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saline groundwater systems at depths of 5-6 km where temperatures are in the
range of growth and/or survival of hyperthermophiles [46].
3.2 DNA from Mid-Cretaceous Black Shales (Ocean
Anoxic Events)
One of the most enigmatic phenomena of the Cretaceous period is that of
the episodic deposition of black shale layers. These occurrences, referred to as
ocean anoxic events or OAEs, are thought to have occurred during warm anoxic
periods during the Cretaceous, leaving clearly distinct organic-rich black layers.
The OAEs are considered to have been caused by a complex combination of
temporal warm temperature, high biological productivity and turnover, ocean
current relaxation, methane hydrate explosion, and superplume activities (e.g.
[10, 32, 51]). However, only limited fossils are found in these black shales, and
the factors that led to their formation remain the objects of great debate. With
regard to these issues, and because of the ability of microbes to survive a wide
variety of conditions, we investigated some OAE layers to see whether or not
microbial fossils might be left behind as indicators of their activities during the
periods of shale formation.
In October 2000, in southwestern France, we collected a core from a depth of
338 cm below the surface [20] that contained a extremely black layer defined
as the top of OAE1b formed at 108 million years ago [41]. The innermost
part of the core was aseptically sampled at six vertical depths of the black
shale core. No contamination occurred during drilling as judged by the use
of fluorescence micro-beads [43]. Bacterial 16S rRNA genes were amplified
from the six different horizons associated with an OAE stratum. Although the
core was recovered from a terrestrial environment, the recovered sequences
showed affinity to bacterial communities previously seen in deep-sea sedimen-
tary environments (i.e., the sequence assemblage was easily recognizable as
a marine community). At the non-OAE horizons, bacterial community struc-
tures were mainly composed of the gamma-Proteobacteria, similar to extant
deep-sea genera such as
Shewanella
,
Moritella
,
Psychromonas
,
Halomonas
,
and
Marinobacter
. Some of these sequence were closely related to the mem-
bers of psychrophilic (cold-loving) or piezophilic (pressure-loving or tolerant)
bacteria.
At the OAE horizons, the bacterial populations changed dramatically (Fig.
5), showing a shift towards sulfate-reducing bacteria (SRB) belonging to the
delta-Proteobacteria. In particular, the predominant clone sequences within
the delta-Proteobacteria were affiliated with the
Desulfosarcina/Desulfococcus
cluster and the
Desulfobulbus
cluster [20]. The members within these clusters
are known to be the 'putative anaerobic methane oxidation (AMO) syntrophic
SRB group' [37] and detected from global methane and cold seep environ-
