Influence of submarine topography and sediment environment on microbial assemblages in a coastal lagoon in northeastern Japan

The relationships among eutrophication, anoxia, and microbial distribution were investigated for Nagatsura-Ura Lagoon on the northeastern Pacific coast of Japan. In September 2017, the bottom environment in a small area of the inner part of the lagoon (which has a basin-shaped bottom topology) was eutrophic and anoxic, with high carbon, nitrogen, phosphate, acid-volatile sulfide, and low dissolved oxygen and oxidation-reduction potential. Dissolved oxygen levels improved during the winter. Bacillariophyta (diatoms) were the main organic component according to pigment analysis and next-generation sequencing of nucleic acids in seawater samples. Phylum Proteobacteria was dominant among the bacterial flora in the sediment but the proportions of Class Epsilonproteobacteria and Chlorobium (a green sulfur-utilizing bacterium) were high in the inner part of the lagoon compared to other stations, and these groups were also present in winter. Apparently groups able to thrive in both anoxic and aerobic conditions were predominant in the inner part of the lagoon.