Bacterial carbon sources in coastal sediments: a cross-system analysis based on stable isotope data of biomarkers

Coastal ecosystems are typically highly productive, and the sediments in these systems receive organic matter from a variety of local and imported sources. To assess if general patterns are present in the origin of carbon sources for sedimentary bacteria and their relation to the origin of the sediment organic carbon pool, we compiled both literature and new data on delta C-13 of bacterial biomarkers ( the phospholipid derived fatty acids i+a15:0), along with delta C-13 data on sediment organic carbon (delta C-13(TOC)) and macrophyte biomass from a variety of typical near-coastal systems. These systems included mangroves, salt marshes (both C3 and C4-dominated sites), seagrass beds, and macroalgae-based systems, as well as unvegetated sediments. First, our delta C-13(i+a15:0) data showed large variability over the entire range of delta C-13(TOC), indicating that in many settings, bacteria may depend on carbon derived from various origins. Secondly, systems where local macrophyte production is the major supplier of organic carbon for in situ decomposition are generally limited to organic carbon-rich, peaty sites (TOC > 10 wt%), which are likely to make up only a small part of the global area of vegetated coastal systems. These carbon-rich sediments also provided a field based estimate of isotopic fractionation between bacterial carbon sources and biomarkers (-3.7 +/- 2.1%), which is similar to the expected value of about -3% associated with the biosynthesis of fatty acids. Thirdly, only in systems with low TOC ( below similar to 1 wt%), we consistently found that bacteria were selectively utilizing an isotopically enriched carbon source, which may be root exudates but more likely is derived from microphytobenthos. In other systems with between similar to 1 and 10 wt% TOC, bacteria appear to show on average little selectivity and delta C-13(i+a15:0) data generally follow the delta C-13(TOC), even in systems where the TOC is a mixture of algal and macrophyte sources that generally are believed to have a very different degradability.