Oxidation of polymetal sulfides by chemolithoautotrophic bacteria from deep‐sea hydrothermal vents

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Abstract
Aerobic mesophilic sulfur‐oxidizing bacteria were tested for their ability to utilize a variety of natural and commercial polymetal sulfides as energy sources at near neutral pH. Substantial fixation of 14CO2 by natural microbial populations covering polymetal sulfide deposits was observed in in situ experiments conducted from DSV ALVIN at the Mid‐Atlantic Ridge hydrothermal vent sites at a depth of 3600 m. In shipboard experiments, similar rates were measured with natural populations on intact or pulverized sulfide minerals. In laboratory experiments with four vent isolates, the rate of 14CO2 incorporation depended on the particular strain and the type of polymetal sulfide substrate used. Vent sample material rich in chalcopyrite (CuFeS2) resulted in higher activities than on sphalerite (ZnS), galena (PbS), or chalcocite (CuS2) alone. Growth on polymetal sulfides, determined as protein, and I4CO2 fixation by strain MA‐3 (the most active of four isolates) were in the range of 5–10% of the corresponding values obtained on thiosulfate as the substrate. While the hydrothermal vent isolates lowered the pH to approximately 4.8 in thiosulfate medium, cultures grown on polymetal sulfides plateaued at higher pH values. Growth of isolates on polymetal sulfides was not enhanced by an addition of thiosulfate. Optimal growth occurred at near neutral pH on both the soluble and insoluble fractions of polymetal sulfides. The massive polymetal sulfide deposits at tectonic ocean spreading centers must be considered long‐term sources of electrons for the chemosynthetic production of biomass in the deep sea.