Spatial Heterogeneity of Bacterial Assemblages in Marine Sediments: The Influence of Deposit Feeding byBalanoglossus aurantiacus

Densities and substrate utilization patterns of the bacterial assemblages of three intertidal sandflats were contrasted to investigate spatial heterogeneity. Possible influence of deposit-feeding Balanoglossus aurantiacus (Enteropneusta: Ptychoderidae) was also studied by comparing samples taken from the faecal mounds with surrounding sediments. While there were no significant differences in total bacterial abundance, carbon source utilization patterns, determined using Biolog GN microtitre plates, differed among the sandflats and within the sandflats for samples from grids more than 10 m apart. No consistent quantitative or qualitative differences were detected among clusters of ambient sediment samples taken less than 0·5 m apart. Qualitative differences in microbial assemblages were found between faecal mounds and ambient (>10 cm apart) sediment within a single intertidal flat. These differences were in part due to digestive removal of bacteria, as heterotrophic plate counts and response in Biolog plates (average well colour development, AWCD) were significantly higher for the surrounding sediment. However, when Biolog profiles were normalized to AWCD, it became clear that gut passage resulted in structural shifts in bacterial assemblages. These changes were due to digestion of a subset of ingested bacteria with the concomitant stimulation (or inoculation) of other groups, which were largely unculturable under the conditions employed. These findings illustrate that deposit feeding effects spatial heterogeneity in sedimentary bacterial assemblages. Given the ephemeral nature of these disturbed patches, however, it is not yet clear that deposit feeding is a primary determinant of microbial community structure in this sedimentary landscape.