Synechococcus narB gene (nitrate reductase) diversity, biogeography and expression

Synechococcus is a cyanobacterial genus that significantly contributes to global primary productivity, carbon (C) cycling and microbial food webs. The genus is relatively diverse, and consists of populations which vary in their abilities to assimilate N. In order to study the specific diversity and distribution of nitrate assimilating Synechococcus, the narB gene, encoding for a nitrate assimilatory reductase in cyanobacteria, was targeted for genetics-based studies.;Synechococcus-like narB sequences were obtained from a variety of open-ocean and coastal ocean samples, and the sequences were phylogenetically classified into 8 clades (of related sequences). The described sequence diversity suggests nitrate assimilation is potentially beneficial to open-ocean and coastal Synechococcus. narB mRNA sequences were also recovered indicating marine cyanobacteria are expressing the gene.;Synechococcus populations from different narB -defined phylogenetic clades (called narB subgroups) were tracked using newly developed quantitative PCR (qPCR) assays. Spatially total narB subgroup abundances were highest in the upper water column of coastal to coastal-transition zones of the California Current System (CCS). Patterns in narB subgroup abundances and nitrate concentrations from CCS samples suggest that subgroups have different ecologies related to nutrient concentration (covarying factors). Temporal monitoring of narB subgroup abundances at Monterey Bay mooring M0 indicates that spring upwelling causes seasonal decreases in all subgroups. Individual subgroup abundances patterns at M0 were distinct, probably in part due to the influence of less-commonly measured factors (e.g. mortality, metals, organic nutrients).;narB expression can potentially indicate nitrate assimilation by Synechococcus, but the influence of light and nutrients upon narB expression and its relation to nitrate assimilation are largely unknown. Culture experiments with Synechococcus strain CC9311 indicate that narB expression is altered by (diel and intensity) changes in light and based on previous N-growth status of cells. Also, narB expression and nitrate assimilation rate do not necessarily change in parallel, which makes narB expression is a better indicator of nitrate assimilation activity than nitrate assimilation rate. narB expression has the potential to determine sub-genus specific nitrate assimilation activity, but future efforts to interpret narB expression will need to consider sampling time, irradiance conditions and N-growth status of cells.