The contribution of macrophyte-derived organic matter to microbial biomassin salt-marsh sediments: Stable carbon isotope analysis of microbial biomarkers

Stable carbon isotope ratios of bacterial biomarkers were determined to inf er sources of organic carbon used by bacteria in the sediments of three sal t marshes. Biomarkers studied were polar lipid-derived fatty acids (PLFA), mainly bacteria-specific, methyl-branched i15:0 and a15:0. Experiments show ed that isotopic fractionation between substrate and biomarkers was relativ ely constant (-4 to -6 parts per thousand, on average) compared to the wide range in C-13/C-12 ratios of carbon sources found in the studied marshes. At the Spartina site of the Waarde Marsh (The Netherlands), biomarker C-13/ C-12 ratios were depleted by approximately 6 parts per thousand more than e xpected for bacteria growing on Spartina litter and were similar to an unve getated control sediment. This pattern suggested that local macrophyte prod uction was of little importance and that other material (probably of algal origin) was the dominant carbon source for bacterial growth. Spartina contr ibuted about half of the carbon in bacterial PLEA at the Kattendijke Marsh (The Netherlands) and dominated at the Great Marshes (U.S.). The variation in bacterial carbon sources in these marshes was probably related to estima ted inputs of nonmacrophyte organic matter to the sediment. At the Waarde M arsh, a clear plant species effect was found as coupling between plant and bacteria was more important in Scirpus maritimus than in Spartina anglica. The contribution of local plant production to bacterial biomass in salt-mar sh sediments is highly variable between marshes and depends on the input of nonmacrophyte material by sedimentation in comparison to local plant input , which in turn may differ among plant species.