Microphytobenthic extracellular polymeric substances (EPS) in intertidal sediments fuel both generalist and specialist EPS‐degrading bacteria

Abstract

Microphytobenthic biofilms contain high concentrations of carbohydrate‐rich extracellular polymeric substances (EPS) that are important in sediment carbon cycling. Field measurements at two locations in the Colne Estuary, U.K., showed that a significant curvilinear relationship explained 50% of the variability in chlorophyll a and EPS content. Estimates of EPS production, based on field data and published rates of production by diatoms, revealed that EPS turnover of 52% to 369% over the tidal cycle was required to account for field standing stocks. We investigated EPS degradation in sediment slurries using purified 13C‐EPS produced by the diatom Nitzschia tubicola. Although EPS constituted only 5% of the sediment dissolved organic carbon (DOC) pool, 100% of the added EPS was utilized within 30 h, before decreases in other sediment‐carbohydrate fractions and DOC concentrations. A general 13C enrichment of phospholipid fatty acids (PLFAs), representative of Gram‐positive and Gram‐negative bacteria, occurred within 6 h, with the PLFAs a15:0, i15:0, and 18:1ω7c being highly enriched. The diatom PLFA 20:5ω3 had relatively low but significant 13C enrichment. Stable isotope probing of 16S ribosomal ribonucleic acid (RNA‐SIP) at 30 h revealed 13C‐enriched sequences from the diatom genus Navicula; further evidence that diatoms assimilated the EPS, or EPS‐breakdown products, from other diatom taxa. RNA‐SIP also demonstrated a diverse range of highly 13C‐enriched bacterial taxa, including a distinct subset (Alphaproteobacteria and Gammaproteobacteria) found only in the heavily labeled microbial assemblages. Thus, cycling of diatom EPS is rapid, and involves a wide range of microbial taxa, including some apparent specialists.