Longitudinal changes in bacterial community composition in river epilithic biofilms: influence of nutrients and organic matter

Epilithic bacterial communities play a major role in biogeochemical cycles of rivers; however, distributional patterns and controls of epilithic communities remain unclear. The objective of the present study was to examine possible environmental factors that affect longitudinal distributional patterns of epilithic bacterial communities in 2 rivers (Yasu and Ado Rivers) draining the Lake Biwa basin, Japan. Phylogenetic analyses were conducted using denaturing gradient gel electrophoresis (DGGE) of 16S rRNA genes and fluorescence in Situ hybridization (FISH) with oligonucleotide probes targeted to small subunit rRNA. Non-metric multidimensional scaling and canonical correspondence analysis of the DGGE profile indicated that a clear shift in community composition occurred at the middle reach of the Yasu River. This shift was most closely related to variation in the nitrogen stable isotope ratio (delta N-15) of epilithic materials (BIOENV analysis, rho(w) = 0.512), which is an indicator of the extent of anthropogenic nitrogen loading. The concentration of dissolved organic carbon (DOC) also affected community composition (rho(w) = 0.510 for the combination of delta N-15 and DOC). The longitudinal pattern of the DGGE profile was less evident in the Ado River. In both rivers, FISH results indicated that Alphaproteobacteria and Betaproteobacteria generally accounted for a large fraction (37 to 71%) of total bacterial abundance, indicating that species and clones affiliated with a limited number of the major phylogenetic groups occupy diverse epilithic habitats. Our results are consistent with the hypothesis that nutrients and organic matter of anthropogenic origin are major determinants of epilithic bacterial community structure in rivers.