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3.1 EPR Vent Ecosystems
A schematic representation of the general appearance of vent ecosystems on the
northern EPR is shown in Figure 3. More vent species are known from sites on the
northern EPR than from any other spreading ridge on the planet. This may in part be due
to the longer history of research in the area, but there are legitimate theoretical
arguments that relate this greater biodiversity to the history of seafloor spreading and the
abundance of vent habitat along the northern EPR 9,10. Chemosynthetic microbial growth
provides the primary production of biomass, and occurs in three distinctly different
habitats: endosymbioses, ectosymbioses, and free-living on animal and mineral surfaces.
Endosymbioses
There are three general models for the functioning of the most common forms of
host-symbiont associations at EPR vents: vestimentiferan worms, clams and mussels.
Microbial symbionts directly nourish their hosts through lysis (digestion of symbionts
within host cells) or through secretion of organic matter that is subsequently absorbed by
host tissues 4.
Type 1 - The most evolved symbiosis occurs in the vestimentiferan tube worms -
which have no mouth or digestive system, and are entirely reliant on their symbiotic
bacteria for nutrition. In the tubeworms the symbiotic bacteria are housed in a
specialized organ known as the trophosome. Substrates for microbial metabolism (HS - ,
CO2, O2, etc) are taken up at the gills and transported to the trophosome by the worm's
blood 4. Physiological and biochemical aspects of this symbiosis have been extensively
studied in the vestimentiferan Riftia pachytila 4.
Type 2 - Vesicomyid clams found at vent sites are filter-feeding animals, but
their digestive tract is highly reduced and experiments have shown that they are unable
to survive without a supply of hydrogen sulphide for their symbionts. They host their
symbionts in the tissue of their large modified gills. CO2 and O2 diffuse directly into the
gills from the external environment, while the clams take up H2S into their blood through
their foot, which they extend into fractures, or sediments where there is diffuse
hydrothermal flow 4. The large size attained by the giant vent clam Calyptogena magnifica
attests to the productive nature of this symbiosis.
Type 3 - Like the clams, the EPR vent mussel Bathymodiolus thermophylus also
houses its symbionts in gill tissue, is a filter feeder and has a functional digestive system.
Unlike the clams, filtration of particles of organic matter from the surrounding water
appears to provide a significant supplement to the mussel's nutrition. When
experimentally moved away from active venting these mussels do survive, although
considerable weight loss occurs. The mussel blood lacks specific proteins for the binding
and transport of sulphide and oxygen that are found in the vestimentifera and clams 4.
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