Biogeography and Diversity of Freshwater Bacteria on a River Catchment Scale

To illustrate how freshwater bacterial community changes with geographic gradient, we investigated the spatial changes of bacterial abundance and community structures from over 200 samples on a catchment scale in the Songhua River using heterotrophic plate counts, flow cytometry, denaturing gradient gel electrophoresis, and pyrosequencing analysis. The results showed that the mainstream had higher cultivable bacteria and total bacterial concentration than tributaries in the Songhua River catchment. Response model analysis demonstrated that the bacterial community exhibits a biogeographical signature even in an interconnected river network system, and the total bacterial concentration and biodiversity were significantly correlated to latitude (p < 0.001) and longitude (p < 0.001). Multivariate redundancy analysis indicated that temperature was the most important factor driving bacterial community structure in the Songhua River, which accounts for 35.30% variance of communities, then dissolved oxygen (17.60%), latitude (17.60%), longitude (11.80%), and pH (5.88%). High-throughput pyrosequencing revealed that at the phylum level, Proteobacteria was numerically dominant (89.6%) in river catchment, followed by Bacteroidetes (8.1%) and Cyanobacteria (1.2%). The overall results revealed that the bacterial community was driven by geographical distance regardless of the continuum of the river on a catchment scale.