Influence of nitrate availability on the distribution and abundance of heterotrophic bacterial nitrate assimilation genes in the Barents Sea during summer

In a transect across Norwegian coastal waters and the Barents Sea to approximately 78 degrees N, distinct patterns in the distribution and abundance of bacterial assimilatory nitrate reductase (nasA) genes were observed in relation to NO3- availability and bacterial dissolved inorganic nitrogen utilization. A real-time PCR assay, developed for a group of nasA genes characteristic of Marinobacter sp., which are a common group of nitrate-assimilating bacteria in the marine environment, indicated that the nasA gene abundance of Marinobacter sp. was positively correlated with NO3- concentration. At 5 stations sampled, Marinobacter sp. nasA gene abundance was, on average, 8-fold higher at 80 m compared to 5 m depth, relative to total bacteria. Bacterial productivity, bacterial biomass, chlorophyll a, NH4+, and NO3- were modeled as independent variables in a partial least-squares regression model to determine how well each variable predicted the variation in nasA community structure, defined by terminal restriction-length-fragment polymorphism analysis. NO3- concentration was the best predictor, by a factor of 10, of the variability associated with nasA community structure. In a companion study of (NO3-)-N-15 and (NH4+)-N-15 uptake across the same transect, conducted at the same time as this study, bacteria were relatively more important in terms of total community uptake in the marginal ice zone, where NO3- levels were high, compared to samples from the North Atlantic, where NO3- concentrations were lower. Results presented here indicate that NO3- availability and patterns of NO3- utilization are correlated with nasA community structure variability and abundance.