FORAMINIFERAL COLONIZATION OF HYDROCARBON-SEEP BACTERIAL MATS AND UNDERLYING SEDIMENT, GULF-OF-MEXICO SLOPE

Around bathyal hydrocarbon seeps of Green Canyon, Gulf of Mexico, a co mmunity of foraminifera has colonized mats of Beggiatoa, a chemolithot rophic, sulfide-oxidizing bacterium. The O-2-H2S boundary zone in thes e mats is likely to be within 1-2 mm of the sediment-water interface. Judging by Rose-Bengal staining, small populations of some foraminifer al species survive under the Beggiatoa mats, down to a substrate depth of at least 2 or 3 cm, where the sediment is black and the pore fluid is sulfidic, These species, including Bolivina albatrossi, Bolivina o rdinaria, Cassidulina neocarinata, Gavelinopsis translucens, Osangular ia rugosa, and Trifarina bradyi, are microaerophiles or facultative an aerobes, The subsurface distributions and densities of Bolivina albatr ossi suggest that it is the most tolerant of anoxia and sulfide toxici ty. Transmission electron microscopy of 12 individuals of diverse spec ies failed to confirm the presence of live foraminifera under the bact erial mats, The cytoplasm, when present, was degraded, although the st ructure of some organelles suggests that some of the individuals had d ied only recently, Initial stable-isotope analyses of empty tests of t hree species (Uvigerina peregrina, Bolivina subaenariensis, and Lentic ulina sp.) from previously studied seep sites show anomalously negativ e and wide-ranging delta(13)C values (-1.3 parts per thousand to -3.6 parts per thousand PDB) that are typical of seep CO2 plumes, but not o f associated sediment-pore fluids, However, the carbon-isotopic signat ure in the dominant species (especially in Bolivina albatrossi) living under Beggiatoa mats in deep-bathyal Green Canyon is presently unknow n, Thus, whether species of foraminifera invading the anoxic layers un der Beggiatoa mats can construct tests in these microhabitats remains unresolved.