Determination of the trophic mode of Beggiatoa spp. found at hydrocarbon seeps on the continental slope of the northern Gulf of Mexico

Abstract

Pigmented and non-pigmented bacterial mats, which consist of filamentous, sulfur oxidizing Beggiatoa, are prevalent at hydrocarbon seeps located on the continental slope of the northern Gulf of Mexico. Samples of both filament types were collected via the slurp mechanism of the DSV Johnson Sea Link. Microscopic analysis was used to determine filament width and morphology. C02 incorporation experiments, RuBisCo assays, and DNA extraction, amplification, and probing experiments were used to determine the trophic mode of the filaments. A non-pigmented sample that contained only one filament width class represented, most likely, a single species. The wide range of filament widths found in a second non-pigmented sample and one pigmented sample indicated the presence of more than one species. Very wide filaments observed in both pigmented and nonpigmented samples may represent a species endemic to hydrocarbon seeps and hydrothermal vents. Microscopic analysis revealed pigmented and non-pigmented filaments to have rounded ends, cells which appear hollow, and other characteristic traits of Beggiatoa. Sulfur granules in both pigmented and non-pigmented cells indicated the capacity for H2S oxidation. Incorporation Of C02 by whole, live cells, and RuBisCo assays using cell free extract, showed non-pigmented filaments to be capable of significant C02 fixation. This ability to use C02 as the primary carbon source along with the ability to oxidize H2S for energy, categorizes the non-pigmented filaments as chemoautotrophic. Pigmented filaments were shown to possess little C02 incorporation ability. Their consequent dependence on organic compounds as a carbon source precludes chemoautotrophic capability. Whether they are mixotrophic or heterotrophic depends on the source of energy used and was not clearly determined in this study. PCR using primers from the large subunit of the RuBisCo gene from Zea showed possible amplification of the RuBisCo gene in pigmented and non-pigmented Beggiatoa DNA. However, the non-pigmented DNA did not hybridize with a Pisum sativum probe. The geochemical nature of hydrocarbon seeps creates an environment capable of supporting a Beggiatoa population functioning in three distinct trophic modes: chemoautotrophic, mixotrophic, and heterotrophic.