The physiology and biochemistry of many taxa inhabiting deep-sea hydrothermal vents have been elucidated; however, the physicochemical factors controlling the distribution of these organisms at a given vent site remain an enigma after 20 years of research. The chemical speciation of particular elements has been suggested as key to controlling biological community structure in these extreme aquatic environments. Implementation of electrochemical technology has allowed us to make in situ measurements of chemical speciation at vents located at the East Pacific Rise (9 degrees 50' N) and on a scale relevant to the biology. Here we report that significant differences in oxygen, iron and sulphur speciation strongly correlate with the distribution of specific taxa in different microhabitats. In higher temperature (> 30 degrees C) microhabitats, the appreciable formation of soluble iron-sulphide molecular clusters markedly reduces the availability of free H2S/HS- to vent (micro)organisms, thus controlling the available habitat.

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Sulfur speciation was determined in real time in salt marsh microbial mats, subtidal sediments and hydrothermal vent diffuse flow waters using solid state gold-amalgam voltammetric microelectrodes. Chemical species were measured in situ without any sample manipulation or processing. The partially oxidized sulfur species detected were polysulfides, thiosulfate, elemental sulfur and tetrathionate. Fe(III) oxidation of hydrogen sulfide does not occur within the mats where microbially mediated processes are responsible for oxidation of H2S. In sediments and diffuse flow vent waters, Fe(III) phases are the direct oxidant of H2S. Sulfur speciation determined in this work is due to in situ biogeochemical processes and is not due to artefacts of sample manipulation. The voltammetric data show that polysulfides are the first detectable intermediate during sulfide oxidation which is consistent with previous laboratory studies.

Saline and warm Mediterranean water flowing through the Bosporus Strait maintains a permanent pycnocline with vertical separation of oxic (O 2 ), suboxic (absence of O 2 and H 2 S), and anoxic (H 2 S) zones in the Black Sea. The stable suboxic zone implies restricted vertical mixing of the upper oxic and lower anoxic layers and limited vertical flux of oxygen that cannot balance the upward flux of sulfide. We report data that directly confirm massive lateral injections (Ͼ200 km from the Bosporus) of oxygen-enriched waters of the Bosporus plume, created by the mixing of shallow, cold, intermediate-layer Black Sea water with Mediterranean water. These plume waters are laterally injected into the oxic layer and, more importantly, into the suboxic and anoxic layers over several small vertical scales (''fingers'' of ϳ5 m) at water densities ( t ) from 15.0 to 16.4. O 2 injection oxidizes Mn(II) to Mn(III,IV), which then oxidizes H 2 S. The onset of H 2 S detection occurs in deeper waters in the southwest (Ͼ170 m; t ഠ 16.4) relative to the west central Black Sea (110 m; t ഠ 16.2) and coincides with increased MnO 2 and S 8 formation in the southwest.

Solid-state microelectrodes have been used in situ in Raritan Bay, NJ to measure pore water profiles of dissolved O2, Mn, Fe, and sulfide at (sub)millimeter resolution by voltammetric techniques. The voltammetric sensor was positioned with microprofiling instrumentation mounted on a small remote operated vehicle (ROV). This instrumentation and the sensor were controlled and monitored in real time from a research vessel anchored at the study site. The voltammetric analyzer was connected to the electrodes of the voltammetric cell with a 30 m cable which also bridged receiver-transmitter transducers to ensure signal quality along the cable. Single analyte O2, pH, and resistivity microsensors were operated alongside the voltammetric sensor. We report on the technology of the system and the concentration changes of redox species observed from 2 to 3 cm above to approximately 4 cm below the sediment−water interface during three deployments. O2 measurements from both Clark and voltammetric electrodes were in excellent agreement. The profiles obtained show that there is no detectable overlap of O2 and Mn2+ in the sediments which is similar to previous reports from other continental margin sediments which were cored and analyzed in the laboratory. These data indicate that O2 is not a direct oxidant for Mn2+ when diffusive (rather than advective) processes control the transport of solutes within the sediment. Subsurface Mn2+ peaks were observed at about 2 cm and coincide with a subsurface pH maximum. The data can be explained by organic matter decomposition with alternate electron acceptors and by the formation of authigenic phases containing reduced Mn at depth.