Bioturbation activity of three macrofaunal species and the presence of meiofauna affect the abundance and composition of benthic bacterial communities.

Given concerns of increasing rates of species extinctions, the relationship between biodiversity and ecosystem functioning has become a major research focus over the past two decades. Many studies have shown that biodiversity per se (e.g. species richness) or species-specific traits may be good predictors of changes in ecosystem function. Although numerous studies on this subject have focused on terrestrial systems, few have evaluated benthic marine systems. We used the Limecola balthica community as a model to test whether the number or identity of three well-studied macrofaunal species influence the sediment bacterial compartment, which drives important biogeochemical processes and influence ecosystem functioning. We also investigated the poorly known role of meiofauna in the interactions between macrofauna and bacteria. Eight combinations of 0-3 species were maintained in microcosms for 34 days in the presence or absence of meiofauna. The abundance and composition of the bacterial community, defined by the relative percentage of cells with a high (HNA) vs low (LNA) nucleic acid content, were measured. Species identity of macrofauna was a better indicator of changes in the microbial compartment than was species richness per se. In particular, the gallery-diffuser behaviour of the polychaete Alitta virens likely induced strong changes in sediment physical and geochemical properties with a major impact on the bacterial compartment. Moreover, the presence of meiofauna modulated the influence of macrofauna on bacterial communities. This study provides evidence that species identity provides greater explanatory power than species richness to predict changes in the bacterial compartment. We propose that multi-compartment approaches to describe interactions amongst different size classes of organisms and their ecological roles should be further developed to improve our understanding of benthic ecosystem functioning.