Mats of giant sulphur bacteria on deep-sea sediments due to fluctuating hydrothermal flow

FILAMENTOUS sulphide-oxidizing bacteria, Beggiatoa spp., com-monly grow as submillimetre-thin white films on anoxic marine sediments. Unusually thick mats (>1 cm) of giant Beggiatoa filaments, 41–120 μm wide and 2–10 mm long, were observed at 2,000 m water depth in the hydrothermal vent fields of Guaymas Basin, Gulf of California 1–4 . We investigated how such dense communities of the largest known bacteria overcome severe diffusion limitation of their substrate supply, and what advantage they may have by developing such large cell sizes. Oxygen, sulphide, pH and temperature were therefore measured in Beggiatoa mats directly on the sea floor. We report here the discovery of small-scale hydrothermal fluid circulations around patches of the bacteria, causing a pulsatory seawater flow into the mats and thereby enhancing the supply of oxygen and sulphide to the bacteria.