Effect of Seawater–Sewage Cross-Transplants on Bacterial Metabolism and Diversity

Bioassays experiments were conducted to determine the metabolic and community composition response of bacteria to transplants between relatively pristine coastal seawater and sewage-impacted seawater. There were four treatments: (1) pristine seawater bacteria + pristine seawater (Pb + Pw), (2) sewage-impacted bacteria + sewage-impacted water (Sb + Sw), (3) pristine seawater bacteria + sewage-impacted water (Pb + Sw), and (4) sewage-impacted bacteria + pristine seawater (Sb + Pw). Sewage-derived DOC was more labile and readily utilized by bacteria, which favored the growth of high nucleic acid (HNA) bacteria, resulting in high bacterial production (BP, 113 ± 4.92 to 130 ± 15.8 μg C l−1 day−1) and low respiration rate (BR, <67 ± 11.3 μg C l−1 day−1), as well as high bacterial growth efficiency (BGE, 0.68 ± 0.09 to 0.71 ± 0.05). In contrast, at the relatively pristine site, bacteria utilized natural marine-derived dissolved organic matter (DOM) at the expense of lowering their growth efficiency (BGE, <0.32 ± 0.02) with low BP (<62 ± 6.3 μg C l−1 day−1) and high BR 133 ± 14.2 μg C l−1 day−1). Sewage DOM input appeared to alter the partitioning of carbon between respiration and production of bacteria, resulting in a shift toward higher BGE, which would not enhance oxygen consumption. Taxonomic classification based on 454 pyrosequencing reads of the 16S rRNA gene amplicons revealed that changes in bacterial community structure occurred when seawater bacteria were transferred to the eutrophic sewage-impacted water. Sewage DOM fueled the growth of Gammma-proteobacteria and Epsilson-proteobacteria and reduced the bacterial richness, but the changes in the community were not apparent when sewage-impacted bacteria were transferred to pristine seawater.