Marine Ecosystems Research Department - jamstec japan agency ...

JAMSTEC 2002 Annual Report
Frontier Research System for Extremophiles
be used to study hyperthermophiles even under subcritical
aqueous conditions, where TDT is supposedly
very short.
4. Subsurface Microbiology
4.1. Biomapping of Subsurface Biosphere
(a) "Subvent Biosphere" in a deep-sea hydrothermal
field in the Central Indian Ridge
Subsurface microbial communities supported by
geologically derived hydrogen and carbon dioxide from
the earth's interior have been of great interest since
their finding as a potentially analogous model in the
earth, and could facilitate the search for extraterrestrial
life in Mars and Europa. Despite a recent report on
occurrence of Archaea-dominating, subsurface microbial
community in groundwater system beneath Idaho
hot springs, the microbial ecosystem primarily based on
photosynthesis-independent, lithospheric energy and
carbon sources has been still unidentified. Here we
present for the first time strong geochemical and microbiological
evidences pointing to the existence of hyperthermophilic
subsurface lithoautotrophic microbial
ecosystem (HyperSLiME) dominated by hyperthermophilic
methanogens beneath an active deep-sea
hydrothermal field in the Central Indian Ridge.
Geochemical and isotopic analyses of gas components
in hydrothermal fluids revealed heterogeneity of concentration
and isotopic property of methane (. to .
mM and d C(CH
) = –. to -.PDB) between
the main hydrothermal vent and adjacent divergent vent
sites, representing potential subsurface microbial
methanogenesis at least in the branched vent emitting
more C-depleted methane. Extremely high abundance
of magmatic energy sources such as hydrogen (.
mM) in the fluids also encouraged the hydrogen-based,
lithoautotrophic microbial activity. Finally, both cultivation
of microbes and culture-independent molecular
analyses demonstrated the predominance of
Methanococcales members in the superheated
hydrothermal emissions and chimney interiors along
with the other major microbial components of
Thermococcales members. These results imply that a
HyperSLiME, consisting of methanogens and fermentors,
occurs in this tectonically active subsurface zone.
(b) "Subvent Biosphere" in a deep-sea hydrothermal
field in the Mid Okinawa Trough
The spatial heterogeneity in viable population of
microorganisms was evaluated by using liquid serial
dilution culture technique in natural and anthropogenic
hydrothermal niches. These included chimney structures,
vent fluids, fluid-seawater mixing regions,
hydrothermal plume, ambient seawater, and in situ colonization
systems (ISCSs) in a sediment-hosted
backarc hydrothermal system, Iheya North in the Mid-
Okinawa Trough, Japan. To examine the colonization
process of microorganisms discharged by the
hydrothermal emissions, the ISCSs were deployed near
and into the vent orifices of three spatially separated
vent sites for several days or two years. Culturable population
was equivalent up to . % of the total microscopic
population. Phylogenetic positions and physiological
traits were determined on the isolates obtained
from the terminal positive tubes of the dilution experiments.
On the basis of S rDNA sequence, a total of
isolates belonged to the orders of Thermococcales,
Aquificales, Methanococcales, Archaeoglobales,
Thermales, and Clostridiales, and Group A, B, D, F,
and G of epsilon subclass of Proteobacteria, those covered
almost all phylogenetic groups that had been
detected in global hydrothermal environments.
Culturable population of epsilon subclass of
Proteobacteria with versatile energy metabolisms
was most widely distributed. The phylogenetic Groups
were highly related to the growth temperatures. The
major members of Group B of epsilon subclass of
Proteobacteria could impartially oxidize both hydrogen
and elemental sulfur with nitrate or oxygen, although
members of other Groups generally preferred hydrogen
oxidizing with nitrate. Habitats for cultured thermophiles
were strictly restricted to high temperature
environments, and only a few culturable thermophile
were dispersed into the ambient seawater. The cultur-
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