rich in faults constituting a percolation system. The mafic (contraction of magnesium
- ferric, although the iron is actually ferrous) and ultramafic (more Mg, very low
silicate) basalts are subject to serpentinization by seawater moving through the fault
cracks. That is, the water oxidizes the iron to the ferric (+3) state, releasing hydrogen
and substantial heat that drives the hydrothermal flow and dissolves calcium. The
rock left behind is serpentine (which you can look up). The rising water is basic (pH >
10), and when mixed with cool deep-sea water containing carbon dioxide, it deposits
calcium carbonate (both aragonite and calcite) in remarkably lovely white towers. At
least the initial structures are complex matrices of stony tubes conducting flow
through the extending mass (pictures in Kelley et al. 2005). Temperatures in the tube
systems are 40–90°C.
(^) Lost City towers are not inhabited by abundant macrofauna supported by
chemosynthetic bacteria, either free-living or symbiotic with animals. Likely that is
because of the near absence of carbon dioxide and low sulfide levels in the venting
water. There is a surprisingly rich meiofauna in the pores and cracks of the towers,
both actively venting and cold; some larger snails and crustaceans are fairly abundant
on the towers, and non-endemic epibenthos are present in the surrounding area. The
moderately hot, basic water is loaded with molecular hydrogen and contains some
methane that serves as a microbial substrate. The matrix of active Lost City vents
harbors a mixture of microbes dominated by archaea of the order Methanosarcinales,
many of which are methanogens of anoxic habitats and some of which can grow by
reducing CO 2 with H 2 (one form of chemoautotrophy). Some of the archaeal–
bacterial consortium depends upon oxidation of hydrogen and methane for energy,
and a fraction of oxidative metabolism by the microbial consortium is supported by
sulfate reduction, producing some sulfide. Deborah Kelley is confident that more
calcium carbonate tower systems will be found, but, given the rate of new discoveries,
they must be relatively few.
Chemosynthesis
(^) Sulfide-driven chemosynthesis was not newly discovered with the finding of the
deep-sea vents. Chemosynthetic sulfur bacteria have been known since the 19th
century (Winogradsky 1887), and they are particularly active in sediments. Sulfur is
abundant in seawater as the sulfate ion, . Wherever oxygen concentration drops to
very low levels (<0.1 ml liter−1), bacteria oxidizing organic matter will turn to sulfate
and nitrate to obtain oxygen to metabolize organic matter. The products are sulfide,
S2−, and ammonium, NH 4 +. These reduced compounds represent a store of energy, of
chemical potential. When oxygen is again available, chemosynthetic bacteria can
oxidize NH 4 + and S2−, obtaining energy to drive a sequence of carbon-assimilating