Diversity and distribution of microbes involved in marine nitrogen cycle

Abstract

Marine nitrogen (N) fixation and anaerobic ammonium oxidation (anammox) represent by far the major N input and loss pathways in marine environments. Together, these two ecologically important N cycling processes can regulate the total N contents in the ocean. Previous studies have detected diverse microbes involved in these processes with functional genes based molecular analysis. However, the spatial or/and temporal variations in composition and distribution of these microbial communities in some special environments remain unknown. In this study, the phylogenetic diversity and abundance of diazotrophs were examined along a transect from Pearl River plume to oceanic waters of the South China Sea (SCS) in both summer and winter, where biological N fixation is thought to be an important N source to its oligotrophic surface waters. Highly diverse diazotrophs were recovered, including proteobacteria, cyanobacteria and cluster III diazotrophs, with γ-proteobacteria dominating the entire diazotrophic community. Cyanobacterial diazotrophs were only detected in oceanic stations and the disappearance of cyanobacterial diazotrophs in the plume influenced station was suggested to be driven by low salinity and high nutrient concentrations. Compared with spatial variations, seasonal variations of diazotrophic communities were less apparent. Trichodesmium spp. was the most abundant cyanobacterial diazotroph and potentially the most important nitrogen fixer in this environment. Marine oxygen minimum zone (OMZ) represents another habitat for active microbial N transformations, where substantial N loss through anammox occurs. Novel hzo sequences in cluster 2 were detected in the OMZ of the Costa Rica Dome (CRD), with the simultaneous presence of the well-known hzo cluster 1 sequences. The novel hzo sequences were much more abundant and more widespread than cluster 1. More cDNA transcripts of the novel sequences were also obtained, indicating their potentiality to expressing the hydrazine oxidoreductase. The novel cluster 2 hzo sequences are suggested to be more ecologically important due to their higher abundance and wider distribution. This study firstly provided insight on the community compositions and distributions of diazotrophs in SCS and anammox bacteria in the CRD-OMZ, which is essential for understanding and predicting local and global N budget