Effect of Macroinvertebrates on Detachment of Bacteria from Biofilms in Stream Microcosms

Abstract

Transport patterns of bacteria in streams depend largely on processes of detachment from and attachment to surfaces. Stream substrata are coated with various microorganisms, including bacteria, embedded in a matrix of extracellular material. These complex communities, called biofilms, may be disrupted by movement and feeding activities of macroinvertebrates causing bacterial cells to detach. Laboratory experiments were conducted to assess the role of stream macroinvertebrates in detachment of bacteria from biofilms. Marked bacteria that had a rare combination of antibiotic resistances were used in microcosms as tracers of bacterial exchanges. The role of macroinvertebrates in movement of bacteria from leaves to water was investigated by including either mayfly, stonefly, or dragonfly nymphs or glass shrimp in microcosms. The presence of macroinvertebrates did not alter bacterial exchange between habitats. There were considerable variation among replicates for some macroinvertebrate treatments, indicating invertebrates may cause large, catastrophic releases of bacteria from biofilms on leaves or fecal pellets. Alternatively, invertebrates may not be directly involved in these releases if pulses of bacteria result from disintegration of fecal pellets. The effect of invertebrate density on exchange was measured by varying mayfly nymph density. No significant differences among densities were detected. The ability of macroinvertebrates to serve as vectors for transfer of bacteria between leaf packs was also investigated. Although transfer by invertebrates was detected in some experiments, transfer was not consistently observed.