Occurrence of benthic microbial nitrogen fixation coupled to sulfate reduction in the seasonally hypoxic Eckernförde Bay, Baltic Sea

Abstract

Despite the worldwide occurrence of marine hypoxic regions, benthic nitrogen (N) cycling within these areas is poorly understood and it is generally assumed that these areas represent zones of intense fixed N loss from the marine system. Sulfate reduction can be an important process for organic matter degradation in sediments beneath hypoxic waters and many sulfate-reducing bacteria (SRB) have the genetic potential to fix molecular N (N₂). Therefore, SRB may supply fixed N to these systems, countering some of the N lost via microbial processes, such as denitrification and anaerobic ammonium oxidation. The objective of this study was to evaluate if N₂ fixation, possibly by SRB, plays a role in N cycling within the seasonally hypoxic sediments from the Eckernförde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure nitrogenase activity (NA) and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geochemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, the active N-fixing community was examined via molecular tools. Integrated rates of N₂ fixation (approximated from NA) and sulfate reduction showed a similar seasonality pattern, with highest rates occurring in August (approx. 22 and 880 nmol cm⁻³ d⁻¹ of N and SO₄^2&minus;, respectively) and October (approx. 22 and 1300 nmol cm⁻³ d⁻¹ of N and SO₄^2&minus; respectively), and lowest rates occurring in February (approx. 8 and 32 nmol cm⁻³ d⁻¹ of N and SO₄^2&minus;, respectively). These rate changes were positively correlated with bottom water temperatures and previous reported plankton bloom activities, and negatively correlated with bottom water oxygen concentrations. Other variables that also appeared to play a role in rate determination were bioturbation, bubble irrigation and winter storm events. Molecular analysis demonstrated the presence of <i>nifH</i> sequences related to two known N₂ fixing SRB, namely <i>Desulfovibrio vulgaris</i> and <i>Desulfonema limicola</i>, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Overall, our data show that Eckernförde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling

Water column biogeochemistry of oxygen minimum zones in the eastern tropical North Atlantic and eastern tropical South Pacific oceans

“…Evidence for co-occurrence of denitrification and N 2 fixation has previously only been documented for an anoxic lake (Halm et al, 2009) and for cyanobacterial aggregates in the Baltic Sea (Klawonn et al, 2015). Recent investigations from Baltic Sea sediments on N 2 fixation and diazotrophic abundance in sediments show, however, that a very close spatial link between N loss and N 2 fixation might exist (Bertics et al, 2013). Still, too little is currently known about the interactions among the stoichiometry of inorganic nutrient supply, primary production, N 2 fixation, and remineralization under anoxic conditions to allow for a definite characterization of the conditions leading to fixed-nitrogen exhaustion in the OMZs.…”

Microbial methanogenesis in the sulfate-reducing zone of sediments in the Eckernförde Bay, SW Baltic Sea

“…Several previous studies confirmed the high activity of sulfate reduction in the surface sediment of Eckernförde Bay, revealing rates up to 100-10 000 nmol cm −3 d −1 in the upper 25 cm b.s.f. (Treude et al, 2005a;Bertics et al, 2013;. The microbial fermentation of organic matter was probably high in the organic-rich sediments of Eckernförde Bay (POC contents of around 5 %; Figs.…”

“…At this sediment depth either the availability of noncompetitive substrates, including methanol, was highest (derived from fresh organic matter), or the usage of noncompetitive substrates was increased due to the high competitive situation as sulfate reduction is most active in the 0-5 cm b.s.f. layer (Treude et al, 2005a;Bertics et al, 2013). It should be noted that even though methanogenesis rates were calculated assuming a linear increase in methane concentration over the entire incubation to make a better comparison between different treatments, the methanol treatments generally showed a delayed response in methane development (Figs.…”

Summer upwelling at the Boknis Eck time-series station (1982 to 2012) – a combined glider and wind data analysis

“…Schweiger et al, 2007;Dale et al, 2011;Bange et al, 2011;Bertics et al, 2013;Dähnke and Thamdrup, 2013). Variability from seasonal to long term have been described for a number of parameters sampled at BE (e.g.…”

“…It is a ship survey-based time series with approximately monthly resolution, and was first surveyed in April 1957. Details on data collection and parameters have been described elsewhere Dale et al, 2011;Bertics et al, 2013;Hoppe et al, 2013;Lennartz et al, 2014). Here we will use bottle data acquired at standard depth (1, 5, 10, 15, 20, 26/25 m).…”