CONCENTRATION AND TRANSPORT OF NITRATE BY THE MAT-FORMING SULFUR BACTERIUM THIOPLOCA

MARINE species of Thioploca occur over 3,000 km along the continental shelf off Southern Peru and North and Central Chile(1-4). These filame ntous bacteria live in bundles surrounded by a common sheath and form thick mats on the sea floor under the oxygen-minimum zone in the upwel ling region, at between 40 and 280 m water depth. The metabolism of th is marine bacterium(5,6) remained a mystery until long after its disco very(1,7). We report here that Thioploca cells are able to concentrate nitrate to up to 500 mM in a liquid vacuole that occupies >80% of the cell volume. Gliding filaments transport this nitrate 5-10 cm down in to the sediment and reduce it, with concomitant oxidation of hydrogen sulphide, thereby coupling the nitrogen and sulphur cycles in the sedi ment.