Environmental Engineering Reference
In-Depth Information
and bacterial 16S rRNA gene using clone library sequence comparisons and
T-RFLP analyses. The results showed that archaeal communities in the rock
interior were different from those near the rock surface. In the surface sections,
almost all archaeal 16S rRNA gene sequences were affiliated within the MGI,
while the inner parts of the deep-sea rock were predominantly composed of
hyperthermophilic Archaea, in the genus
Thermococcus
[15]. A similar transi-
tion was observed in the bacterial 16S rRNA genes, with the outermost samples
being composed of three clusters;
Cytophaga-Flexibacter-Bacterioides
(CFB),
alpha- and gamma-Proteobacteria, and the innermost samples being dominated
by the gram-positive bacteria and the genus
Burkholderia
within the beta-
Proteobacteria [15].
Rocks of the kind reported above offer a situation that is different from
that seen in sediments that are examined via core analysis. In the case of the
rock, the microbial populations have presumably been incorporated during the
formation of the sediment, taken up residence, and then the rock transported
to another environment where further sedimentation does not occur, but rather,
an exchange with the exterior environment drives any population change that
is seen. That is, the shift of the prokaryotic community structures we observed
may be associated with the historical circumstances of the rock, with the outer
communities (MGI and the gamma Proteobacteria) indicating the outer envi-
ronment that slowly infiltrates the rock surface, and the
Thermococcus
, Gram
positives, and
Burkholderia
representing organisms associated with the sedi-
mentary history of the rock.
With regard to this notion, the outer regions of dropstone were more porous
(13%) than the inner regions (7.5% approximately 15 cm from the rock surface)
[15]. Thus, if communities were in place in the interior of the sedimentary stone,
they would be expected to still be in place and detectable via our molecular
approaches.
The existence of the
Thermococcus
signals in the interior of the dropstone
is in keeping with a similar shift of the archaeal community structure reported
above, and also described in deep subseafloor sediments recovered by ODP
Leg 190 from the Nankai Trough [28]. As with the sediments described above,
one imagines these organisms being transported to the sediments as they were
forming. In contrast, the bacterial community in the innermost section is dif-
ficult to explain. Indeed, the genus
Burkholderia
is commonly found in soils
and terrestrial subsurface environments [34]. If the rock interior represents a
paleome, then one must invoke a transport event that brought a major input of
terrestrial or soil microbes to the environment sometime in the distant past.
