LARKIN, JOHN, Department of Microbiology, Louisiana State Univ., Baton Rouge, LA; PAUL AHARON, Department of Geology and Geophysics, Louisiana State Univ., Baton Rouge, LA; and J. CORBETT and W. STEELE, Department of Microbiology, Louisiana State University, Baton Rouge, LA

ABSTRACT: Beggiatoa in Microbial Mats at Hydrocarbon Seeps in the Gulf of Mexico

Microbial mats have been collected from a variety of sites near hydrocarbon seeps along the slope in the Gulf of Mexico over a period of four years. A predominant microorganism in each of the mats is the giant bacterium Beggiatoa. They are located at the sediment-water interface where they form a boundary layer between the underlying anaerobic sediment and the overlying aerobic water column. In several locations, the Beggiatoa populations were dominated by filaments of only a single diameter, providing nearly a monoculture. At other sites there were from two to four size-species of Beggiatoa. At monoculture sites the range of sizes was from a low of about 6 micrometers in diameter, to a high of about 30 micrometers. At other sites the diameters ranged from about 10 micrometers to appr ximately 200 micrometers. The latter organisms are the largest procaryotic organisms yet found. In electron micrographs, it was found that all filaments over about 10 micrometers in diameter contained a large central vacuole, which effectively decreased the cytoplasmic volume of each cell. In addition, it was found that the beggiatoas from some sites were pigmented, either shades of yellow, orange, or green. Pigmentation correlated with the nature of the site. In general, the white-gray mats occur at the periphery of the seepage sites containing crude oil, whereas the yellow-orange pigmentation occurs at the center. Determinations of (isotope){13}C in Beggiatoa mats yield values in the range -27.2 to 27.9 o/oo suggesting they metabolize CO2 derived from bacterial sulfate reduction of cru e oil. The presence of S* (elemental sulfur) within cells of Beggiatoa resulting from oxidation of H2S support the importance of bacterial sulfate reduction processes in the underlying seeps for the sustenance of the Beggiatoa mats. The beggiatoas have been kept alive in the laboratory for long periods of time, but have not yet been obtained in

pure culture. It appears from studies of their nutrition that they may be chemoautotrophic.

AAPG Search and Discovery Article #90987©1993 AAPG Annual Convention, New Orleans, Louisiana, April 25-28, 1993.