Evidence of bacterial activity from micrometer-scale layer analyses of black-smoker sulfide structures (Pito Seamount Site, Easter microplate)

We report the observation of an external layer, less than 20 μm thick, attached to hydrothermal chimney fragments of black smokers collected from the Pito Seamount site on the Easter microplate. Scanning electron microscope observations revealed bacterial imprints located on and within the layer, which was associated with underlying layers of thin jarosite deposits and iron sulfides displaying corrosion features. X-ray, electron microprobe, DRIFT and HPLC measurements were used to determine the precise chemical composition and nature of compounds in the layer. The external layer was composed mainly of a combination of hydrated ferric sulfate, ferrous sulfate, iron oxi-hydroxide and elemental sulfur species. A jarosite-like mineral was found deeper in the layer. We suggest that this layer may not be the result of an abiotic thermochemical precipitation, but instead, was formed chemically by bacterial processes. These processes were (1) elemental sulfur oxidation which represents major energetic exchanges within the external layer and (2) bioleaching processes inducing iron oxidation. Originally, elemental sulfur probably resulted from thermochemical precipitation. The results of this study give new insight on the possible role and impact of related acidophilic microorganisms living in hot vent environments of the deep sea.