Heavy metal concentrations in fauna from hydrothermal fields of the Mid-Atlantic Ridge

Distributions of Fe, Mn, Zn, Cu, Ni, Co, Cr, Pb, As, Ag, Cd, Se, Sb, and Hg in 128 samples of tissues of organisms that inhabit hydrothermal vent fields of the Mid-Atlantic Ridge (Menez Gwen, Snake Pit, and Rainbow) depending on the abiotic environmental parameters were studied. The majority of the elements studied showed direct correlations between their concentrations in fluids released and in tissues of hydrothermal organisms. A higher degree of bioaccumulation of metals was revealed in Bathymodiolus mussels and Rimicaris shrimps from the Rainbow hydrothermal vent field as compared to their analogues from the Menez Gwen and Snake Pit fields. This corresponds to maximal concentrations of the majority of the metals studied in the Rainbow high-temperature hydrothermal fluids. The highest degree of bioaccumulation of heavy metals was found in gills of symbiotrophic mussels Bathymodiolus and in maxillipeds of ectosymbiotic shrimps Rimicaris, i.e., in organs that function in dependence on chemosynthetic bacteria. Within the Rainbow vent field, the shrimps, which inhabit in biotopes with more high-temperature conditions and therefore are more strongly subjected to influence of fluids, were found to contain higher metal contents than mollusks. Fe-Mn hydroxide films that cover mussel shells might serve as important reservoirs of other metals related to Fe and Mn.

In situ measurements of hydrogen sulfide, oxygen, and temperature in diffuse fluids of the ultramafic-hosted Logatchev hydrothermal vent field (Mid-Atlantic Ridge)

The Logatchev hydrothermal vent field (14°45'N, Mid-Atlantic Ridge) is located in a ridge segment characterized by mantle-derived ultramafic outcrops. Compared to basalt-hosted vents, Logatchev high temperature fluids are relatively low in sulfide indicating that the diffuse, low temperature fluids of this vent field may not contain sufficient sulfide concentrations to support a chemosymbiotic invertebrate community. However, the high abundances of bathymodiolin mussels with bacterial symbionts related to free-living sulfur oxidizing bacteria suggested that bioavailable sulfide is present at Logatchev. To clarify if diffuse fluids above mussel beds of Bathymodiolus puteoserpentis provide the reductants and oxidants needed by their symbionts for aerobic sulfide oxidation, in situ microsensor measurements of dissolved hydrogen sulfide and oxygen were combined with simultaneous temperature measurements. High temporal fluctuations of all three parameters were measured above the mussel beds. H2S and O2 co-existed with mean concentrations between 9-31 µM (H2S) and 216-228 µM (O2). Temperature maxima (<= 7.4°C) were generally concurrent with H2S maxima (<= 156 µM) and O2 minima (>= 142 µM). Long-term measurements for 250 days using temperature as a proxy for oxygen and sulfide concentrations indicated that the mussels were neither oxygen- nor sulfide-limited. Our in situ measurements at Logatchev indicate that sulfide may also be bioavailable in diffuse fluids from other ultramafic-hosted vents along slow- and ultraslow-spreading ridges.