Bertics, V. J., Löscher, C. R., Salonen, I., Dale, A. W., Schmitz, R. A., and Treude, T.
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 Eckernforde Bay, Baltic Sea. Monthly samplings were performed over the course of one year to measure N2-fixation and sulfate reduction rates, to determine the seasonal variations in bioturbation (bioirrigation) activity and important benthic geo-chemical profiles, such as sulfur and N compounds, and to monitor changes in water column temperature and oxygen concentrations. Additionally, at several time points, rates of benthic denitrification were also measured and the active N-fixing community was examined via molecular tools. Integrated rates of N2-fixation 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 SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., respectively) and October (approx. 22 and 1300 nmol cm-3 d-1 of N and SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., respectively), and lowest rates occurring in February (approx. 8 and 32nmol cm-3 d-1 of N and SO Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed., 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 winterstorm events. Molecular analysis demonstrated the presence of nifH sequences related to two known N2-fixing SRB, namely Desulfovibrio vulgaris and Desulfonema limicola, supporting the hypothesis that some of the nitrogenase activity detected may be attributed to SRB. Denitrification appeared to follow a similar trend as the other microbial processes and the ratio of denitrification to N2-fixation ranged from 6.8 in August to 1.1 in February, indicating that in February, the two processes are close to being in balance in terms of N loss and N gain. Overall, our data show that Eckernforde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling. [ABSTRACT FROM AUTHOR]