Oxygen-minimum zone sediments in the northeastern Arabian Sea off Pakistan: a habitat for the bacterium Thioploca

Filamentous sulfur-oxidizing bacteria and geochemical parameters of sedimen ts at the Makran accretionary wedge in the northeastern Arabian Sea off Pak istan were studied. The upper continental slope between 350 and 850 m water depth, which is in the center of the oxygen-minimum zone, is characterized by numerous sites of small-scale seeps of methane- and sulfide-charged por ewater. White bacterial mats with diameters <1 m were discovered at the sur face of these sites using a photo-TV sled. Seep sediments, as well as non-s eep sediments, in the vicinity were characterized by the occurrence of the bacterium Thioploca in near-surface layers between 0 and 13 cm depth. Thiop loca bundles were up to 20 mm in length and contained up to 20 filaments of varying diameters, between 3 and 75 <mu>m. Up to 169 ind, cm(-2) were coun ted. Maximum numbers occurred in the top 9 cm of sediment, which contained very low concentrations of soluble sulfide (<0.2 <mu>M) and high amounts of elemental sulfur (up to 10 mu mol cm(-3)). Moderate sulfate reduction acti vity (between 20 and 190 nmol cm(-3) d(-1)) was detected in the top 10 cm o f these sediments, resulting in a gradual downcore decrease of sulfate conc entrations. CO2 fixation rates had distinct maxima at the sediment surface and declined to background values below 5 cm depth. The nutritional implica tions of the distinct morphology of Thioploca and of the geochemical settin g are discussed and compared to other sites containing Thioploca communitie s.