Limited coupling of macrophyte production and bacterial carbon cycling in the sediments of Zostera spp. meadows

Two approaches were used to study coupling between seagrass production and bacterial processes in the rhizosphere. In the first approach, stable carbo n-isotope ratios of bacteria, sediment organic matter and plants were deter mined to infer sources of organic carbon used by bacteria in the sediments of 4 European Zostera marina and 2 Z. noltii meadows. Bacterial isotope rat ios were derived from bacteria-specific polar lipid-derived fatty acids (PL FA), mainly methyl-branched i15:0 and a15:0. Bacterial delta(13)C ratios in the sediment from both vegetated and unvegetated sites were mostly similar and did not resemble Z. marina ratios, suggesting that seagrass material w as of limited importance as a bacterial carbon source. Bacterial ratios wer e in most cases similar to benthic macroalgae and did correlate well with r atios of diatom biomarkers. Sediment organic matter inside the meadows had delta(13)C ratios similar to those of nearby unvegetated sites, and ratios were clearly different from the material produced by seagrasses, indicating that little seagrass material accumulated. Results from the 2 Z, noltii si tes were less conclusive, as there was no clear difference in delta(13)C ra tios between the potential source materials. In addition, bacterial delta(1 3)C ratios were highly variable at one Z, noltii site. In the second approa ch, cultured Z, marina was labeled with C-13-bicarbonate to study the short -term transfer of label from plants to bacteria in the rhizosphere. However , no label was detected in bacterial PLFA after 20 h of incubation. In conc lusion, a close coupling between macrophyte production and bacterial carbon cycling could not be detected in the sediment of Z. marina meadows, and be nthic production by algae was probably the main carbon source for bacterial growth.