Search for: All records

ABSTRACT The diversity of the marine cyanobacteriumSynechococcuscan be broadly separated into clades, with clade II typically present in warm oligotrophic water, and clades I and IV found in cooler coastal water. We found amplicon sequence variants (ASVs) belonging to clade II in the nutrient‐replete waters of San Diego Bay (SDB). Using the 16S rRNA gene, 18S rRNA gene and internal transcribed spacer region sequencing, we analysed multiple locations in SDB monthly for over a year, with additional samples dating back to 2015.Synechococcuscommunity composition differed from the nearby coast into SDB in terms of dominant clade and ASVs. Specific clade II ASVs became relatively more abundant towards the back of the bay and showed seasonality, with higher relative abundance in the warm months. Select ASVs group phylogenetically and show similar seasonal and spatial distribution patterns, indicating these ASVs have adapted to SDB. Isolates matching clade II ASVs from SDB show pigment composition that is better adapted to the green light available in SDB, further supporting our findings. Other microbial taxa also show SDB enrichment, providing evidence that SDB is a chemostat‐like environment where circulation, temperature, light and other environmental conditions create a zone for microbial evolution and diversification. 

Abstract Biological nitrogen fixation is a major important source of nitrogen for low-nutrient surface oceanic waters. Nitrogen-fixing (diazotrophic) cyanobacteria are believed to be the primary contributors to this process, but the contribution of non-cyanobacterial diazotrophic organisms in oxygenated surface water, while hypothesized to be important, has yet to be demonstrated. In this study, we used simultaneous 15 N-dinitrogen and 13 C-bicarbonate incubations combined with nanoscale secondary ion mass spectrometry analysis to screen tens of thousands of mostly particle-associated, cell-like regions of interest collected from the North Pacific Subtropical Gyre. These dual isotope incubations allow us to distinguish between non-cyanobacterial and cyanobacterial nitrogen-fixing microorganisms and to measure putative cell-specific nitrogen fixation rates. With this approach, we detect nitrogen fixation by putative non-cyanobacterial diazotrophs in the oxygenated surface ocean, which are associated with organic-rich particles (<210 µm size fraction) at two out of seven locations sampled. When present, up to 4.1% of the analyzed particles contain at least one active putative non-cyanobacterial diazotroph. The putative non-cyanobacterial diazotroph nitrogen fixation rates (0.76 ± 1.60 fmol N cell −1 d −1 ) suggest that these organisms are capable of fixing dinitrogen in oxygenated surface water, at least when attached to particles, and may contribute to oceanic nitrogen fixation. 

Abstract Biological dinitrogen (N2) fixation supplies nitrogen to the oceans, supporting primary productivity, and is carried out by some bacteria and archaea referred to as diazotrophs. Cyanobacteria are conventionally considered to be the major contributors to marine N2 fixation, but non-cyanobacterial diazotrophs (NCDs) have been shown to be distributed throughout ocean ecosystems. However, the biogeochemical significance of marine NCDs has not been demonstrated. This review synthesizes multiple datasets, drawing from cultivation-independent molecular techniques and data from extensive oceanic expeditions, to provide a comprehensive view into the diversity, biogeography, ecophysiology, and activity of marine NCDs. A NCD nifH gene catalog was compiled containing sequences from both PCR-based and PCR-free methods, identifying taxa for future studies. NCD abundances from a novel database of NCD nifH-based abundances were colocalized with environmental data, unveiling distinct distributions and environmental drivers of individual taxa. Mechanisms that NCDs may use to fuel and regulate N2 fixation in response to oxygen and fixed nitrogen availability are discussed, based on a metabolic analysis of recently available Tara Oceans expedition data. The integration of multiple datasets provides a new perspective that enhances understanding of the biology, ecology, and biogeography of marine NCDs and provides tools and directions for future research.