Carbon cycling and bacterial carbon sources in pristine and impacted Mediterranean seagrass sediments

Stable carbon-isotope ratios of bacterial biomarkers were studied in Mediterranean seagrass meadows and macroalgae communities to identify the sources of organic carbon used by the sediment bacteria. Bacteria delta(13)C ratios in pristine sediments vegetated by the seagrass Posidonia oceanica were either similar to the seagrass signal or slightly enriched, suggesting that seagrass detritus was of major importance as a bacterial carbon source. There was a shift in bacterial carbon sources in anthropogenic impacted P. oceanica meadows towards seston and macroalgae. The net primary productivity was reduced in these meadows, whereas the rates of mineralization as measured by sulfate reduction rates were enhanced in the sediments. This effect on mineralization was probably due to the input of less refractory organic matter compared to seagrass detritus, which enhances the bacterial decomposition of organic matter. In the fast growing seagrass Cymodocea nodosa meadow, the bacterial carbon sources consisted of a mixture of seagrass detritus and seston, and the mineralization rates were much higher compared to the P. oceanica meadows, indicating that these carbon sources were more labile and easily decomposed by the bacteria. A similar pattern was found in the macroalgae bed with Caulerpa prolifera, where the rates of mineralization were high, similar to findings in organic impacted fish farm sediments. Here C. prolifera detritus was the most important carbon source and accounted for an increase in sediment organic content. The possible impacts of a shift in bacterial carbon sources due to nutrient loading are discussed in relation to the performance of P. oceanica in carbonate sediments.