Differences between aerobic and anaerobic degradation of microphytobenthic biofilm-derived organic matter within intertidal sediments

Abstract

Within intertidal sediments, much of the dissolved organic carbon (DOC) consists of carbohydrate-rich extracellular polymeric substances (EPS) produced by microphytobenthic biofilms. EPS are an important source of carbon and energy for aerobic and anaerobic microorganisms owing to burial of microphytobenthos and downward transport of their exudates. We established slurries of estuarine biofilms to determine the fate of organic carbon and EPS fractions, differing in size and complexity, under oxic and anoxic conditions. DOC and hot-water-extracted organic matter (predominately diatom chrysolaminarin) were utilised rapidly at similar rates in both conditions. Concentrations of insoluble, high-molecular-weight EPS were unchanged in oxic microcosms, but were significantly degraded under anoxic conditions (39% degradation by day 25). Methanogenesis and sulphate reduction were major anaerobic processes in the anoxic slurries, and 16S rRNA gene pyrosequencing revealed that Desulfobacteraceae (relative sequence abundance increased from 1.9% to 12.2%) and Desulfobulbaceae (increased from 1.5% to 4.3%) were the main sulphate reducers, whilst Clostridia and Bacteroidetes were likely responsible for anaerobic hydrolysis and fermentation of EPS. We conclude that a diverse consortium of anaerobic microorganisms (including coexisting sulphate reducers and methanogens) degrade both labile and refractory microphytobenthic-derived carbon and that anaerobic degradation may be the primary fate of more structurally complex components of microphytobenthic EPS.