Environmental Engineering Reference
In-Depth Information
2.3 Are Viable Microbes in Deep Subseafloor Sediment
Potentially Old?
For the most part, the large and varied efforts using culture-independent
molecular genetics techniques to analyze subseafloor communities has not
been accompanied by traditional cultivation efforts. Only a few species have
been cultivated so far from deep subseafloor environments [2, 36]. In no case
the population size, metabolic activity, or ecological significance of any of
these isolates has been clearly delineated. A fair summary of the efforts to
cultivate and evaluate the living microorganisms from the deep subseafloor
core sediments, such as those of Smith [43] might be: (1) the activity of the
cells appears to be extremely low, (2) the growth of the native populations may
be extremely slow, (3) the cells may be very sensitive to oxygen and pressure,
(4) most cells may require special cultivation techniques such as growth in
mixed cultures; and, (5) most cells may be dormant or dead, constituting what
we call here the 'Paleome'. Our own studies have involved both cultivation-
independent, and, when possible cultivation-connected studies, thus raising the
issue of whether there may be environments where truly ancient organisms are
still surviving.
Viable cells in a subseafloor sediment core from the Sea of Okhotsk (IM-
AGES 2001). In August 2001, a sediment core, which extended 58.1 mbsf,
was recovered from the southeastern Sea of Okhotsk by the International Global
Environmental Change Study (IMAGES) Project [17]. The sediment core was
found to be composed of pelagic clay with several volcanic ash layers con-
taining pumice grains. The preliminary identification of the sedimentation age
by the tephra analysis indicated that the bottom of the core sediment was
approximately 130,000 years old, indicating a very high sedimentation rate.
Culture-independent molecular analysis showed clear differences between the
bacterial populations in the pelagic clay environments and the volcanic ash lay-
ers: the pelagic clays were dominated by members within the Green non-sulfur
bacteria and the candidate OP9 division, while the ash layers harbored members
of the genera Halomonas , Psychrobacter and type-I methanotrophs within the
gamma-Proteobacteria and Sulfitobacter sp. within the alpha-Proteobacteria
[17]. Cultivation-dependent analyses also showed clear differences: no growth
was observed from the pelagic clay layers, while abundant populations of viable
heterotrophs were obtained from the ash layers (Fig. 3). With the exception of
a few spore formers and Actinobacteria that were recovered, there was a good
agreement between the populations identified by culture dependent and culture
independent methods [17]. The factors that account for the apparent good via-
bility of the bacteria in this deep and old environment are not clear - it could
be that small organic nutrients and electron acceptors are imported from the
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