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

[摘要] Despite the worldwide occurrence of marine hypoxicregions, benthic nitrogen (N) cycling within these areas is poorlyunderstood and it is generally assumed that these areas represent zones ofintense fixed N loss from the marine system. Sulfate reduction can be animportant process for organic matter degradation in sediments beneathhypoxic waters and many sulfate-reducing bacteria (SRB) have the geneticpotential to fix molecular N (N₂). Therefore, SRB may supply fixed N tothese systems, countering some of the N lost via microbial processes, such asdenitrification and anaerobic ammonium oxidation. The objective of thisstudy was to evaluate if N₂ fixation, possibly by SRB, plays a role inN cycling within the seasonally hypoxic sediments from the Eckernförde Bay,Baltic Sea. Monthly samplings were performed over the course of one year tomeasure nitrogenase activity (NA) and sulfate reduction rates, to determinethe seasonal variations in bioturbation (bioirrigation) activity andimportant benthic geochemical profiles, such as sulfur and N compounds, andto monitor changes in water column temperature and oxygen concentrations.Additionally, at several time points, the active N-fixing community wasexamined via molecular tools. Integrated rates of N₂ fixation(approximated from NA) and sulfate reduction showed a similar seasonalitypattern, with highest rates occurring in August (approx. 22 and 880 nmol cm⁻³ d⁻¹ of N and SO₄²⁻, respectively) and October(approx. 22 and 1300 nmol cm⁻³ d⁻¹ of N and SO₄²⁻respectively), and lowest rates occurring in February (approx. 8 and 32 nmol cm⁻³ d⁻¹ of N and SO₄²⁻, respectively). These ratechanges were positively correlated with bottom water temperatures andprevious reported plankton bloom activities, and negatively correlated withbottom water oxygen concentrations. Other variables that also appeared toplay a role in rate determination were bioturbation, bubble irrigation andwinter storm events. Molecular analysis demonstrated the presence of nifHsequences related to two known N₂ fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supportingthe hypothesis that some of the nitrogenase activity detected may beattributed to SRB. Overall, our data show that Eckernförde Bayrepresents a complex ecosystem where numerous environmental variablescombine to influence benthic microbial activities involving N and sulfurcycling.

[学科分类] 地球化学与岩石