Different types of diatom-derived extracellular polymeric substances (EPS) drive changes in heterotrophic bacterial communities from intertidal sediments

Microphytobenthic (MPB) diatoms exude large quantities of extracellular polymeric substances (EPS) comprising polysaccharides, glycoproteins and other biopolymers, which represent a substantial carbon pool. However, degradation rates of different EPS components, and how they shape heterotrophic communities in sediments, are not well understood. An aerobic mudflat-sediment slurry experiment was performed with two different carbon sources from a diatom-dominated biofilm: colloidal EPS and the more complex hot-bicarbonate-extracted EPS. Degradation rate constants over 9 days for three sediment fractions (dissolved organic carbon , total carbohydrates and colloidal EPS) were generally higher in the colloidal-EPS slurries (0.105 – 0.123 d-1) compared with the hot-bicarbonate-extracted-EPS slurries (0.060 – 0.096 d-1). The hot-bicarbonate-EPS addition resulted in large increases in dissolved nitrogen and phosphorous, indicating that complex EPS is an important source of regenerated inorganic nutrients. Addition of both types of EPS greatly altered the bacterial community composition, with Bacteroidetes increasing in abundance. There were also significant differences between the two types of EPS treatment; specifically Verrrucomicrobia were enriched only by colloidal EPS. These differential effects of EPS fractions on carbon-loss rates, nutrient regeneration and microbial community assembly improve our understanding of coastal-sediment carbon cycling and demonstrate the importance of diverse microbiota in processing this abundant pool of organic carbon.