Surface adhesion of microphytobenthic biofilms is enhanced under Hediste diversicolor (O.F. Müller) trophic pressure

In soft-bottom tidal flats, sediment stability is one of the crucial parameters modulating the abundance and composition of benthic assemblages. It is dependent on a wide range of variables, both abiotic and biotic. Investigating how these variables and their interactions influence sediment stability is therefore essential to understand how benthic assemblages are distributed in their environment. In this context, we designed a microcosm study to examine how microorganisms and macrofauna interact to alter sediment stability. We cultured a natural microbial community, enriched with diatoms, both alone and together with the common ragworm Hediste diversicolor, and monitored their effects on photosynthetic biomasses, bacterial abundances, exopolymer secretions and sediment stability. We also assessed the consumption of biofilm by worms using fatty acid biomarkers. Our results demonstrate that even if H. diversicolor fed on diatoms, they stimulated biofilm development, in terms of photosynthetic biomass and exopolymer production. Also, sediment cohesiveness was enhanced when both diatoms and H. diversicolor were cultured together; this result was unexpected since macrofauna, through consumption of microorganisms and modification of sediment properties, is often considered to have a destabilising effect on sediment. Predicting the effect of macrofauna on microphytobenthic biofilms and their associated influence of sediment stability is therefore not straightforward. Similar experiments including different types of organisms or more complex assemblages might help to further characterise the effect of biota on sediment stability