Chapter 4
REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
Verena TUNNICLIFFE, S. Kim JUNIPER and Myriam SIBUET
INTRODUCTION
The deep ocean harbours several types of habitat in
which dissolved oxygen is low or virtually absent.
While this condition is problematic for most marine
organisms, many have adapted to dysaerobic or reducing conditions. Procaryotic life emerged on Earth
under anaerobic circumstances, and many of these
archaebacterial lineages remain active. These habitats
are characterized by the presence of hydrogen sulphide
and, sometimes, methane. Sulphide originates from
sulphate reduction either by microbes or inorganic processes beneath the seafloor. Methane is usually derived
from reduction of organic matter either by thermogenic
processes or by biogenic processes (methanogens)
although, at ridge crests, this volatile can also be
inorganic in origin. Throughout this chapter, unless
otherwise stated, the term “sulphide” refers to the
sum of all dissolved chemical species of hydrogen
sulphide.
Much attention has focused on these habitats, even
though they occupy a very small proportion of the
sea floor, because of the unusual nature of many
inhabitants. With the discovery of hydrothermal vents
in the eastern Pacific in the late 1970s, came the
recognition that complex ecosystems can be sponsored
on Earth using energy that is not solar. Geothermal
processes supply reduced chemical species which,
through microbial mediation, provide chemical energy
for production of organic carbon. Other features of vent
communities captured the attention of biologists: the
dense biomass, unusual symbioses, novel systematic
relations, among others. The remarkable visual impact
of giant tube worms (vestimentiferans) and “black
smokers” is truly captivating – and not only to
the “general public”. Much justification for work in
these habitats lies in elucidation of how biological
systems can adapt to extremes. The potential of
biotechnological exploitation has figured in some of
this work. Investigations into evolutionary relationships
of both procaryotes and eucaryotes have given rise to
numerous hypotheses, not the least of which is the
development of scenarios around the origin of life in
such habitats (see inter alia Corliss et al., 1981; Holm,
1992; Nisbet and Fowler, 1996).
Researchers realized, with further seafloor exploration, that a community based on chemotrophs is not
unique to vents, nor are many of the organisms. Other
sulphide- and methane-rich habitats such as seeps
on continental margins foster communities that share
some characters with hydrothermal vent communities
on spreading ridges. While knowledge of the varied
seeps and organic-enriched habitats in the deep sea is
still limited, cold-seep ecology is expanding, and links
between the faunas of seeps and hydrothermal vents
continue to be identified (Sibuet and Olu, 1998).
Ecosystem characters in these habitats are understood incompletely – for some properties, very poorly.
Often, workers are reduced to documentation of species
present and visual description of the habitat. As deepsea technologies develop to meet requirements to
measure physico-chemical properties and to assess spatial and temporal variability, scientific understanding
continues to grow.
REDUCING ENVIRONMENTS IN THE DEEP SEA
Hydrothermal vents
The release of heat from the Earth’s interior dictates the
nature and behaviour of the overlying crust. Hydrothermal circulation represents an important mechanism
for cooling newly generated crust. Vigorous surface
81
REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
Verena TUNNICLIFFE, S. Kim JUNIPER and Myriam SIBUET
INTRODUCTION
The deep ocean harbours several types of habitat in
which dissolved oxygen is low or virtually absent.
While this condition is problematic for most marine
organisms, many have adapted to dysaerobic or reducing conditions. Procaryotic life emerged on Earth
under anaerobic circumstances, and many of these
archaebacterial lineages remain active. These habitats
are characterized by the presence of hydrogen sulphide
and, sometimes, methane. Sulphide originates from
sulphate reduction either by microbes or inorganic processes beneath the seafloor. Methane is usually derived
from reduction of organic matter either by thermogenic
processes or by biogenic processes (methanogens)
although, at ridge crests, this volatile can also be
inorganic in origin. Throughout this chapter, unless
otherwise stated, the term “sulphide” refers to the
sum of all dissolved chemical species of hydrogen
sulphide.
Much attention has focused on these habitats, even
though they occupy a very small proportion of the
sea floor, because of the unusual nature of many
inhabitants. With the discovery of hydrothermal vents
in the eastern Pacific in the late 1970s, came the
recognition that complex ecosystems can be sponsored
on Earth using energy that is not solar. Geothermal
processes supply reduced chemical species which,
through microbial mediation, provide chemical energy
for production of organic carbon. Other features of vent
communities captured the attention of biologists: the
dense biomass, unusual symbioses, novel systematic
relations, among others. The remarkable visual impact
of giant tube worms (vestimentiferans) and “black
smokers” is truly captivating – and not only to
the “general public”. Much justification for work in
these habitats lies in elucidation of how biological
systems can adapt to extremes. The potential of
biotechnological exploitation has figured in some of
this work. Investigations into evolutionary relationships
of both procaryotes and eucaryotes have given rise to
numerous hypotheses, not the least of which is the
development of scenarios around the origin of life in
such habitats (see inter alia Corliss et al., 1981; Holm,
1992; Nisbet and Fowler, 1996).
Researchers realized, with further seafloor exploration, that a community based on chemotrophs is not
unique to vents, nor are many of the organisms. Other
sulphide- and methane-rich habitats such as seeps
on continental margins foster communities that share
some characters with hydrothermal vent communities
on spreading ridges. While knowledge of the varied
seeps and organic-enriched habitats in the deep sea is
still limited, cold-seep ecology is expanding, and links
between the faunas of seeps and hydrothermal vents
continue to be identified (Sibuet and Olu, 1998).
Ecosystem characters in these habitats are understood incompletely – for some properties, very poorly.
Often, workers are reduced to documentation of species
present and visual description of the habitat. As deepsea technologies develop to meet requirements to
measure physico-chemical properties and to assess spatial and temporal variability, scientific understanding
continues to grow.
REDUCING ENVIRONMENTS IN THE DEEP SEA
Hydrothermal vents
The release of heat from the Earth’s interior dictates the
nature and behaviour of the overlying crust. Hydrothermal circulation represents an important mechanism
for cooling newly generated crust. Vigorous surface
81