DEEP SEA MINERALS - Vol 1 - Sea-Floor Massive Sulphides

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2.1

Habitats and biodiversity associated with sea-floor massive sulphide deposits

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

Energy

PHOTOSYNTHESIS PHOTOSYNTHE IS

CHEMOSYNTHESIS CHEMOSYNTHE IS

Food chain

F od chain

Sunlight

Algae

CO 2

Coral

Sunlight

Energy

CO 2

+R Organic

molecules +R Organic

molecules

CO 2

- 2 SO 4

- 2

HS - +2O 2

SO 4

Food chain

F od chain

Animal tissue

Energy

Organic molecules

CO 2

Animal tissue

Mussels and snails

Bacteria

Hydrogen Sulphide Hydrogen Sulphide

Source:TBC

Reduced chemicals Reduced chemicals

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.

THE GEOLOGY OF SEA-FLOOR MASSIVE SULPHIDES 20

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