Do
the effects of the bacterial processes depend
on
the geomorphology of the atolls ?
Based on the differences observed between most of the chemical and
biological parameters studied so far, the trophic
conditions can be said to vary
considerably from one atoll to another.
Differences have also been observed between the
lagoons in terms of the variables characterising the
development of the plankton (their biomass, production
rates, growth rates, etc.), depending on the size of the
channels and apertures connecting the lagoons to the
open sea. Generally speaking, the atolls which are the
most widely open to the sea have the smallest bacterial
populations.
 |
Fig.
:
Bacterial production in various Tuamotu lagoons
|
The ratio between the exoproteolytic activity (EPA)
and the bacterial production indicates the extent to
which bacteria depend on the process of peptide
hydrolysis. A high EPA ratio shows that substrates with
a high molecular weight are being preferentially used.
These ratios were found to be (1) negatively correlated
with the primary production and (2) positively
correlated with the thickness of the coral layer
at 9 of the lagoon test sites (see figure). It is
rather tempting to conclude that little use of
EPA is necessary when the primary production is
able to provide sufficiently large amounts of simple
compounds in the form of dissolved organic matter. When
the levels of these compounds are low, however,
corresponding to the formation of thick coral
layers, the bacterioplankton inhabiting the atoll
lagoons with shallow, frequently renewed waters adopt
the more energy-saving strategy consisting of
synthesizing higher levels of exoenzymes in order to be
able to degrade the coral mucus and other products
excreted by the benthos.
 |
Fig.
: Changes
in the ratio between the potential
exoproteolytic activity and the bacterial
production (PEA/BSP), depending on the
chlorophyll a level, the primary production rate
and the abundance of the coral, after adjusting
in each case for the volume of the lagoon
waters. On Hikueru, due to an epidemic of
dystrophy, there is practically no longer any
living coral to be found
. |
...
two
special cases.
|
On
Tekokota, the
lagoon looks more like a coral reef than an
atoll lagoon. It is widely open to the
surrounding sea, quite shallow and more densely
covered with coral than the other atolls, while
containing significantly lower bacterial
biomasses than the oceanic waters it receives.
This pattern, which is frequently observed on
the coral reefs, is attributable to the fact
that benthic organisms such as sponges and some
corals, feed actively on bacteria (Ferrier-Pagès
et al. 1998).The
ratio between the
exo-proteolytic activity and the
bacterial biomass production suggests that high
molecular weight organic compounds, such as
those present in the coral mucus, contribute
decisively to the
bacterial requirements as far as organic matter
is concerned. Since
the bacterioplankton no longer responded to additional
inorganic nutrients, their growth seems to
depend on the exportation process from
the coral reef ecosystem. Lastly,
the existence of close phylogenetic
similarities between the bacterial populations
living in the Tekokota lagoon and those
collected at the surface of the nearby ocean
confirms that a high rate of replenishment of
the lagoon waters occurs on this atoll (see the
section on Communities
inhabiting the Tuamotu atolls/ the
planktonic/ bacterial organisms). |
|
The
Reka-Reka lagoon
is even shallower than the Tekekota lagoon. It
is a closely confined lagoon and its floor is
covered with sediments. This lagoon was found to
have the highest bacterial
biomasses and rates of activity among all
those studied, and contrary to what occurs in
the other lagoons, the bacterioplankton are controlled
here by the phosphorus levels. The
phylogenetic differences observed between the
bacterial populations identified on Reka-Reka
and those specific to all the other atolls
confirm that this atoll is quite unlike
all the others. The sediments present on its
floor are characterised by a high rate of
atmospheric nitrogen fixation. The ratio between
the surface area of this sediment and the volume
is the highest among all the lagoons studied,
and these conditions obviously favour the supply
of nitrogen to this ecosystem via the
benthos. |
The studies carried out on the bacterial ecology of the atoll lagoons
have therefore yielded several paradoxical findings as
regards the characteristics of the microbial populations
inhabiting these oligotrophic environments : (1)
contrary to what the allometric
relations previously seemed to indicate, the
mean growth rate of the bacteria in the lagoons is lower
than that of the phytoplankton, (2) there is a large
bacterial biomass but the bacteria are not very active,
and (3) the phytoplankton are exposed to nutritional
shortages, which are probably aggravated by
bacterial competition. It is not possible in the scope
of the present outline to discuss these paradoxes in
greater detail. It can be stated, however, that the
mainly bottom-up type of control exerted on the
bacterial communities in the lagoon environments makes
them particularly sensitive to the availability of
nutrients. This characteristic will obviously be worth
taking into account in any future development projects.
This
page was
based on :
Torréton
J-P, Pagès J, Talbot V (soumis.) Bacterioplankton
and phytoplankton biomass and production in Tuamotu
atoll lagoons.
References:
Ferrier-Pagès
C, Allemand D, Gattuso JP, Jaubert J (1998)
Microheterotrophy in the zooxanthellate coral Stylopora
pistillata: effects of light and ciliate density. Limnol Oceanogr 43:1639-1648
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Atoll_site_