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 μM, 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.IMPORTANCE In the terminal steps of methane formation in freshwater lake sediments, fermenting bacteria cooperate syntrophically with methanogens and homoacetogens at minimum energy increments via interspecies electron transfer. The energy yields of the partner organisms in these cooperations have so far been calculated based mainly on in situ hydrogen partial pressures. In the present study, we also analyzed pools of formate as an alternative electron carrier in sediment cores of an oligotrophic lake. The formate and hydrogen pools appeared to be energetically nearly equivalent and are likely to be used simultaneously for interspecies electron transfer. Calculations of reaction energies of the partners involved suggest that propionate degradation may also proceed through the Smithella pathway, which converts propionate via butyrate and acetate to three acetate residues, thus circumventing one energetically difficult fatty acid oxidation step.

Keywords: energetics; fatty acids; methanogenesis; pool sizes; secondary fermentations; syntrophy.

FIG 1

Physicochemical properties and pool sizes of fermentation intermediates in sediment cores taken in Lake Constance at three different water depths (2 m, 10 m, and 80 m).

FIG 2

Gibbs free energy changes of (hypothetical) conversion of formate to hydrogen plus CO₂ in sediment cores taken in Lake Constance at three different water depths (2 m, 10 m, and 80 m).

FIG 3

Gibbs free energy changes of secondary fermentation reactions as specified in Table 1, based on interspecies hydrogen or formate transfer in sediment cores taken in Lake Constance at three different water depths (2 m, 10 m, and 80 m). In panels f and g, energetics of fermentations according to Syntrophobacter are shown with solid lines, and those according to Smithella are shown with dashed lines.