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

Bertics, Victoria J., Löscher, C. R., Salonen, I., Dale, Andy W., Gier, Jessica, Schmitz, R.A. and Treude, Tina (2013) Occurrence of benthic microbial nitrogen fixation coupled to sulfate reduction in the seasonally hypoxic Eckernförde Bay, Baltic Sea. Open Access Biogeosciences (BG), 10 (3). pp. 1243-1258. DOI 10.5194/bg-10-1243-2013.

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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 Eckernförde 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 geochemical 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 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 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 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 Eckernförde Bay represents a complex ecosystem where numerous environmental variables combine to influence benthic microbial activities involving N and sulfur cycling.

Document Type: Article
Funder compliance: info:eu-repo/grantAgreement/EC/FP7/226213
Additional Information: WOS:000317010600001
Keywords: Biogeochemistry: Sediment; Environmental Microbiology; R/V Alkor; RC Littorina; RB Polarfuchs; ANAEROBIC AMMONIUM OXIDATION; COASTAL MARINE-SEDIMENTS; OXYGEN MINIMUM ZONE; SULPHATE-REDUCING BACTERIA; OCEANIC FIXED NITROGEN; SHALLOW-WATER STATION; WESTERN KIEL BIGHT; ESTUARINE SEDIMENTS; AEROBIC RESPIRATION; CYANOBACTERIAL MAT
Research affiliation: OceanRep > SFB 754 > B6
OceanRep > GEOMAR > FB2 Marine Biogeochemistry > FB2-MG Marine Geosystems
OceanRep > SFB 754 > B4
OceanRep > The Future Ocean - Cluster of Excellence
OceanRep > SFB 754
OceanRep > SFB 754 > B1
Kiel University > Kiel Marine Science
Refereed: Yes
Open Access Journal?: Yes
DOI etc.: 10.5194/bg-10-1243-2013
ISSN: 1726-4170
Related URLs:
Projects: HYPOX, Boknis Eck, Future Ocean
Date Deposited: 26 Sep 2012 09:14
Last Modified: 12 Jan 2015 12:16
URI: http://oceanrep.geomar.de/id/eprint/15427

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