reactions producing organic matter, exactly the same reactions as the light-
independent reactions of photosynthesis mediated by the Calvin–Benson cycle and by
other enzymes. Plants use light to reduce NAD+ to NADH and to generate ATP.
Chemosynthetic bacteria use energy from oxidizing ammonium or sulfide with
oxygen to carry out these reactions. Their enzyme, called ATP sulfurylase, mediates
production of ATP from sulfide oxidation.
(^) It is the simultaneous requirement for oxygen and sulfide that determines which
sites will support sulfide chemosynthesis, including sites other than vents. In
sediment, for example, these two solutes are available at the interface between anoxic
(deeper) and oxic (shallower) layers, usually a rather narrow zone. In some organic-
rich salt marshes this interface may be almost exactly at the sediment surface, where
sulfur bacteria can form a pink scum. Near deep-sea vents, the zones sustaining such
chemosynthesis are where the sulfide-rich vent water mixes with the relatively
oxygen-rich deep-sea water. Oxidation of sulfide is thermodynamically favorable with
a very low energy barrier, and it proceeds spontaneously in oxic solution. That makes
the zones small where both oxygen and sulfide are at sufficient concentrations for
chemosynthesis.
(^) The basic reaction is:
(^) producing −790 kJ mol−1 (free energy of formation).
(^) Only bacteria perform this as a biochemical feat, capturing a fraction of the energy
for chemosynthesis. So, for animals to take advantage, they must either graze on those
bacteria right at the interface (or as they are swept away from it), or they must harbor
the bacteria symbiotically in or on their bodies. In the case of vent animals, symbioses
have produced the most dramatic life forms. A few modes of symbiosis, those of some
“charismatic” invertebrates, are reviewed below.
(^) In addition to sulfide-based chemoautotrophs, bacterial varieties have been found in
hydrothermal vents that utilize other reduced species (often referred to as electron
donors because they lose electrons as they are oxidized in energy-yielding reactions),
particularly elemental hydrogen, H 2 :
(^) There are also bacteria, “methanotrophs”, or “methylophiles”, which oxidize
methane, obtaining energy from:
(^) and converting the carbon to complex organic matter. Both hydrogen and methane are
dissolved in vent effluent. Different bacteria in vent plumes and deposits derive