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2.1 Symbiotic and free-living chemosynthesis
Seafloor vents
At vent openings, some chemosynthetic microorganisms live in symbiotic
associations within the tissues of giant worms and bivalves 4, converting CO2 into organic
matter to nourish themselves and their hosts. Others grow freely as biofilms and
filamentous mats on mineral and animal surfaces, providing food for grazing and
deposit feeding animals. Predators and scavengers - some vent specialists, others
attracted from the surrounding deep sea - complete the food web (Fig. 2). It is likely that
at least some biological productivity from seafloor vents is exported to the surrounding
deep sea through predation and advection of organic detritus but this food pathway
remains unquantified (Fig. 2).
Other hydrothermal habitats
Microbial chemosynthesis also occurs in subsurface hydrothermal conduits 5, and
in the plumes 6 that overlie vent fields, but faunal exploitation of microbial biomass
produced in these two environments has not been extensively examined. Interaction
between biological and geochemical processes has been most clearly documented in
hydrothermal plumes. For example, recent studies show that oxidation of dissolved
manganese in hydrothermal plumes is mostly microbially mediated 6,7. High
concentrations of zooplankton at the upper boundary of hydrothermal plumes in the
northeast Pacific indicate that plume productivity may feed plankton food chains in the
water column 8; this remains to be quantified.
3. VENT ECOSYSTEMS
Mid-ocean ridges reveal an astonishing diversity of styles of seafloor spreading
and associated geology and biology. One of the key variables that affect spreading style
is the plate divergence rate: slow spreading ridges have very different characteristics
from fast spreading ridges. Readers are referred to a theoretical paper 9 for detailed
consideration of the relation between seafloor spreading rate, vent habitat frequency and
vent faunal diversity. This section will briefly compare vent ecosystems on two parts of
the ridge system that have formed the basis of much of our understanding: the East
Pacific Rise (EPR), between 8° 30’ N and 13° 30’ N, which is spreading at approximately
100 mm/yr; and the Mid-Atlantic Ridge (MAR) between 33° N and 38° N, which is
spreading at about a quarter of that rate.
104 International Seabed Authority