Distinct pigmentation and trophic modes in Beggiatoa from hydrocarbon seeps in the Gulf of Mexico

Abstract

Bacterial mats, which are comprised of spatially distinct, pigmented and non-pigmented filamentous Beggiatoa, are abundant at hydrocarbon seeps on the continental slope of the northern Gulf of Mexico. Samples of both filament types were collected, using the submarine 'Johnson Sea Link', from seeps at water depths of ~550 m. The water-soluble pigment of colored strains was inter- nal to the cells and had an absorbance peak of approximately 390 nm. Sulfur granules in both pig- mented and non-pigmented cells indicated that these Beggiatoa had the capability of oxidizing H2S. Non-pigmented filaments were capable of significant CO2 fixation based on incorporation of CO2 by whole, live cells and ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) assays using cell-free extracts. RuBisCO activities for extracts from non-pigmented cells ranged from 9.92 to 135.35 nmol CO2 fixed mg protein -1 min -1 . Activities varied significantly with temperature and pH. This ability to use CO2 as the primary carbon source, along with the ability to oxidize H2S for energy, suggests that these non-pigmented filaments were chemoautotrophic. Pigmented filaments, in contrast, had little CO2 incorporation ability. RuBisCO activities from pigmented mats ranged from 0.17 to 0.92 nmol CO2 fixed mg protein -1 min -1 . These results suggest that geochemical processes at hydrocarbon seeps create an environment capable of supporting separate chemoautotrophic and heterotrophic (presumably organo-heterotrophic) Beggiatoa populations.