AME 53:257-269(2008)  -  doi: 10.3354/ame01248

ABSTRACT: The pathways and fate of C and N in the microbial compartment of a coastal permeable sandy sediment in the photic zone were studied in a pulse-chase experiment. ¹³C-bicarbonate and ¹⁵N-nitrate were added to the water column on top of 5 sediment cores incubated in 4 transparent and 1 opaque laboratory chambers. After 9 h of labelling in the light and dark, stable isotope incorporation by microphytobenthos (MPB) and bacteria was quantified over a period of 4 d through the analysis of phospholipid-derived fatty acids (PLFA) and hydrolysable amino acids (HAA). ¹³C was fixed by MPB, and more than 50% was directed to the production of extracellular polymeric substance (EPS). MPB ¹⁵N incorporation was similar in the dark and in the light. Bacterial activity appeared to have 2 effects that depended on sediment depth: at the sediment surface, there was a steady increase of label in the bacterial PLFA, suggesting metabolism of label-containing EPS; in the subsurface layers, uniform enrichment started immediately after the labelling procedure, indicating continuous incorporation of inorganic C by chemoautotrophic bacteria (Cbac). This experiment demonstrates the efficient transfer of inorganic C and N to the benthic community through the activities of photo- and chemoautotrophic microorganisms, and the role of EPS as a carrier of energy to the benthic microbial food web.

KEY WORDS: Microphytobenthos · Cyanobacteria · EPS · PLFA · Biomarkers · Amino acids · Chemoautotrophy · Permeable sediment · ¹⁵N · ¹³C · Stable isotopes · Nitrate