Illustrating the importance of particulate organic matter to pelagic microbial abundance and community structure—an Arctic case study

The pelagic bacterial community composition of the summertime Chukchi Sea (coastal Alaskan Arctic) was investigated under varying particle concentrations. Free-living and whole-community assemblages were compared using PCR-amplified 16S rDNA with denaturing gradient gel electrophoresis (PCR/DGGE) alongside traditional geochemical and microbial inventories. Algal blooms were characterized by increased microbial abundance, decreased species richness, and decreased similarity between free-living and whole-community bacterial assemblages. Bacterial and viral abundance correlated positively with a bloom index identified by principle components analysis that included particulate organic carbon and nitrogen, total organic carbon, and chlorophyll a, but not dissolved organic carbon or inorganic nutrients. The species richness of the free-living community correlated negatively with microbial abundance; differences between free-living and whole-community assemblages correlated positively with viral abundance. Algal blooms may therefore increase microbial abundance while stimulating a succession of the bacterial assemblage to fewer free-living species and more specialized particle-associated bacteria. Such community-level shifts are very likely to impact the fate of carbon in high production and export regions like the coastal Arctic. We speculate about the importance of viral lysis to such a succession.