THE GEOLOGY OF SEA-FLOOR MASSIVE SULPHIDES
20
Habitats and biodiversity
associated with sea-floor
massive sulphide deposits
2.1
Hydrothermal vents occur in areas of undersea volcanic activi-
ty, most commonly associated with plate boundaries. The same
geochemical processes that contribute to the formation of SMS
deposits also bring hot fluid, rich in reduced chemicals, to the
sea-floor. These chemicals are toxic to most animals, but they
can be used as an energy source for growth by chemoautotrophic
bacteria, organisms that get energy through a chemical process
rather than by photosynthesis. As a result, hydrothermal vents
provide an abundant source of bacteria-based food in a chemical
and thermal habitat that most animals cannot tolerate (Figure 9).
However, a diverse array of animals has evolved the adapta-
tions necessary to tolerate this extreme habitat and thrive. The
list includes at least 600 species, called vent-endemic species,
that are only known to exist at hydrothermal vents (Desbruyères
et al
. 2006a). Many vent-endemic species have evolved a sym-
biotic, or mutually beneficial, relationship with chemoautotro-
phic bacteria, which allows them to benefit directly from the
energy in hydrothermal vent fluids and reach densities of hun-
dreds to thousands of individuals per square metre. Vent-en-
demic species, along with a limited number of other species
that can tolerate and indirectly benefit from the extreme envi-
ronment, form distinct vent communities.
In addition to the physiological challenges associated with ex-
posure to extreme chemistry and widely varying temperatures,
the vent-endemic species must also evolve adaptations to al-
low them to exploit a habitat that is very patchy and ephemeral,
likely to disappear suddenly. Hydrothermal venting is not con-
tinuous along the plate margins. For example, vent site spacing
along a spreading centre can range from a few kilometres to
hundreds of kilometres. Within a site, sources of venting fluid
Mussels
and snails
Hydrogen Sulphide
HS - +2O
2
SO
4
-
2
CO
2
+R
CO
2
CO
2
Organic
molecules
Animal tissue
Bacteria
Algae
Coral
Food chain
Food chain
Energy
Reduced chemicals
Energy
Sunlight
CHEMOSYNTHESIS
PHOTOSYNTHESIS
Source:TBC
CO
2
+R Organic
molecules
Animal tissue
Energy
Sunlight
Mussels
and snails
Hydrogen Sulphide
HS - +2O
2
SO
4
-
2
CO
2
+R
CO
2
CO
2
Organic
molecules
Animal tissue
Bacteria
Algae
Coral
F od chain
F od chain
Energy
Reduced chemicals
Energy
Sunlight
CHEMOSYNTHE IS
PHOTOSYNTHE IS
Source:TBC
CO
2
+R Organic
molecules
Animal tissue
Energy
Sunlight
Figure 9. Chemoautotrophic symbiotic relationships.
These relationships are similar to the symbiosis between shallow-water reef
corals and their photosynthesizing algae. Like some species of corals, which must be exposed to sunlight to reap the benefits of their
algal partners, vent animals must live exposed to hydrothermal vent fluids in order to benefit from their bacterial symbionts.
