Abstract
Metals play an important role in microbial metabolism by acting as cofactors for many enzymes. Supplementation of biological processes with metals may result in improved performance, but high metal concentrations are often toxic to microorganisms. In this work, methanogenic enrichment cultures growing on H2/CO2 or acetate were supplemented with trace concentrations of nickel (Ni) and cobalt (Co), but no significant increase in methane production was observed in most of the tested conditions. However, high concentrations of these metals were detrimental to methanogenic activity of the cultures. Cumulative methane production (after 6 days of incubation) from H2/CO2 was 40% lower in the presence of 8 mM of Ni or 30 mM of Co, compared to controls without metal supplementation. When acetate was used as substrate, cumulative methane production was also reduced: by 18% with 8 mM of Ni and by 53% with 30 mM of Co (after 6 days of incubation). Metal precipitation with sulfide was further tested as a possible method to alleviate metal toxicity. Anaerobic sludge was incubated with Co (30 mM) and Ni (8 mM) in the presence of sulfate or sulfide. The addition of sulfide helped to mitigate the toxic effect of the metals. Methane production from H2/CO2 was negatively affected in the presence of sulfate, possibly due to competition of hydrogenotrophic methanogens by sulfate-reducing bacteria. However, in the enrichment cultures growing on acetate, biogenically produced sulfide had a positive effect and more methane was produced in these incubations than in similar assays without sulfate addition. The outcome of competition between methanogens and sulfate-reducing bacteria is a determinant factor for the success of using biogenic sulfide as detoxification method.
Highlights
Heavy metals are common pollutants in wastewaters, such as those from metal plating, mining, or pulp and paper industries (Sarioglu et al, 2010)
Positive effects of metal supplementation on the anaerobic digestion of wastewaters have been reported, especially for Ni and Co (Gustavsson et al, 2011, 2013; Karlsson et al, 2012). This is explained by the importance of these metals in enzymes that are involved in methanogenesis; Ni is present in Ni-Fe hydrogenases and in the cofactor F430, while Co is present in cobalamides (DiMarco et al, 1990)
Cumulative methane production values measured after 20, 60, and 120 h of incubation of the anaerobic sludge are shown in Table 1 and Table 2
Summary
Heavy metals are common pollutants in wastewaters, such as those from metal plating, mining, or pulp and paper industries (Sarioglu et al, 2010). Positive effects of metal supplementation on the anaerobic digestion of wastewaters have been reported, especially for Ni and Co (Gustavsson et al, 2011, 2013; Karlsson et al, 2012). This is explained by the importance of these metals in enzymes that are involved in methanogenesis; Ni is present in Ni-Fe hydrogenases and in the cofactor F430 (the prosthetic group of methyl coenzyme M reductase), while Co is present in cobalamides (DiMarco et al, 1990). Methyl-H4MPT:CoM-SH methyltransferase has in one of its subunits a cob(I)amide prosthetic group that is believed to be important for the enzymatic function (Hedderich and Whitman, 2006)
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