REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
87
1963; Rhoads and Morse, 1971; Tunnicliffe, 1981).
Generally, taxonomic diversity decreases with oxygen
level; in sediments, the polychaetes, nematodes and
crustaceans appear to be most resistant. Rhoads and
Morse (1971) have presented a model for response to
different oxygen levels and drew interesting analogies
to the evolution of benthos in oxygen-poor waters in the
early Phanerozoic. In many basins, mats of filamentous
sulphur-oxidizing microbes are seen where oxygen is
highly depleted, and macrofaunal abundance decreases
in their presence (Rosenberg et al., 1983; Juniper and
Brinkhurst, 1986).
Subsurface reducing habitats
The most voluminous reducing habitat in the deep-sea
lies below the seafloor in anoxic sediments and crustal
rocks. As far as can be determined, this environment is
populated only by micro-organisms capable of anaerobic metabolism, although zones of aerobic microbial
growth may exist in near-surface rocks. In sediments,
anoxia is created during early stages of organic-matter
degradation by the consumption of available pore-water
oxygen. The depth below the sediment/water interface
at which anoxia occurs is determined by the rate of
organic-matter sedimentation; this depth varies from
a few millimetres in carbon-rich sediments near the
continents to tens of centimetres in the oligotrophic
environment of the open ocean. Microbial life extends
for hundreds of metres below the seafloor in oceanic
sediments (Parkes et al., 1994, 2000), approaching the
zone where hydrocarbon formation occurs (Wellsbury
et al., 1997). At these depths, microbial communities
are millions of years old.
For the most part, deep-living sediment microorganisms are deriving energy and nutrients from
fossil organic material, and do not interact with life
in the overlying ocean. An exception occurs in the
seep environment where metabolic products of deepliving sediment microbes are discharged at the seafloor
surface and fuel chemosynthesis. An important local,
subsurface microbial process at seeps is the anaerobic
oxidation of methane, which is coupled to sulphate
reduction. Indications of microbial life have also been
found in drill holes below the seafloor which have
penetrated through sediments into crustal rock. In this
nutrient-poor environment, chemosynthetic microbes
can derive energy from oxidation of hydrogen generated by sea-water/rock interactions, as long as there
are fractures and pore spaces for sea water. Signs of
dissolution of basaltic glass by microbes have also
been noted in drill-hole samples (Furnes et al., 1996),
although it is not clear how this phenomenon is related
to energy metabolism.
The most dynamic subsurface microbial habitat may
be within the crustal rocks at mid-ocean ridges. The
heat-driven circulation of energy-rich fluids within a
large volume of porous and permeable rock should
be very favorable to microbial growth. The potential
for microbial life in the subsurface hydrothermal
environment has been a source for considerable speculation (Gold, 1992; Deming and Baross, 1993), but
direct observation is difficult. Present technology for
deep-sea drilling does not permit penetration into the
basaltic rock at mid-ocean ridges so observation of
subsurface microbial growth must depend on analysis of material discharged from the subsurface by
hydrothermal systems. The venting of microbial floc
from so-called ‘snowblower vents’ that appear in the
weeks and months following seafloor eruptions on
ridge crests (Haymon et al., 1993; Juniper et al.,
1995) indicates that microbial growth is occurring
below the seafloor, although much of the solid material
may be metabolic waste rather than biomass (Taylor
and Wirsen, 1997). The isolation of hyperthermophilic
microbes (growing at 90ºC and above) from venting
fluids following seafloor eruptions (Tunnicliffe et al.,
1997; Holden et al., 1998) provides evidence for a hightemperature subsurface microbial habitat. In addition
to the exportation of biomass, subsurface microbial
growth at ridge crests may have a significant impact
on the chemical composition of hydrothermal fluids
(Lilley et al., 1982, 1983). This chemical evolution
may, in turn, influence the colonization of vents by freeliving micro-organisms and vent animals (Tunnicliffe
et al., 1997). Deming and Baross (1993) have pointed
out that hydrothermal vents provide “windows” into a
subsurface biosphere that cannot be otherwise studied.
Occasional glimpses of subsurface life have been
provided by eruptive events, but a systematic approach
to gather chemical and microbial evidence remains
elusive.
HABITAT CONDITIONS
Substrata and supply of reducing substances
For benthic organisms in reducing habitats, substratum
and supply of reducing substances are intimately
linked. Substrata are colonized because they serve as
87
1963; Rhoads and Morse, 1971; Tunnicliffe, 1981).
Generally, taxonomic diversity decreases with oxygen
level; in sediments, the polychaetes, nematodes and
crustaceans appear to be most resistant. Rhoads and
Morse (1971) have presented a model for response to
different oxygen levels and drew interesting analogies
to the evolution of benthos in oxygen-poor waters in the
early Phanerozoic. In many basins, mats of filamentous
sulphur-oxidizing microbes are seen where oxygen is
highly depleted, and macrofaunal abundance decreases
in their presence (Rosenberg et al., 1983; Juniper and
Brinkhurst, 1986).
Subsurface reducing habitats
The most voluminous reducing habitat in the deep-sea
lies below the seafloor in anoxic sediments and crustal
rocks. As far as can be determined, this environment is
populated only by micro-organisms capable of anaerobic metabolism, although zones of aerobic microbial
growth may exist in near-surface rocks. In sediments,
anoxia is created during early stages of organic-matter
degradation by the consumption of available pore-water
oxygen. The depth below the sediment/water interface
at which anoxia occurs is determined by the rate of
organic-matter sedimentation; this depth varies from
a few millimetres in carbon-rich sediments near the
continents to tens of centimetres in the oligotrophic
environment of the open ocean. Microbial life extends
for hundreds of metres below the seafloor in oceanic
sediments (Parkes et al., 1994, 2000), approaching the
zone where hydrocarbon formation occurs (Wellsbury
et al., 1997). At these depths, microbial communities
are millions of years old.
For the most part, deep-living sediment microorganisms are deriving energy and nutrients from
fossil organic material, and do not interact with life
in the overlying ocean. An exception occurs in the
seep environment where metabolic products of deepliving sediment microbes are discharged at the seafloor
surface and fuel chemosynthesis. An important local,
subsurface microbial process at seeps is the anaerobic
oxidation of methane, which is coupled to sulphate
reduction. Indications of microbial life have also been
found in drill holes below the seafloor which have
penetrated through sediments into crustal rock. In this
nutrient-poor environment, chemosynthetic microbes
can derive energy from oxidation of hydrogen generated by sea-water/rock interactions, as long as there
are fractures and pore spaces for sea water. Signs of
dissolution of basaltic glass by microbes have also
been noted in drill-hole samples (Furnes et al., 1996),
although it is not clear how this phenomenon is related
to energy metabolism.
The most dynamic subsurface microbial habitat may
be within the crustal rocks at mid-ocean ridges. The
heat-driven circulation of energy-rich fluids within a
large volume of porous and permeable rock should
be very favorable to microbial growth. The potential
for microbial life in the subsurface hydrothermal
environment has been a source for considerable speculation (Gold, 1992; Deming and Baross, 1993), but
direct observation is difficult. Present technology for
deep-sea drilling does not permit penetration into the
basaltic rock at mid-ocean ridges so observation of
subsurface microbial growth must depend on analysis of material discharged from the subsurface by
hydrothermal systems. The venting of microbial floc
from so-called ‘snowblower vents’ that appear in the
weeks and months following seafloor eruptions on
ridge crests (Haymon et al., 1993; Juniper et al.,
1995) indicates that microbial growth is occurring
below the seafloor, although much of the solid material
may be metabolic waste rather than biomass (Taylor
and Wirsen, 1997). The isolation of hyperthermophilic
microbes (growing at 90ºC and above) from venting
fluids following seafloor eruptions (Tunnicliffe et al.,
1997; Holden et al., 1998) provides evidence for a hightemperature subsurface microbial habitat. In addition
to the exportation of biomass, subsurface microbial
growth at ridge crests may have a significant impact
on the chemical composition of hydrothermal fluids
(Lilley et al., 1982, 1983). This chemical evolution
may, in turn, influence the colonization of vents by freeliving micro-organisms and vent animals (Tunnicliffe
et al., 1997). Deming and Baross (1993) have pointed
out that hydrothermal vents provide “windows” into a
subsurface biosphere that cannot be otherwise studied.
Occasional glimpses of subsurface life have been
provided by eruptive events, but a systematic approach
to gather chemical and microbial evidence remains
elusive.
HABITAT CONDITIONS
Substrata and supply of reducing substances
For benthic organisms in reducing habitats, substratum
and supply of reducing substances are intimately
linked. Substrata are colonized because they serve as