Minerals Report - International Seabed Authority

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
tubeworms - 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 blood4 .
Physiological and biochemical aspects of this symbiosis have been extensively
studied in the vestimentiferan Riftia pachytila4 .
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 flow4 .
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
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