Stable isotope evidence for alternative bacterial carbon sources in the Gulf of Mexico

Abstract

In temperate coastal waters, it is generally assumed that carbon cycling is primarily supported by phytoplankton production, having δ13C values ranging from −22 to −18‰. In a transect leading out from the Mississippi River, riverine and seawater δ13C endmembers of particulate organic matter have previously been measured at −25.5 and −20.0‰, respectively. In addition, δ13C values of dissolved organic carbon in the northern Gulf of Mexico range from −24.7 to −19.6‰, with the more 13C‐depleted values from fresher waters. Assumptions about coastal transport of dissolved organic matter predict that the bacterial δ13C values should fall along the conservative salinity mixing line between terrestrial and marine carbon sources. However, in the field survey presented here, δ13C values of bacteria in coastal regions of the Gulf of Mexico adjacent to the Mississippi River are considerably 13C‐depleted, with values as low as −33‰. These isotope values suggest that carbon from sources other than phytoplankton production or terrestrial organic matter are supporting the production of the bacterial assemblage. Possibilities include the incorporation of carbon derived from light hydrocarbons from seep areas and the chemoautotrophic processes of methane oxidation and nitrification. These 13C‐depleted stable isotope data are evidence that bacterially assimilated carbon in the northern Gulf of Mexico may be seasonally uncoupled to surface phytoplankton production.