ABSTRACT
Microbial
communities from a subseafloor sediment core from the southwestern Sea
of Okhotsk were evaluated by performing both cultivation-dependent and
cultivation-independent (molecular) analyses. The core, which extended
58.1 m below the seafloor, was composed of pelagic clays with
several volcanic ash layers containing fine pumice grains. Direct cell
counting and quantitative PCR analysis of archaeal and bacterial 16S
rRNA gene fragments indicated that the bacterial populations in the ash
layers were approximately 2 to 10 times larger than those in the clays.
Partial sequences of 1,210 rRNA gene clones revealed that there were
qualitative differences in the microbial communities from the two
different types of layers. Two phylogenetically distinct archaeal
assemblages in the
Crenarchaeota
, the miscellaneous
crenarchaeotic group and the deep-sea archaeal group, were the most
predominant archaeal 16S rRNA gene components in the ash layers and the
pelagic clays, respectively. Clones of 16S rRNA gene sequences from
members of the gamma subclass of the class
Proteobacteria
dominated the ash layers, whereas sequences from members of the
candidate division OP9 and the green nonsulfur bacteria dominated the
pelagic clay environments. Molecular (16S rRNA gene sequence) analysis
of 181 isolated colonies revealed that there was regional proliferation
of viable heterotrophic mesophiles in the volcanic ash layers, along
with some gram-positive bacteria and actinobacteria. The porous ash
layers, which ranged in age from tens of thousands of years to hundreds
of thousands of years, thus appear to be discrete microbial habitats
within the coastal subseafloor clay sediment, which are capable of
harboring microbial communities that are very distinct from the
communities in the more abundant pelagic
clays.