title: Formate, acetate, and propionate as substrates for sulfate reduction in sub-arctic sediments of Southwest Greenland.
abstract: Volatile fatty acids (VFAs) are key intermediates in the anaerobic mineralization of organic matter in marine sediments. We studied the role of VFAs in the carbon and energy turnover in the sulfate reduction zone of sediments from the sub-arctic Godthåbsfjord (SW Greenland) and the adjacent continental shelf in the NE Labrador Sea. VFA porewater concentrations were measured by a new two-dimensional ion chromatography-mass spectrometry method that enabled the direct analysis of VFAs without sample pretreatment. VFA concentrations were low and surprisingly constant (4-6 μmol L(-1) for formate and acetate, and 0.5 μmol L(-1) for propionate) throughout the sulfate reduction zone. Hence, VFAs are turned over while maintaining a stable concentration that is suggested to be under a strong microbial control. Estimated mean diffusion times of acetate between neighboring cells were <1 s, whereas VFA turnover times increased from several hours at the sediment surface to several years at the bottom of the sulfate reduction zone. Thus, diffusion was not limiting the VFA turnover. Despite constant VFA concentrations, the Gibbs energies (ΔGr) of VFA-dependent sulfate reduction decreased downcore, from -28 to -16 kJ (mol formate)(-1), -68 to -31 kJ (mol acetate)(-1), and -124 to -65 kJ (mol propionate)(-1). Thus, ΔGr is apparently not determining the in-situ VFA concentrations directly. However, at the bottom of the sulfate zone of the shelf station, acetoclastic sulfate reduction might operate at its energetic limit at ~ -30 kJ (mol acetate)(-1). It is not clear what controls VFA concentrations in the porewater but cell physiological constraints such as energetic costs of VFA activation or uptake could be important. We suggest that such constraints control the substrate turnover and result in a minimum ΔGr that depends on cell physiology and is different for individual substrates.
authors.affiliation: Department of Bioscience, Center for Geomicrobiology, Aarhus University Aarhus, Denmark.
authors.forename: Clemens
authors.initials: C
authors.lastname: Glombitza
authors.affiliation: Department of Bioscience, Center for Geomicrobiology, Aarhus University Aarhus, Denmark.
authors.forename: Marion
authors.initials: M
authors.lastname: Jaussi
authors.affiliation: Department of Bioscience, Center for Geomicrobiology, Aarhus University Aarhus, Denmark.
authors.forename: Hans
authors.initials: H
authors.lastname: Røy
authors.affiliation: Department of Bioscience, Arctic Research Center, Aarhus University Aarhus, Denmark ; Department of Geoscience, Centre for Past Climate Studies, Aarhus University Aarhus, Denmark.
authors.forename: Marit-Solveig
authors.initials: MS
authors.lastname: Seidenkrantz
authors.affiliation: Department of Bioscience, Center for Geomicrobiology, Aarhus University Aarhus, Denmark ; Section for Microbiology, Department of Bioscience, Aarhus University Aarhus, Denmark.
authors.forename: Bente A
authors.initials: BA
authors.lastname: Lomstein
authors.affiliation: Department of Bioscience, Center for Geomicrobiology, Aarhus University Aarhus, Denmark.
authors.forename: Bo B
authors.initials: BB
authors.lastname: Jørgensen
keywords: Gibbs energy; acetate diffusion; energy turnover; minimum energy requirement; porewater; thermodynamics; turnover rates; volatile fatty acids