Formate and hydrogen as electron shuttles in terminal fermentations in an oligotrophic freshwater lake sediment

Abstract

The energetic situation of terminal fermentations in methanogenesis was analyzed by pool size determinations in sediment cores taken in the oligotrophic Lake Constance, Germany. Distribution profiles of fermentation intermediates and products were measured at three different water depths (2, 10, and 80 m). Methane concentrations were constant below 10 cm of sediment depth. Within the methanogenic zone, concentrations of formate, acetate, propionate, and butyrate varied between 1 and 40 -M, and hydrogen was between 0.5 and 5 Pa. From the distribution profiles of the fermentation intermediates, Gibbs free energy changes for their interconversion were calculated. Pool sizes of formate and hydrogen were energetically nearly equivalent, with -5 ± 5 kJ per mol difference of free energy change (ΔG) for a hypothetical conversion of formate to hydrogen plus CO2. The ΔG values for conversion of fatty acids to methanogenic substrates and their further conversion to methane and CO2 were calculated with hydrogen and with formate as intermediates. Syntrophic propionate oxidation reached energetic equilibrium with formate as the sole electron carrier but was sufficiently exergonic if at least some of the electrons were transferred via hydrogen. The energetic consequences of formate versus hydrogen transfer in secondary and methanogenic fermentations indicate that both carrier systems are probably used simultaneously to optimize the energy yields for the partners involved.