Reactivity of Iron Minerals in the Seabed Toward Microbial Reduction – A Comparison of Different Extraction Techniques

Abstract

Dissimilatory iron reduction and sulfate reduction are the most important processes for anaerobic mineralization of organic carbon in marine sediments. The thermodynamics and kinetics of microbial Fe(III) reduction depend on the characteristics of the Fe(III) minerals, which influence the potential of Fe(III)-reducers to compete with sulfate-reducers for common organic substrates. In the present study, we tested different methods to quantify and characterize microbially reducible Fe(III) in sediments from a transect in Kongsfjorden, Svalbard, using different standard sequential endpoint extractions and time-course extractions with either ascorbate or a Fe(III)-reducing microbial culture. Similar trends of increasing ‘reactive Fe’ content of the sediment along the fjord transect were found using the different extraction methods. However, the total amount of ‘reactive Fe’ extracted differed between the methods, due to different Fe dissolution mechanisms and different targeted Fe fractions. Time-course extractions additionally provided information on the reactivity and heterogeneity of the extracted Fe(III) minerals, which also impact the favorability for microbial reduction. Our results show which fractions of the existing Fe extraction protocols should be considered ‘reactive’ in the sense of being favorable for microbial Fe(III) reduction, which is important in studies on early diagenesis in marine sediments.

Keywords:

• Arctic
• Fe reduction
• iron
• sediment
• sulfate reduction
• Svalbard

Acknowledgments

The authors thank the Alfred Wegener Institute-Institute Paul Emile Victor (AWIPEV) station and staff for housing and excellent logistics support. They furthermore thank MS Teisten captain Roar Strand. They appreciate the technical assistance of Karina Bomholt Oest at Aarhus University. The Polar Geospatial Center and Brad Herried provided geospatial support under NSF OPP awards 1043681 & 1559691. They thank all the participants of the 2017 Svalbard KOP 56/RiS 10528 expedition for help with sample collection and providing stimulating discussions.

Disclosure statement

No potential conflict of interest was reported by the authors.

Additional information

Funding

The study was supported by an ERC Advanced Grant given to BBJ [MICROENERGY, grant #294200, EU 7th FP], by the Danish National Research Foundation [DNRF grant #104], and by the Danish Council for Independent Research [DFF – 7014-00196], as well as by postdoctoral fellowships from the US-National Science Foundation [EAR-PF1625158 to ABM], and the German Research Foundation [DFG 389371177 to KL].