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nature of the vent organisms, most previously unknown to science and many exhibiting
unusual adaptations to the severe, potentially toxic nature of hydrothermal fluids. High
animal density and the presence of unusual species are now known to be common
characteristics of deep-sea hydrothermal vents all over the globe, with the composition of
the fauna varying between sites and regions.
The presence of hydrogen sulphide in hydrothermal fluids and an abundance of
sulphide-oxidizing bacteria were the first clues that led to the development of the
hypothesis whereby biological productivity at hydrothermal vents is sustained not by
photosynthetic products arriving from the sunlit surface ocean, but rather by the
chemosynthesis of organic matter by vent microorganisms 2, using energy from chemical
oxidations to produce organic matter from CO2 and mineral nutrients. Hydrogen
sulphide and other reducing substances present in hydrothermal fluids provide the 'fuel'
for organic matter synthesis (Fig. 1). Since hydrothermal fluids are formed by reaction of
seawater with hot rock, researchers quickly realized that vent ecosystems were
ultimately powered by heat from the earth’s mantle. This was a startling conceptual
challenge to the long held view that all of our planet’s ecosystems require sunlight and
photosynthesis to create new biomass and nourish animal food chains.
Vent ecosystems are not completely independent of sunlight. All animals and
many microorganisms at vents require dissolved oxygen for their metabolism (Fig. 1).
Since dissolved oxygen in the world's oceans is a by-product of photosynthesis, there is a
critical biogeochemical link between the vent ecosystems and the photosynthetic
ecosystems in the upper layers of the sea and on land. Had photosynthesis not evolved
on earth, hydrothermal vents would only be populated by micro-organisms that do not
require dissolved oxygen - such as methanogens that derive energy for growth by
converting hydrogen into methane using CO2 as an oxidant.
2. VENT PRODUCTIVITY
At vent openings, local ecosystems are nourished by microbial growth that is
coupled to the oxidation of H2S, CH4, H2, Fe, Mn and other reducing substances. A
recent thermodynamic modelling study 3 identifies hydrogen sulphide (H2S) as the most
important potential energy source for microbial growth in seafloor hydrothermal
systems. Since both H2S and dissolved oxygen are usually available in the mixing zone
around vent openings, it is not surprising that the most visibly dominant forms of
microbial growth around vent openings are dependent on hydrogen sulphide oxidation.
What is unusual is the diversity of sources and locations of chemosynthetic activity
within the hydrothermal system.
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