Geobacter sulfurreducens metabolism at different donor/acceptor ratios

Abstract

Geobacter species have great application potential in remediation processes and electrobiotechnology. In all applications, understanding the metabolism will enable target-oriented optimization of the processes. The typical electron donor and carbon source of the Geobacter species is acetate, while fumarate is the usual electron acceptor. Here, we could show that depending on the donor/acceptor ratio in batch cultivation of Geobacter sulfurreducens different product patterns occur. With a donor/acceptor ratio of 1:2.5 malate accumulated as an intermediate product but was metabolized to succinate subsequently. At the end of the cultivation, the ratio of fumarate consumed and succinate produced was approximately 1:1. When fumarate was added in excess, malate accumulated in the fermentation broth without further metabolization. After the addition of acetate to stationary cells, malate concentration decreased immediately and additional succinate was synthesized. Finally, it was shown that also resting cells of G. sulfurreducens could efficiently convert fumarate to malate without an additional electron donor. Overall, it was demonstrated that by altering the donor/acceptor ratio, targeted optimization of the metabolite conversion by G. sulfurreducens can be realized.central metabolism, donor/acceptor ratio, Geobacter sulfurreducens, organic acid production | INTRODUCTIONGeobacter strains are deltaproteobacteria and can be found in different habitats, for example, aquatic systems, deep aquifer sediments, subsurface sediments, as well as several contaminated sites. Geobacter cells are Gram-negative, nonspore-forming, and obligate heterotrophs (Straub, 2011;Zhang et al., 2020). In general, Geobacter species grow only under strictly anoxic conditions and are unable to grow by fermentation (Straub, 2011). Geobacter species predominate under iron-reducing conditions (Straub, 2011). Acetate is a common electron and carbon donor among Geobacter species (Straub, 2011) and the majority of species could also metabolize alternative electron donors (e.g., ethanol, pyruvate, lactate, hydrogen, and formate). Geobacter species are using different electron acceptors, for example, Fe(III), nitrate, elemental sulfur (S 0 ), fumarate, malate, Mn(IV), U(VI), Co(III), and humic substances.Geobacter species appear to be the primary agents for coupling the oxidation of organic compounds to the reduction of insoluble Fe(III) and Mn(IV) oxides in many soils and sediments (Lovley et al., 2011).Different Geobacter species can oxidize aromatic hydrocarbons under anaerobic conditions. In addition, Geobacter species are important for aromatic hydrocarbon removal in contaminated