Differential inhibition by allylsulfide of nitrification and methane oxidation in freshwater sediment

Abstract

Addition of nitrapyrin, allylthiourea, C2H2, and CH3F to freshwater sediment slurries inhibited CH4 oxidation and nitrification to similar extents. Dicyandiamide and allylsulfide were less inhibitory for CH4 oxidation than for nitrification. Allylsulfide was the most potent inhibitor of nitrification, and the estimated 50% inhibitory concentrations for this process and CH4 oxidation were 0.2 and 121 μM, respectively. At a concentration of 2 μM allylsulfide, growth and CH4 oxidation activity of Methylosinus trichosporium OB3b were not inhibited. Allylsulfide at 200 μM inhibited the growth of M. trichosporium by approximately 50% but did not inhibit CH4 oxidation activity. Nitrite production by cells of M. trichosporium was not significantly affected by allylsulfide, except at a concentration of 2 mM, when growth and CH4 oxidation were also inhibited by about 50%. Methane monooxygenase activity present in soluble fractions of M. trichosporium was not inhibited significantly by allylsulfide at either 200 μM or 2 mM. These results suggest that the partial inhibition of CH4 oxidation in sediment slurries by high allylsulfide concentrations may be caused by an inhibition of the growth of methanotrophs rather than an inhibition of methane monooxygenase activity specifically. We conclude that allylsulfide is a promising tool for the study of interactions of methanotrophs and nitrifiers in N cycling and CH4 turnover in natural systems.