The physiological effect of heavy metals and volatile fatty acids on Methanococcus maripaludis S2

Abstract

Background: Methanogenic archaea are of importance to the global C-cycle and to biological methane (CH4) production through anaerobic digestion and pure culture. Here, the individual and combined effects of copper (Cu), zinc (Zn), acetate, and propionate on the metabolism of the autotrophic, hydrogenotrophic methanogen Methanococcus maripaludis S2 were investigated. Cu, Zn, acetate, and propionate may interfere directly and indirectly with the acetyl-CoA synthesis and biological CH4 production. Thus, these compounds can compromise or improve the performance of M. maripaludis, an organism which can be applied as biocatalyst in the carbon dioxide (CO2)-based biological CH4 production (CO2-BMP) process or of methanogenic organisms applied in anaerobic digestion. Results: Here, we show that Cu concentration of 1.9 μmol L-1 reduced growth of M. maripaludis, whereas 4.4 and 6.3 μmol L-1 of Cu even further retarded biomass production. However, 1.0 mmol L-1 of Zn enhanced growth, but at Zn concentrations > 2.4 mmol L-1 no growth could be observed. When both, Cu and Zn, were supplemented to the medium, growth and CH4 production could even be observed at the highest tested concentration of Cu (6.3 μmol L-1). Hence, it seems that the addition of 1 mmol L-1 of Zn enhanced the ability of M. maripaludis to counteract the toxic effect of Cu. The physiological effect to rising concentrations of acetate (12.2, 60.9, 121.9 mmol L-1) and/or propionate (10.3, 52.0, 104.1 mmol L-1) was also investigated. When instead of acetate 10.3 mmol L-1 propionate was provided in the growth medium, M. maripaludis could grow without reduction of the specific growth rate (μ) or the specific CH4 productivity (qCH4). A combination of inorganic and/or organic compounds resulted in an increase of μ and qCH4 for Zn/Cu and Zn/acetate beyond the values that were observed if only the individual concentrations of Zn, Cu, acetate were used. Conclusions: Our study sheds light on the physiological effect of VFAs and heavy metals on M. maripaludis. Differently from μ and qCH4, MER was not influenced by the presence of these compounds. This indicated that each of these compounds directly interacted with the C-fixation machinery of M. maripaludis. Until now, the uptake of VFAs other than acetate was not considered to enhance growth and CH4 production of methanogens. The finding of propionate uptake by M. maripaludis is important for the interpretation of VFA cycling in anaerobic microenvironments. Due to the importance of methanogens in natural and artificial anaerobic environments, our results help to enhance the understanding the physiological and biotechnological importance with respect to anaerobic digestion, anaerobic wastewater treatment, and CO2-BMP. Finally, we propose a possible mechanism for acetate uptake into M. maripaludis supported by in silico analyses.