MEPS 211:27-42 (2001)  -  doi:10.3354/meps211027

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

Rolf Schmaljohann1,*, Manuela Drews1, Sylvia Walter1, Peter Linke2, Ulrich von Rad3, Johannes F. Imhoff1

1Institut für Meereskunde an der Universität Kiel, Marine Mikrobiologie, Düsternbrooker Weg 20, 24105 Kiel, Germany
2GEOMAR Forschungszentrum für Marine Geowissenschaften, Wischhofstr. 1-3, 24148 Kiel, Germany
3Bundesanstalt für Geowissenschaften und Rohstoff, Postfach 510153, 30631 Hannover, Germany

ABSTRACT: Filamentous sulfur-oxidizing bacteria and geochemical parameters of sediments at the Makran accretionary wedge in the northeastern Arabian Sea off Pakistan 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 porewater. White bacterial mats with diameters <1 m were discovered at the surface of these sites using a photo-TV sled. Seep sediments, as well as non-seep sediments, in the vicinity were characterized by the occurrence of the bacterium Thioploca in near-surface layers between 0 and 13 cm depth. Thioploca bundles were up to 20 mm in length and contained up to 20 filaments of varying diameters, between 3 and 75 µm. Up to 169 ind. cm-2 were counted. Maximum numbers occurred in the top 9 cm of sediment, which contained very low concentrations of soluble sulfide (<0.2 µM) and high amounts of elemental sulfur (up to 10 µmol cm-3). Moderate sulfate reduction activity (between 20 and 190 nmol cm-3 d-1) was detected in the top 10 cm of these sediments, resulting in a gradual downcore decrease of sulfate concentrations. CO2 fixation rates had distinct maxima at the sediment surface and declined to background values below 5 cm depth. The nutritional implications of the distinct morphology of Thioploca and of the geochemical setting are discussed and compared to other sites containing Thioploca communities.

KEY WORDS: Thioploca · Beggiatoa · Arabian Sea · Cold seeps · Oxygen-minimum zone sediments · Filamentous sulfur-oxidizing bacteria · Sulfate reduction rates · CO2 fixation rates

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