Microbial carbon sources on the shelf and slope of the northwestern Gulf of Mexico

Over the past five years, gas chromatography/combustion/isotope ratio mass spectrometry (GC/C/IRMS) analyses of phospholipid fatty acids (PLFAs) has been increasingly used to link organic matter (OM) sources with sedimentary bacteria. This technique has been applied across diverse estuarine and coastal sediments, including lower Laguna Madre, TX, an oligotrophic, coastal lagoon dominated by a single OM source, seagrasses; shelf stations, a eutrophic coastal region receiving multiple sources of OM, hypoxic regions that occur seasonally and deep slope and abyssal plain sediments of the Gulf of Mexico (GOM). Previous reports using the Laguna Madre data as examples, have been used to make comparisons of PLFA 16:0 and PLFA 15:0 isotope ratios and PLFA 16:0 and total organic carbon isotope ratios. Deviations from the 1:1 line in the former indicate living or recently senescent sources of organic matter are not predominantly bacterial. Deviations from the 1:1 line in the latter indicate living or recently senescent sources of organic matter differ isotopically from detrital or older OM in sediments. Prior to the work of Goni et al. (1998), carbon isotope ratios of OM in GOM sediments were interpreted as marine in origin. Based on a series of geochemical measurements, Goni et al. suggested that GOM sediments are largely composed of terrestrial organic carbon (OCterr). Furthermore, They went on to show that shelf and slope sediments were primarily C3 and C4 respectively. I report on the preferential utilization of autochthonous OM by sedimentary bacteria at the sediment surface and the shift to recalcitrant, terrestrially derived OM with depth.