Differential effects of microorganism-invertebrate interactions on benthic nitrogen cycling

Infaunal invertebrate activity can fundamentally alter physicochemical conditions in sediments and influence nutrient cycling. However, despite clear links between invertebrate activity and microbially mediated processes such as nitrification, the mechanisms by which bioturbating macrofauna affect microbial communities have received little attention. This study provides strong evidence for differential stimulation of microbial nitrogen transformations by three functionally contrasting species of macrofauna (Hediste diversicolor, Corophium volutator, Hydrobia ulvae). Despite increased nitrification, abundance of ammonia-oxidising bacteria (AOB) and ammonia-oxidising archaea (AOA) at the sediment-water interface did not significantly change in the presence of macrofauna. However, species-specific differences in macrofaunal activity did influence ammonia oxidiser community structure, increasing AOB abundance relative to AOA in the presence of C. volutator or H. ulvae, but with no change in H. diversicolor and no-macrofauna treatments. Denaturing gradient gel electrophoresis profiles were similar between macrofaunal treatments, although one AOB band increased in relative intensity in the presence of C. volutator, decreased in the H. diversicolor treatment and was unchanged in the H. ulvae treatment. These data suggest that links between bioturbating macrofauna and nutrient cycling are not expressed through changes in the abundance of ammonia oxidisers in surface sediments, but are associated with changes in the AOA : AOB ratio depending on the invertebrate species.