Bacteria
General
Background
Heterorotrophic
bacteria obtain their energy requirements via the oxidaton of
organic matter. These bacteria account for a large proportion of
the total planktonic biomass, and this proportion increases with
the level of oligotrophy (Cho & Azam 1990). In ecosystems
where there is no organic input from extraneous sources, the
production of the bacterial biomass accounts for approximately
10 to 30 % of the total primary production (Ducklow &
Carlson 1992). As
the bacterial yield is much less than 50% (
del
Giorgio & Cole 1998), the flux of matter
through the bacterial compartment (i.e., the total heterotrophic
activity) is one of the main fluxes occurring in aquatic
ecosystems. In ecosystems which receive extraneous organic
supplies, the rate of bacterial biomass production can be almost
as high as the primary production rate. The bacterial growth
rates depend on the richness of the surroundings and the
temperature, and range from 1- to
5-fold (White et al 1991).
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The contribution
made by heterotrophic bacteria to the flux of organic
matter in the water column can be depicted in the form
of a "microbial loop" (as shown in the
accompanying diagram) inserted into the classical
food-web. Here the dissolved organic matter, which is
partly excreted by the phytoplankton and partly produced
by the feeding activities of the zooplankton (phytoplankton
debris, excretions, fecal droppings), is efficiently
scavenged by the heterotrophic bacteria.
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The
resulting bacterial biomass is consumed by heterotrophic
nanoplankton and thus re-enters the food-web. The mineral
elements released from organic matter thus begin a new life in
the euphotic layer, where they contribute to the excess primary
production: this process is known as regeneration. The pattern
described above varies greatly, however, depending on the
trophic state of the ecosystem in question.
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The
effects of bacteria on oligotrophic systems
- One
of the main effects of this bacterial activity is
therefore that it contributes to the production of an
excess of useful biomass in comparison with that resulting
from the primary producers. This bacterial
biomass results from the particulate or dissolved organic
carbon which would otherwise be exported from the euphotic
layer by a process of sedimentation or convection (Carlson
et al 1994). The bacterial production occurring in the
euphotic layer prevents the organic matter from being
exported, and thus contributes decisively to the carbon
dioxide levels sequestrated in the deep ocean layers,
where they are fixed
by photosynthesis.
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The
other important effect of the microbial loop is that it
contributes to the regeneration of mineral elements.
Bacteria have lower carbon to nitrogen and carbon to
phophorus ratios than other planktonic organisms (Lee
& Fuhrman 1987), and do not usually release these
elements themselves. However, since they are actively
consumed by protists, they participate in the regeneration
of the nutrient elements originating from the organic
debris consumed by scavengers. The recycling of some of
the nutrient elements which would otherwise be
exported into the deeper layers in the form of dead
organisms, faecal droppings or dissolved organic matter
constitutes a further regenerative production process in
addition to the "de novo" production process
whereby nutrients arise from the deep ocean layers.
However,
there are some more negative aspects which complicate this
simple, rather ideal picture. The bacterioplankton sometimes
reach their high nitrogen and phosphorus intake levels to the
detriment of the phytoplankton (Kirchman 1994, Suttle et al
1990). The food shortage which is thus created for the
phytoplankton, and aggravated by bacterial competition, can lead
to the occurrence of a so-called "surflux" mechanism.
Since the processes of cell growth and division are inhibited by
the lack of nutrients, and since the process of photosynthesis
continues to occur, the phytoplankton cells have been reported
to excrete large quantities of dissolved organic carbon (Karl et
al 1998), which stimulate the bacterial activity and thus
increase the stress levels to which the phytoplankton are
exposed due to the lack of nutrients.
The
effects of bacteria on eutrophic systems
In
eutrophic ecosystems, which usually receive organic inputs
from external sources (these can be of either terrestrial or
benthic origin, and can be produced either naturally or as the
result of human activities), the rate of bacterial production
tends to increase considerably and can exceed the primary
production rate. The bacterioplankton consist of particularly
large cells, many of which are attached to particulate matter.
These cells can therefore be consumed directly by ciliated or
meso-zooplankton, and thus increase the final productivity of
these already highly productive ecosystems. Bacteria and
protists together regenerate the mineral elements present in
the organic waste material. Along with the direct
contributions, they thus aggravate the eutrophisation of the
environment. In some extreme cases, the bacterial activity can
lead to such large oxygen requirements that it results in a
state of anoxia, in which procaryotes are practically the only
organisms able to survive .
Conclusion
The
above examples of aquatic ecosystems at both ends of the trophic
scale show that heterotrophic bacterioplankton play a decisive
and variable role in the functioning of the ecosystems and their
macroscopic properties such as the productivity,
recycling rates, retention rates and exportation rates. It has
by now been clearly established that the functional effects of
the microbial loop largely depend on the environmental
conditions (the trophic state, growth-limiting factors, size
distribution, etc., see Legendre & Rassoulzadegan 1995).
Research in microbial ecology is therefore essential to
understanding the bio-geochemical cycles occurring in aquatic
ecosystems.
The
bacteria in the atoll
lagoons
The
bacteria present in the Tuamotu lagoons are extremely small (they
measure only approximately 0.4 µm in diameter on average)
and their density ranges between 220 000 and 2 000 000 cells per
millilitre, depending on the atoll.
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In
most of the atoll lagoons, the bacteria therefore form a
large biomass (there are usually between 0.9 et 2 million
bacterial cells per millilitre), which is 1
to 3 times larger than that of the phytoplankton
cells in terms of their carbon content, and even
larger in terms of their nitrogen and phosphorus
contents |
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The
distinction has been made between heterotrophic bacteria and cyanobacteria,
which are autotrophic bacteria: these are larger (they measure
roughly 1 µm in diameter, and have been defined from the
functional point of view as phytoplankton), and between
heterotrophic bacteria and their predators such as flagellated
bacteria.
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The
composition of bacterial populations has been assessed using
molecular techniques. Briefly, this
type ofanalysis consists of screening a few litres of
water through a special filter cartridge to collect the bacteria.
The cartridges are then filled with DNA extraction medium and
stored at a temperature of –20°C. The DNA is then amplified
at the laboratory using PCR procedures with universal
primers specific to a variable region of the gene coding for the
RNA 16S. The amplification products are then separated by
electrophoresis (in a denaturing gradient, for example, as in
figure 1A). A correlation analysis
is carried out in order to classify the samples based on
the similarities observed between them as regards both the
position (which characterises the species), and the
intensity (which characterises its abundance) of the bands.
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Based
on an analysis of this kind on the Tuamotu lagoons, the
following facts emerged:
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A
high level of similarity (82%) was found to exist
between the bacterial populations collected at 4
experimental stations on the same atoll (Tikehau 1
to 4), which confirmed that the atoll populations
are fairly homogeneous.
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A
high level of similarity (73% all periods combined)
was found to exist between the bacterial populations
observed at various experimental stations located on
the ocean coasts near the atolls under investigation.
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A
high level of similarity (90%) was found to exist
between the bacterial populations observed on one
very widely open atoll (Tekokota) and those
collected at the ocean surface at the same time of
year (for further details, see the section entitled:
"Do the
effects of the bacterial processes depend on the
geomorphology of the atolls?").
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Fig. 1A)
A
DGGE gel image of a few speciments collected
during the TYP4 campaign (in March 1996) |
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Fig. 1B)
Dendrogram
showing the similarity between samples collected during
3 different field campaigns (in October 1994, November
1995 and March 1996). Tekokota (March) and Ocean 6 (November)
(below) were found to be contaminated |
This
page was
based on :
Hollibaugh
JT, J Pagès, JP Torréton and PS Wong (In press) Phylogenetic
variation in bacterial populations from 10 atoll lagoons in the
Tuamotu archipelago, French Polynesia. In: MJ Brylinsky (Ed.) Trends
in Microbial Ecology. In press.
Torréton
J-P, Pagès J, Talbot V (soumis.) Bacterioplankton and
phytoplankton biomass and production in Tuamotu atoll lagoons.
References
:
Cho
BC, Azam F (1990) Biogeochemical significance of bacterial in
the ocean’s euphotic zone. Mar
Ecol Prog Ser 63: 253-259
Ducklow
H.W. and C.A. Carlson (1992) Oceanic bacterial production. Advances
in Microbial Ecology, 12: 113-181
Del
Giorgio P, Cole JJ (1998) Bacterial growth efficiency in natural
aquatic systems. Annu Rev
Ecol Syst 29 :503-541
Carlson
CA, HW Ducklow, AF Michaels (1994) Annual flux of dissolved
organic carbon from the euphotic zone in the northwestern
Sargasso sea Nature
371: 6496 : 405-408
Lee
S, Fuhrman JA (1987). Relationships between biovolume and
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Kirchman
DL(1994) The uptake of inorganic nutrients by heterotrophic
bacteria Microbial Ecology
28: 2 : 255-271
Suttle
CA, Fuhrman JA, Capone DG (1990) Rapid ammonium cycling and
concentration-dependent partitioning of ammonium and phosphate:
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DM, Hebel DV, Bjorkman K, Letelier RM (1998) The role of
dissolved organic matter release in the productivity of the
oligotrophic North Pacific Ocean. Limnol
Oceanogr 43: 1270-1286
Legendre
L, Rassoulzadegan F (1995) Plankton and nutrient dynamics in
marine waters. Ophelia
41:153-172
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