
numbers up to 23 cells cmP2of algal surface were
recorded. This implies that littoral algal stands
can support populations of up to 3.2 X lo6
amoebae m-2. A total of 27 different species were
recognized and, of these, six species were less than
10 pm in length, a size class overlooked in previous studies. Damaged tissue contained higher
numbers of amoebae, up to 43 cells cm-2, presumably due to higher bacterial densities and increased shelter at these sites. Some amoebae may
have been utilizing algal carbon exudates directly,
especially isolates of Trichosphaerium, which
showed evidence of algal digestion. The numbers
of amoebae found in this study suggest that these
Correspondence ro: A. Rogerson, University Marine Biological
Station Millport, Isle of Cumbrae, Scotland KA28 OEG, U.K.
protozoa may play a significant, and previously
overlooked, role in the cycling of estuarine carbon.
2. INTRODUCTION
Although the ecological role of protozoa in the
marine environment is currently the focus of much
research effort, the naked, lobose amoeboid protozoa belonging to the class Lobosea, subclass
Gymnamoebia [13] have been largely ignored. Recent work has concentrated instead on the flagellate and ciliate protists. Unlike amoebae, these
highly motile protozoa are readily detected in fresh
samples and research has shown that they are
important microbial grazers in the much publicized DOM-bacteria-flagellate-ciliate microbial
loop system [2]. However, despite the fact that
gymnamoebae can be isolated from virtually any
seawater sample (191 and despite a wealth of papers
reporting their presence in a diversity of marine
locations including open water sites, the ocean
surface microlayer, suspended flocs and benthic
and shore sediments (1,6-8,15,24,26,28], any involvement by naked amoebae withm this scheme
302
has been overlooked. Admittedly, most of these
papers are descriptive and deal with the presence,
identification, or classification of amoebae rather
than any quantitative data that can be used to
elucidate their ecological role.
The reluctance of marine ecologists to cope
objectively with this group of protists is due in
part to the mistaken belief that they are difficult
to identify. Tlus is an unfounded concern since
keys are available which, in most cases, permit
identification of amoebae to the level of genus or
species by light microscopy [4,19]. Perhaps a more
defendable reason for excluding amoebae from
ecological studies is the fact that they are virtually
invisible in fresh samples. Because they are translucent and are frequently associated with the
surfaces of debris and other particulate matter,
they cannot be successfully enumerated by direct
counting. Before amoebae can be enumerated in
field samples their numbers must be amplified by
enrichment cultivation which, although not entirely satisfactory, is not enough of a drawback in
itself to further delay their ecological study. They
are a neglected group of protists deserving of
at tention.
The present study is the first to estimate the
numbers of amoebae on the surfaces of macroalgae. There are several reasons why amoebae may
be an important part of the microbial consortia on
macrophytes. It is known that seaweeds with heavy
epiphyte wt-nmunities are excellent Sources of inocula for culturing amoebae [6]. The surfaces of
macroalgae would appear to be an attractive site
for colonisation and growth of amoebae since they
are rich in bacteria (121 and offer a large surface
area for amoeba1 attachment. It is also expected
that organisms living at or on the surface of
macrophytes would benefit, either directly or indirectly, from the release of substantial levels of
dissolved organic carbon from intact and decaying
algae [23,25]. At least one genus of marine amoeba,
Trichosphaerium, has been shown to utilize
seaweed wall extracts in its diet [21,22] and heterotrophic estuarine flagellates have been shown to
use high-molecular mass dissolved organic matter,
at least under laboratory conditions [27], as an
energy source. Trichosphaetium can also digest
macroalgal cell walls implying that this and per-
haps other species of amoebae may be herbivorous
and consequently instrumental in causing tissue
damage to seaweeds.
Ths paper takes a step towards clarifying the
ecological role of a previously overlooked group of
marine protists and presents data on their seasonal abundance within macroalgal stands.
3. MATERlALS AND METHODS
Populations of amoebae inhabiting the surfaces
of the following five species of algae were examined: Fucus serratus, Laminaria saccharins,
Porphyra umbiiicafis, Dilsea carnosa and UtUa
lactuca. Attached specimens of Fucus, Porphyra
and Uha were collected from the intertidal zone
directly across from the Marine Station, Millport
(37'7"W, 54'5'") while the Laminaria and Di[sea specimens were collected by divers in 3.5 and
4.2 m of water respectively, off the Station's Keppel Pier. Samples were collected once a month
over a one-year period (August 1989 to July 1990).
Numbers of amoebae on the surfaces of
seaweeds were estimated following enrichment
culture. Amoebae were grown in polyxenic Culture
on MY75S agar plates (75% seawater with 0.1
malt and yeast extract [19]). This medium is suitable for isolating amoebae which feed on the
mixed bacterial populations and occasionally on
eukaryotic protists which develop on the plates.
Culture plates were inoculated by dissecting
squares of algal tissue from random areas Over the
algal blade and placing them on the agar surface.
The edges of plates were wrapped in Clingfilm to
prevent moisture loss. Amoebae present on either ,
of the algal surfaces (top or bottom) reproduced
and migrated onto the agar surface. On each Sam- ,
piing occasion and for each algal species ten replicate algal squares, 2 m m x 2
(8 mm2 total
surface area), were cut from areas of intact tissue,
and from areas of damaged tissue, where the numbers of microorganisms were expected to be
greater. Usually this damaged tissue accounted for
less than 5% of the total macroalgal blade and was '
caused by grazing herbivores, physical damage by
wave action or by disintegration of tissue during
reproduction. To obtain a more accurate estimate
303
of species diversity over the entire algal blade, ten
larger squares ( 5 mm X 5 mm) were also dissected
from intact tissue and inoculated onto the agar
surface.
All plates were examined at XlOO magnification after 20 days and 30 days incubation at 18" C
and each zone around the algal tissue was scored
for the presence or absence of amoebae. Positive
results implied that there was at least one amoeba
cell per tissue section surface. In these cases cells
were washed off the agar surface with sterile
seawater and identified by light microscopy. Most
isolates were identified to the level of genus and in
some cases to the level of species using published
keys [4,13,19,20].
SEM was used to examine the surface microbial
populations on three of the intertidal algal species,
Fucus, UIva and Porphyra. Samples were collected
on two occasions in the summer months July and
August 1990. Segments of intact and wounded
tissue were fixed in cacodylate buffered (pH 7.2)
glutaraldehyde (4%) and osmium tetroxide (1%)at
4°C for 1 h. Samples were washed in distilled
water, dehydrated through an acetone series, critical point dried and sputter coated with gold. These
were viewed on a JEOL JSM-5200 SEM. Counts
of bacteria were made on 15 randomly selected
areas of algal surface, each area equalling 364
pm2. Fucus had noticeable amounts of loose floc
material adhering to its polysaccharide-rich surface
which waj lost during preparation for SEM. This
material was examined separately for evidence of
'lost' microbial prey. Washings from Fucus were
pelleted by centifugation, fixed and dehydrated as
detailed above. Material was embedded in Spurr
resin and thick-sectioned (1 pm) for LM.
4. RESULTS
The numbers of naked amoebae on the surfaces
of the five algal species sampled are shown in Fig.
1 (A to E). It is important to note that because an
indirect counting method was used, these counts
are conservative for the following reasons. Firstly,
it was assumed that if positive growth was attained this was from replication of a single amoeba
on the algal tissue. No allowance was made for the
A
L
301
E
30t
Month Sampled
Fig. 1. Seasonal changes in the numbers of naked amoebae on
the surfaces of five species of macroalgae: F u m serratus (A),
Laminaria saccharina (B). UIva lactuca (C), Porphyra umbilicalis (D). Dilsea carnosa (E). Open symbols are damaged
regions on the algal blade, closed symbols are intact tissue.
fact that several amoebae may have been present
on the original algal square. Secondly, it is unrealistic to assume that the culture conditions used
in this study were appropriate for the growth of all
amoebae; an unknown percentage of species must
have failed to grow out on the agar plates used.
Thirdly, rapidly growing amoebae may have
masked or competitively excluded the establishment of slower growing populations. Finally, the
smallest forms of amoebae found, those less than
10 p m in length, were difficult to detect even after
enrichment culture and it must be assumed that
these, perhaps more than other species, have been
304
underestimated. Even so, significant numbers of
amoebae were found on all occasions throughout
this one-year study. Over the year, amoebae were
most abundant on areas of wounded algal tissue
when compared to equivalent areas of intact tissue. In one case, there were as many as 43 amoeA
a3
z9$
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100
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Species Number
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Species Number
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23
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I
Species Number
Fig. 2. Relative abundance of different species of amoebae (see Table 1) on the surface of five macroalgae: Fucus serrarw (A),
Larninorio saccharina (B). Ulua lacruca (C), Porphyra umbilicalis (D). Dike0 carnoso (E).Closed bars represent amoebae from intact
algal tissue, open bars represent amoebae from wounded tissue.
305
D
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5
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Species Number
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al
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Species Number
Fig. 2 continued.
bae cm-2 (Fig. 1A) although generally maxima
were around half this number.
Amoebae were least abundant during the winter
months of January to March when water temperatures were between 7.0-8.7"C and most abundant
during the summer months. There were, however,
several exceptions to this trend, notably the high
peaks in June (Fucus damaged), November (Ulva
damaged) and December ( Laminaria and Dilsea).
These high counts coincided with periods of high
amoeba1 species diversity with eight or nine different species coexisting on the blade surface (Fig.
2A-E). On the other hand, the anomolously high
numbers of amoebae found on Porphyra in April
were due to only three species and coincided with
marked disintegration of the tissue during sporulation. Presumably, under these conditions the high
levels of algal exudates and bacterial prey promoted luxuriant growth of amoebae.
Over the one-year sampling period, 27 different
species of amoebae were recognized. These are
listed in Table 1. The relative abundance of different species within the ten replicate samples is
given in Fig. 2A-E; a 100% abundance indicates
that a species of amoeba was present in all ten
replicates and implies that this species was widely
distributed over the algal blade. The commonest
amoebae were Flabellula demetica (Fig. 3),
Neoparamoeba pemaquidensis (Fig. 4), Vannella
septentrionalis (Fig. 5 ) and Stygamoeba sp. 23
(Figs. 6 and 7). These species were particularly
abundant on the two brown algae sampled, Fucus
and Laminaria, where in some cases single species
of amoebae were distributed over the entire surface
307
Table 1
List of amoebae isolated from seaweeds giving identification
numbers as used in the text and mean sizes (maximum dimension in pm, n =15)
Species/genus
Number
Size
(w)
Flabellula citafaSchaeffer 1926
F. demetica Page 1980
F. trinovantia Page 1980
Flabellula Schaeffer 1926
Flabellula Schaeffer 1926
Hartmannello Alexeieff 1912
Mayorella Schaeffer 1926 a
Mayorella a
Neoparamoeba pemaquidensis (Page 1970)
Paraflahelluia reniformis (Schmoller 1964)
Paramoeba erlhardi Schaudinn 1896
Platyamoeba Page 1969
Pla fyamoeba
Rhrzamoeba saxonica Page 1974
Rhrzamoeba Page 1972
Thecamoeba orbis Schaeffer 1926
Trichosphaerium sieboldi Schneider 1878
T. sieboldi
Vannella aberdonica Page 1979
V. septenrrionalis Page 1980
Vannella Bovee 1965
Vexi1tife.a Schaeffer 1926
Srygamoeba Sawyer 1974
Unidentified amoeba
Unidentified amoeba
Unidentified amoeba
Unidentified amoeba
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
24
4.5
15
33
17
19
25.5
44.5
25
36
69
12
28.5
21
9
19.5
100
108
85
14.5
23
17.5
13
9.5
7.5
5
30
These Muyorella-like amoebae may belong to the genus
Dactylamoeba Korotneff 1880. Correct identification relies
on EM examination of the surface coat; the surface of
Mayorella has a cuticle while that of Dactylamoeba has
microscales.
This was a spicule-bearing Trrchosphaerium. The validity of
classifying any Trichosphaerrum as a ‘naked‘ amoeba is uncertain; see comments in discussion.
of the macroalgal tissue. It is important to note
that F. demetica is a ‘small’ amoeba, i.e. with a
locomotive form less than 10 p m in length, and
that five other species isolated throughout the year
fell into this category (Table 1 and Fig. 2).
Most species of amoebae were found in low
abundance throughout the year. For example, the
easily identified amoeba Thecamoeba orbis with
its characteristic longitudinal surface wrinkles (Fig.
8) was found only sporadically (Fig. 2, sp. number
16) with no clear seasonality or association with a
particular algal type. Some of the more abundant
species did show evidence of seasonality. Flabellula demetica and Stygamoeba sp. were found on
all algae throughout the year but were generally
less abundant during the winter months December
to March. Similarly, N. pemaquidensis was common throughout the year on Fucus, Laminaria
and Dilsea but less so during the winter months.
Paramoeba eilhardi also showed some evidence of
seasonality, being most abundant between May
and August on Fucus.
There was some evidence of lower amoeba1
species abundance on algae high on the intertidal
zone. Although N. pernaquidensis was found on all
five algal species, it was least abundant on
Porphyra which was found high on the shore. This
alga also had fewer species associated with it; a
total of 17 species of amoebae were isolated from
Porphyra throughout the year compared with 19,
21, 20 and 20 species on Ulua, Fucus, Laminaria
and Dilsea, respectively. The amoebae, Mayorella
sp., P. eilhardi, Platyamoeba sp. and T. orbis were
present on all algal species except Porphyra.
Since many of the species encountered were
rare it was difficult to detect any obvious specific
amoeba/alga associations. However, T. sieboldi
(spiculed form) was restricted to Dilsea, Plaryamoeba sp. number 13 to the Rhodophytes, and
an Hyatodiscus-like amoeba (sp. number 30) was
only found on Ulua. There was also little evidence
of any species exclusively inhabiting damaged tissue. Although numbers of amoebae were higher at
Figs. 3-15. Micrographs of algal-associated amoebae. 3, LM of Flabellula demefzca. X 1OOO: 4. LM of Neoparamoeba pemuqurdensls
X 1OOO; 5, LM of Vannella septentrronalis. X 1OOO; 6, LM of Stygamoebo sp. x 500; I , SEM of Srygarnoeba sp. X 2600; 8, LM of
Thecamoeba orbis. X600; 9, disrupted agar surface with Trichosphaerrum sieboldi (arrowed). X40; 10, LM of T. sieboldi showing
dactylopodium (arrowed). X200; 11, faecal pellets from T. sieboldi grown on an agar surface. X700; 12, LM of N. pemaqurdensrs
showing bacteria in food vacuole (arrowed). X 1200; 13, LM of F. cirara with ingested diatom. x 1500; 14, LM of T. sieboldi with
ingested diatoms. Dactylopodium arrowed. X 500; 15, LM of thick section through surface flw material on Fucu. X 1500. Note
associated bacteria (arrowed).
308
damaged sites, reflecting higher prey availability,
the same species were usually also present on the
surface of intact blades. There were some exceptions. For example, the Rhiramoeba and Rhizamoeba-like amoebae on UIua and Dilsea were
found only at wounded sites.
Some amoebae consistently showed evidence of
agar digestion on the culture plates, similar to the
disruption shown by T. sieboldi in Fig. 9. These
amoebae were N. pemaquidensis, Mayorella sp. 8,
unidentified Nolandella-like sp. 26, Flabellula trinouantia, F. citata, Trichosphaerium sieboldi,
Stygamoeba sp. 23, Vexillflera sp. 22, and Vannella sp. 21. It is important to note that since
cultures were polyxenic, disruption of the agar
surface is not, in itself, conclusjve proof that
amoebae were manufacturing extracellular enzymes. But in the case of T. sieboldi (Fig. lo),
which has already been shown to digest phycocolloids [21,22], agar digestion was almost certainly
due to amoeba1 enzyme rather than to bacterial
enzymes particulary since this amoeba egested faecal pellets resembling partially-digested agar (Fig.
11).
The majority of amoebae in this study were
bactivorous as evidenced by bacteria in their food
vacuoles (e.g. N. pemaquidensis, Fig. 12). But F.
d a t a and T. seiboldi (Figs. 13 and 14) were frequently observed to preferentially prey on diatoms
and T. orbis (Fig. 8) and Mayorelfa sp. number 8
on fungal spores, a t least in the laboratory. It
should be noted that for diatoms and fungal spores
to have grown out in culture they must have been
present with the amoebae on the algal inoculum.
Thus, the distribution of preferred prey may have
affected the distribution of amoebae on algal
surfaces. For example, in Ulua, the amoeba F.
citata was only found among damaged tissue where
diatoms were abundant (Fig. 21).
The numbers of bacteria on the surfaces of
three algae were estimated by SEM (Figs. 16 to
21). The numbers of bacteria are compared in
Table 2. In the case of Porphyru and Ulua there
were significantly more bacteria per unit area at
damaged sites. This was not the case for FucW
where SEM examination suggested that the numbers of bacteria in both regions were equivalent.
However, t h s may be misleading since the surface
of FUCUShad a loose mucous/floc layer, obvious
when plants were viewed in situ. This microbe-rich
material was lost from both intact and damaged
tissue during specimen preparation (Fig. 16). The
majority of bacteria retained on the surface of
FUCUS were those embedded in the remnant
mucous layer in the crevices between adjacent
cells. In Porphyra, bacteria on normal tissue were
distributed in patches while the microbial population around damaged sites formed a dense m a t
dominated by filamentous bacteria (Figs. 18 and
19). Bacteria were more or less evenly distributed
over the surface of normal U/ua tissue; at damaged
sites the microbial mats were denser and fiequently rich in diatoms (Figs. 20 and 21).
Amoebae were common on the surface of algae,
yet they were not obvious in the SEM. This is n o t
surprising since most amoebae have no striking
surface features, and unless they have a characteristic shape, such as the limax amoeba Stygarno&
(Fig. 7). they are overlooked amid surface debris.
Moreover, most cells are lost during specimen
preparation as they are loosely attached and shrink
during specimen fixation. Occasionally, amoeba,
like structures were observed and those shown in
Fig. 19 are presumed to be amoebae which were
-
Figs. 16-21. Scanning electron micrographs of algal surfaces. 16, normal tissue of Fucus with bacteria partially embedded in algal
coat. Most bacteria restricted to areas between cells. x 5000; 17, wounded tissue of F u w wrth bactena. X 5000; 18, norm&
tissue of Porphyra with patch of bacteria. x SOOO; 19, wounded tissue of Porphyra with numerous filamentous bacteria. Tissue edge,
lower right comer. Structures (A) are presumed to be amoebae. x 1000; 20, normal tissue of Ulua with heterogeneous microbial
population. x5OOO; 21, wounded tissue of Ulua showing dense bactenal population and diatoms. Tissue edge, lower right corner.
X 1500.
309
310
Table 2
The numbers of bactena on the surface (intact and wounded)
of three macro-algal specles determined from scanning electron
micrographs (mean of 15 randomly selected sites + SE)
Algal species
Tissue type
Number of bactena
per cm2 ( x lo6)
Fucw
intact
damaged
intact
damaged
intact
damaged
47.0 (5.8)
41.7 (4.5)
16.0 (2.1)
95.3 (10.4)
38.3 (6.8)
84.6 (7.5)
Porphyra
Ulua
retained on the surface because they were entangled in the microbial mat.
5. DISCUSSION
To date, there has been no thorough assessment
of the ecological importance of marine gymnamoebae within the context of the food-web ecology of the seas [6] yet they are ubiquitous and
numerically important at some sites in the ocean.
In the nutrient rich surface microlayer of the sea
there can be as many as 1413 amoebae I - ’ according to Davis et al. [8]. In open water samples,
there are fewer amoebae with estimates averaging
only 1.39 amoebae 1-’ [8] down to a depth of 3090
m, although studies in this laboratory have found
them to be up to 1000 times more abundant than
this in estuarine plankton samples (unpublished
data). The present study has shown that the
surfaces of macroalgae support substantial populations of amoebae. Depending upon the time of
year and species of algae sampled, total numbers
of amoebae ranged between 1-23 amoebae per
cm2 of algal surface. These values can be put into
context using published estimates of macroalgal
standing crop for Fucus (2130 g wet weight m-2
[5]) and Laminaria (6600 g wet weight m-2 [Ill)
and the conversions that 1 g of tissue has a surface
area of 24.6 cm2 and 25.3 cm2 for Fucus and
Laminaria, respectively. Thus there were between
0.2-1.1 ( X 1 0 6 ) amoebae per m2 of intact Fucus
and between 0.5-3.2 ( X l o 6 ) amoebae per m2 of
Laminaria. Taking these estimates one stage further, the total biovolumes (pm’) of amoebae on
Fucus and Laminaria were between 2.8-15.0
( ~ 1 0 and
~ ) 6.7-42.0 ( X l O ’ ) pm3 mP3. These
estimates are tentative since in making the calculations it was assumed that only the four common
species were present and that their volumes approximated cylinders with heights one third their
diameter. Bacterial biovolumes on these algae were
estimated from SEM counts for Fucus, which, as
pointed out in the results, were probably underestimated, and from published counts f o r
Laminaria. In all cases it was assumed that the
mean diameter of a bacterial cell was 0.7 pm. T h e
total biovolume of bacteria in July/August was
4.4 x 10” pm3 m-3 for Fucus and 6.9 x 10” pm3
m P 3 for Laminaria. Over the summer months,
therefore, amoebae constituted as much as 0.34%
and 0.95% of the total microbial (amoebae/
bacteria) biomass on these algae.
Linley et al. [16] have estimated that the Standing crop of bactivorous protozoa (ciliates and
flagellates) constitute some 6-10s of the bacterial
biomass. Amoebae would appear to be a minor
component of the grazing protozoan population,
however, it must be remembered that they are
unique among microconsumers in that they prey
on firmly attached surface bacteria, unlike many
of the ciliates and flagellates which prey on loosely
associated bacteria and/or those associated With
surface flocs. Although many of the amoebae
counted were small with low standing crops, relative to other protozoa, their predatory role may be
substantial since amoebae have been described as
having voracious feeding habits [24]. These small
amoebae, those less than 10 pm in length, are
important since they have not been reported in
previous studies and amoebae in this size range
may constitute an ecologically important and
hitherto neglected group. Finally, it should be
remembered that the abundance-estimates of
amoebae are conservative since they do not take
into account the fact that amoebae were more
abundant in damaged areas of tissue, nor the fact
that the indirect counting method underestimated
the population size.
Sieburth [26] referred to the ocean’s surface
microlayer as a heterotrophic microcosm in the
311
sea where populations can be as rich as in laboratory culture. The surfaces of macroalgae, especially where damaged, are similar in this regard,
with dense populations of amoebae and bacteria.
Amoebae thrive at damaged tissue sites presumably because of the higher bacterial densities at
these locations, as well as being sheltered from
wave action and dessication in these micro-habitats. Some amoebae, such as Rhizamoeba and the
rhizamoeba-like amoebae, were common on
damaged algal tissue. These amoebae have filamentous holdfasts around their periphery which
may facilitate their attachment at damaged sites.
Bacteria thrive on the surface of algae because
they utilize the localized concentrations of dissolved organic material released by macrophytes.
It has been estimated that as much as 25% of the
algal energy production in a kelp bed is released
as dissolved organic matter during fragmentation
of the algal blade [17]. What is unknown at this
time is whether some amoebae also utilize dissolved organic exudates as an energy source or
whether their trophic role is exclusively as micropredators. In the laboratory, amoebae were found
to feed on bacteria, fungal spores and diatoms.
However, some species also gave indications of
possible phycocolloid digestion. One of these
species, T. sieboldi, has already been shown to
digest a range of seaweeds [22] including
Laminaria and Porphyra. This is an unusual
amoeba which appears naked at the LM level, but
has a thin, flexible wall with pores through which
characteristic pseudopodia, termed dactylopodia,
emerge (Figs. 10, 14). Because it possesses a wall it
is considered by some to be a testate amoeba [14],
although this classification is debatable since the
wall is structurally no more elaborate than the
cuticle around the naked amoeba, Thecamoeba.
The function of dactylopodia is unknown; they
have not been found to be involved in either
locomotion or phagocytosis [19]. But since this
amoeba is common among seaweeds and can digest phycocolloids, dactylopodia may be involved
in the localized release of an extracellular, membrane-bound enzyme. This notion was supported
by LM observations made on Trichosphaerium
while it was actively digesting algal tissue. Along
the length of dactylopodia in contact with the
algal surface, considerable cytoplasmic streaming
and vacuole transport was observed.
There were indications that amoebae were less
abundant on algae higher in the intertidal zone.
For example, N. pemaquuidenszs was found
throughout the year on all algae but was least
abundant on Porphyra, which inhabits the exposed upper shores of Cumbrae. In this habitat
the main limiting factors are reduced salinity, dessication effects and predation pressures from
amphpods which dominate grazing activity in the
upper shore [18]. The much smaller amoeba, F.
demetica, was the species most frequently isolated
from Porphyra, suggesting that, because of its
small size, it is better able to cope with the
fluctuating conditions of the upper shore. Unlike
their terrestrial counterparts, marine amoebae do
not encyst in response to adverse conditions and
thus smallness may be the most appropriate adaptation for survival in such locations.
Amoebae
were
numerically
important
throughout the year and it follows that they may
play a significant, but as yet undetermined, role in
the microbial cycling of macroalgal carbon. Their
predatory effect on the indigenous bacterial microflora may help to maintain a physiologically
active bacterial population [3]. This would promote the bacterial conversion of dissolved organic
exudates particulary by those bacteria in intimate
contact with the surface where amoebae are probably exerting most effect. The surface of algae can
often become overgrown with epiphytes to the
point where macrophyte production is reduced [9],
hence the grazing activity of amoebae on epiphytes may enhance algal production. Some
amoebae may also perform a dissimilatory role
within the algal community by utilizing algal
carbon exudates directly or by exploiting particulate carbon, as is the case with Trichosphaerium. Here amoebae may be initiating cell
damage, thereby attracting macrograzers to
damaged sites and rendering tissue susceptible to
invasion by pathogens.
ACKNOWLEDGEMENTS
This study was partially supported by an
Equipment Grant from the Society of General
312
Microbiology, U.K. I am grateful for technical
assistance from Mr. Peter Wilson, diving support
from Mr. Colin Munro and helpful comments
from Kristen Schlech.
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