Controls on volatile fatty acid concentrations in marine sediments (Baltic Sea)

Volatile fatty acids (VFA) are key intermediates in the degradation pathway of organic matter (OM) in marine sediments. They are products of fermentation and are utilized as substrates in the terminal steps of microbial OM mineralization, such as sulfate reduction and methanogenesis. We studied the distribution of formate, acetate, propionate and butyrate concentrations in the pore water throughout 100-m deep sedimentary sequences in the Baltic Sea. The sediments included both sulfate and methane zones and were retrieved during the Integrated Ocean Drilling Program Expedition 347. VFA concentrations were consistently low throughout the sulfate and methane zones, ranging from few hundred nanomolar to about hundred micromolar, thus pointing to efficient microbial utilization in the terminal OM mineralization steps in both zones. Concentrations of formate followed a different depth trend than the other three VFAs suggesting a different mechanistical control of the formate turnover. Calculations of the Gibbs energy reveal that acetoclastic methanogenesis is exergonic (up to -30 kJ (mol CH4)(-1)) throughout the sulfate zone and suggest that methanogens can potentially coexist with sulfate reducers in the sulfate zone while sharing acetate as a substrate. In the deeper methane-rich sediments, the energy yield of acetoclastic methanogenesis is generally very low (less negative than -10 kJ (mol CH4)(-1)). The uniformly low acetate concentrations in the methane zone suggest its active microbial turnover but the pathway from acetate to methane still remains unclear. Interestingly, acetoclastic sulfate reduction still yields at least -20 kJ (mol sulfate)(-1) in the methane zone and, thus, sulfate reducers may utilize acetate as electron donor in the methane zone, which is in agreement with the earlier findings of sulfate reducers in methanogenic sediments. (C) 2019 Elsevier Ltd. All rights reserved.