Search

The wide distribution of diverse nitrogenase ( nifH) genes affiliated with those of heterotrophic bacteria in marine and estuarine waters indicates ubiquity and an ecologically relevant role for heterotrophic N2-fixers (diazotrophs) in aquatic nitrogen ( N) cycling. However, the lack of cultivated representatives currently precludes an evaluation of their N2-fixing capacity. In this study, microoxic or anoxic N-free media were inoculated with estuarine Baltic Sea surface water to select for N2-fixers. After visible growth and isolation of single colonies on oxic plates or in anoxic agar tubes, nifH gene amplicons were obtained from 64 strains and nitrogenase activity, applying the acetylene reduction assay, was confirmed for 40 strains. Two strains, one G ammaproteobacterium affiliated with P seudomonas and one A lphaproteobacterium affiliated with R hodopseudomonas were shown to represent established members of the indigenous diazotrophic community in the Baltic Sea, with abundances of up to 7.9 × 104 and 4.7 × 104 nifH copies l−1 respectively. This study reports media for successful isolation of heterotrophic diazotrophs. The applied methodology and the obtained strains will facilitate future identification of factors controlling heterotrophic diazotrophic activity in aquatic environments, which is a prerequisite for understanding and evaluating their ecology and contribution to N cycling at local and regional scales. [ABSTRACT FROM AUTHOR]

Abstract: Automated measurements of the surface CO2 partial pressure, pCO2, were performed since 2003 on a cargo ship along a transect between Helsinki in the Gulf of Finland and Lübeck/Gdynia in the southwest of the Baltic Sea. The temporal and spatial resolution of the measurements amounted to 2–4days and about 2nautical miles, respectively. Based on temperature and salinity records and on the mean alkalinity, the total CO2 concentrations, CT, were calculated from the mean pCO2 in the northeastern Gotland Sea. The CT data were used to establish a CO2 mass balance for the period from mid-June to the beginning of August in 2005, 2008, 2009 and 2011. Taking into account the air–sea CO2 gas exchange, the mass balance yielded the net organic matter (Corg) production which is fuelled by nitrogen fixation at this time of the year. Several production events were detected with rates up to 8μmol-CL−1 d−1. The production rates were not related to temperature, but showed a distinct correlation with the rate of the temperature increase. This led to the conclusion that the exposure of nitrogen fixing cyanobacteria to irradiance is the dominating control for the Corg production. Therefore, we suggest using the ratio of irradiance to the mixed layer depth as a variable for the parameterization of nitrogen fixation in biogeochemical models. The Corg production and thus the nitrogen fixation rates remained almost constant as long as continuous rising temperatures indicated favorable irradiation conditions. A limitation of the rates by phosphate or any other factor could not be detected. Based on the C/N ratio of particulate organic matter during a cyanobacteria bloom, the Corg production was used to estimate the mid-summer nitrogen fixation. The values varied from 102mmolm−2 to 214mmolm−2 (mean: 138mmolm−2) for the different years and did not show any correlation with the phosphate excess after the spring nitrate depletion. [Copyright &y& Elsevier]

The Baltic Sea receives large nitrogen inputs by diazotrophic (N2-fixing) heterocystous cyanobacteria but the significance of heterotrophic N2 fixation has not been studied. Here, the diversity, abundance and transcription of the nifH fragment of the nitrogenase enzyme in two basins of the Baltic Sea proper was examined. N2 fixation was measured at the surface (5 m) and in anoxic water (200 m). Vertical sampling profiles of >10 and <10 μm size fractions were collected in 2007, 2008 and 2011 at the Gotland Deep and in 2011 in the Bornholm Basin. Both of these stations are characterized by permanently anoxic bottom water. The 454-pyrosequencing nifH analysis revealed a diverse assemblage of nifH genes related to alpha-, beta- and gammaproteobacteria (nifH cluster I) and anaerobic bacteria (nifH cluster III) at and below the chemocline. Abundances of genes and transcripts of seven diazotrophic phylotypes were investigated using quantitative polymerase chain reaction revealing abundances of heterotrophic nifH phylotypes of up to 2.1 × 107 nifH copies l−1. Abundant nifH transcripts (up to 3.2 × 104 transcripts l−1) within nifH cluster III and co-occurring N2 fixation (0.44±0.26 nmol l−1 day−1) in deep water suggests that heterotrophic diazotrophs are fixing N2 in anoxic ammonium-rich waters. Our results reveal that N2 fixation in the Baltic Sea is not limited to illuminated N-deplete surface waters and suggest that N2 fixation could also be of importance in other suboxic regions of the world's oceans. [ABSTRACT FROM AUTHOR]