THE SHALLOW SUBSURFACE AT DEEP-SEA HYDROTHERMAL VENTS: A PRIME HABITAT FOR NOVEL SULFIDE-OXIDIZERS?

We have recently described a chemoautotrophic, sulfide-oxidizing marine bacterium unique among prokaroytes in that it excretes elemental sulfur in the form of long, irregular filaments. These microaerophilic microorganisms are retained in sulfidic, high-fluid flow environments by attachment to and entrainment in the produced filamentous sulfur flocs. Based on its 16S rDNA sequence and FISH with genus and species specific probes, the organism has been identified as an Arcobacter sp., tentatively named Candidatus Arcobacter sulfidicus. Examination of material previously collected from a 1991 "snowblower" event at the 9° N vent site revealed filamentous sulfur morphologically and chemically identical to that produced by the coastal microbe. On follow up cruises the rapid in situ microbial formation of filamentous sulfur mats (3 cm thick in 10 to 11 days) was documented from warm (20-40°C) subsurface discharges at 9° N. These observations and those of similar discharges at a number of vent sites indicate that a shallow non-thermophilic subsurface biosphere dominated by these sulfide oxidizers might be an important component heretofore unconsidered in the overall organic matter production at hydrothermal vents. The microaerophilic growth of this bacterium and its preference for high sulfide concentrations, make this microbe ideally adapted to the conditions created by the mixing of reduced hydrothermal fluid with oxygenated seawater in the shallow subsurface at vents. In addition, it is capable of nitrogen fixation. Interestingly, the described organism does not seem to use the Calvin-Bassham-Benson cycle for CO2 assimilation, which means that a significant portion of the carbon produced de novo at these sites might actually originate from an alternative carbon fixation pathway. This may have important implications for primary production assessments and carbon isotope analyses conducted on hydrothermal vent samples.