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.
Highlights
Mineralization of buried organic matter drives microbial activity and element cycles in sub-seafloor sediments (Reeburgh, 1983; Arndt et al, 2013)
We evaluated the influence of volatile fatty acids (VFAs) diffusion in the sediment on the microbial availability of VFAs and discuss the potential for thermodynamic control of in-situ porewater concentrations
The fjord system is characterized by the inflow of saline, well-oxygenated water from the Labrador Sea (West Greenland Current water) as a subsurface/bottom current filling the deep basins of the fjord, and the outflow of less saline water, primarily derived from glacier meltwater, which exits the fjord as a surface current (Buch, 2002)
Summary
Mineralization of buried organic matter drives microbial activity and element cycles in sub-seafloor sediments (Reeburgh, 1983; Arndt et al, 2013). The initial step in the degradation of high molecular weight organic matter in anoxic sediments is the hydrolytic breakdown of polymers by exoenzymes excreted by heterotrophic bacteria. This produces smaller compounds (mono- and dimers) which can be taken up by fermentative bacteria (e.g., Capone and Kiene, 1988). Fermentation does not result in a net oxidation of the organic matter but produces volatile fatty acids (VFAs), small alcohols, amines, CO2, H2 etc. The VFAs are quantitatively important intermediates and are turned over in the terminal steps of organic matter mineralization (e.g., sulfate reduction) by which they serve as electron donors (Middelburg, 1989)
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have