Thioploca spp.: filamentous sulfur bacteria with nitrate vacuoles

Thioploca spp. are multicellular, filamentous, colorless sulfur bacteria inhabiting freshwater and marine sediments. They have elemental sulfur inclusions similar to the phylogenetically closely related Beggiatoa, but in contrast to these they live in bundles surrounded by a common sheath. Vast communities of large Thioploca species live along the Pacific coast of South America and in other upwelling areas of high organic matter sedimentation with bottom waters poor in oxygen and rich in nitrate. Each cell of these thioplocas harbors a large liquid vacuole which is used as a storage for nitrate with a concentration of up to 500 mM. The nitrate is used as an electron acceptor for sulfide oxidation and the bacteria may grow autotrophically or mixotrophically using acetate or other organic molecules as carbon source. The filaments stretch up into the overlying seawater, from which they take up nitrate, and then glide down 5–15 cm deep into the sediment through their sheaths to oxidize sulfide formed by intensive sulfate reduction. New major occurrences have been found in recent years, both in lakes and in the ocean, and have stimulated the interest in these fascinating bacteria.

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