REDUCING ENVIRONMENTS OF THE DEEP-SEA FLOOR
103
flux of fluid mud to seepage of mineral-rich fluids
may occur over periods of time that represent hundreds
to thousands of years (Roberts and Carney, 1997).
At the scale of millennia, the lifetime of gas hydrate
accumulations on continental margins follows a cycle
at the frequency of sea-level change (Roberts and
Carney, 1997).
Colonization and niche specialization
Comparison of the species composition of benthic
communities invokes questions about niche specialization and controls on colonization. At seeps, where
symbioses are usually a major biomass component,
it is important to understand how environmental
factors influence the establishment of symbiont-bearing
invertebrates. Cold seeps are initially mud bottoms
within which hard substratum is mostly secondarily
formed as carbonate concretions. Sulphide is the
secondary product of anaerobic bacterial oxidation
of methane or higher hydrocarbons using seawater
sulphate as the oxidizing agent. Thus species requiring
hydrogen sulphide and/or hard substrata would likely
appear later in the evolution of a seep site. Faunistic
differences within sites may be explained in this
manner (MacDonald et al., 1990a; Olu et al., 1997).
Colonization of seeps by species not containing
symbionts, endemic or otherwise, contributes to the
development of a complex ecosystem. Seeps represent
localized perturbations of the vast and well-established
soft-bottom benthic environment of the deep sea.
The participation of non-symbiont deep-sea species
in seep food webs is an important feature at several
sites (Carney, 1994; Olu et al., 1996a; Barry et al.,
1996), where extremely high densities of meiofauna,
suspension feeders, deposit feeders and, usually, carnivores occur. Development of this trophic continuity
of seeps with the surrounding environment appears to
depend on the duration of seepage. At early stages
only symbiont-containing species would be present.
Later colonization of seeps by the background fauna,
influenced by the type of substrata, should vary over
the very broad geographic and bathymetric range in
which seeps occur (Sibuet and Olu, 1998). Links with
the surrounding ecosystem are thus more apparent here
than at hydrothermal vents on new oceanic crust.
The relatively high diversity of species at seep sites
can be explained by local variation in the intensity
of fluid flow, long-term stability, presumed continuous
availability of nutrients and the variety of substrata
present (Sibuet and Olu, 1998). Currently, there are
many more symbiont-containing species known from
seeps than from vents; up to 15 such species are known
from a single region (Sibuet and Olu, 1998). Has
the broader range and longer duration of seep habitat
permitted greater specialization? More studies on
systematic and molecular phylogeny will be necessary
to analyse the evolution of seep fauna some of which
(as demonstrated for mussels and vestimentiferans)
may be ancestral to those of hydrothermal vents.
CONCLUDING COMMENTS
Reducing habitats present physiological and ecological
challenges to organisms. In the deep sea, such ecosystems are uncommon, but foster high biomass and many
unusual animals. It is the supply of reduced compounds
that allows chemoautotrophic production by microbes
using either reduction or oxidation reactions. The
extent of production by heterotrophs is unknown,
but likely contributes to the mobilization of organic
compounds in these systems. While the food source is
abundant, the ability to use it or to tolerate the physicochemical conditions is limited to a relatively small
group of organisms. High dissolved sulphide levels,
variable and extreme temperatures, unpredictable fluid
flows, and the patchy, dispersed nature of the habitat
all contribute to the challenges that both macro- and
microfauna face in the vent habitats. Cold seeps
and organic food falls present less extreme physicochemical conditions, but chemical toxicity and high
habitat dispersion are important factors.
Compared to many other areas in the deep sea,
vents and seeps have received much attention from
researchers. Description of some fundamental phenomena such as the major role of Archaea in production,
the highly integrated animal–microbe symbioses, and
discovery of the presence of many novel systematic
groups represent major achievements. Nonetheless,
there remain many gaps in the knowledge of these
ecosystems and their role in deep-sea production,
diversity and functioning. Study of evolutionary relationships among the reducing habitats and many others
of the deep sea may prove particularly exciting. These
systems do not stand alone; a better appreciation of
adjacent deep-sea systems could be of great value to
reassessment of information from vents, seeps, organic
remains and low-oxygen basins.
103
flux of fluid mud to seepage of mineral-rich fluids
may occur over periods of time that represent hundreds
to thousands of years (Roberts and Carney, 1997).
At the scale of millennia, the lifetime of gas hydrate
accumulations on continental margins follows a cycle
at the frequency of sea-level change (Roberts and
Carney, 1997).
Colonization and niche specialization
Comparison of the species composition of benthic
communities invokes questions about niche specialization and controls on colonization. At seeps, where
symbioses are usually a major biomass component,
it is important to understand how environmental
factors influence the establishment of symbiont-bearing
invertebrates. Cold seeps are initially mud bottoms
within which hard substratum is mostly secondarily
formed as carbonate concretions. Sulphide is the
secondary product of anaerobic bacterial oxidation
of methane or higher hydrocarbons using seawater
sulphate as the oxidizing agent. Thus species requiring
hydrogen sulphide and/or hard substrata would likely
appear later in the evolution of a seep site. Faunistic
differences within sites may be explained in this
manner (MacDonald et al., 1990a; Olu et al., 1997).
Colonization of seeps by species not containing
symbionts, endemic or otherwise, contributes to the
development of a complex ecosystem. Seeps represent
localized perturbations of the vast and well-established
soft-bottom benthic environment of the deep sea.
The participation of non-symbiont deep-sea species
in seep food webs is an important feature at several
sites (Carney, 1994; Olu et al., 1996a; Barry et al.,
1996), where extremely high densities of meiofauna,
suspension feeders, deposit feeders and, usually, carnivores occur. Development of this trophic continuity
of seeps with the surrounding environment appears to
depend on the duration of seepage. At early stages
only symbiont-containing species would be present.
Later colonization of seeps by the background fauna,
influenced by the type of substrata, should vary over
the very broad geographic and bathymetric range in
which seeps occur (Sibuet and Olu, 1998). Links with
the surrounding ecosystem are thus more apparent here
than at hydrothermal vents on new oceanic crust.
The relatively high diversity of species at seep sites
can be explained by local variation in the intensity
of fluid flow, long-term stability, presumed continuous
availability of nutrients and the variety of substrata
present (Sibuet and Olu, 1998). Currently, there are
many more symbiont-containing species known from
seeps than from vents; up to 15 such species are known
from a single region (Sibuet and Olu, 1998). Has
the broader range and longer duration of seep habitat
permitted greater specialization? More studies on
systematic and molecular phylogeny will be necessary
to analyse the evolution of seep fauna some of which
(as demonstrated for mussels and vestimentiferans)
may be ancestral to those of hydrothermal vents.
CONCLUDING COMMENTS
Reducing habitats present physiological and ecological
challenges to organisms. In the deep sea, such ecosystems are uncommon, but foster high biomass and many
unusual animals. It is the supply of reduced compounds
that allows chemoautotrophic production by microbes
using either reduction or oxidation reactions. The
extent of production by heterotrophs is unknown,
but likely contributes to the mobilization of organic
compounds in these systems. While the food source is
abundant, the ability to use it or to tolerate the physicochemical conditions is limited to a relatively small
group of organisms. High dissolved sulphide levels,
variable and extreme temperatures, unpredictable fluid
flows, and the patchy, dispersed nature of the habitat
all contribute to the challenges that both macro- and
microfauna face in the vent habitats. Cold seeps
and organic food falls present less extreme physicochemical conditions, but chemical toxicity and high
habitat dispersion are important factors.
Compared to many other areas in the deep sea,
vents and seeps have received much attention from
researchers. Description of some fundamental phenomena such as the major role of Archaea in production,
the highly integrated animal–microbe symbioses, and
discovery of the presence of many novel systematic
groups represent major achievements. Nonetheless,
there remain many gaps in the knowledge of these
ecosystems and their role in deep-sea production,
diversity and functioning. Study of evolutionary relationships among the reducing habitats and many others
of the deep sea may prove particularly exciting. These
systems do not stand alone; a better appreciation of
adjacent deep-sea systems could be of great value to
reassessment of information from vents, seeps, organic
remains and low-oxygen basins.