The only organisms known to co-oxidize methane are ammonia-oxidizing bacteria such as Nitrosomonas and Nitrosospira. The ammonia monooxygenase enzyme is evolutionarily related to pMMO (Holmes et al., 1995) and both enzymes will oxidize both substrates, which are structural analogues of each other (Bedard and Knowles, 1989). However, there is substantial evidence that nitrifiers are not involved in atmospheric methane consumption, and no evidence to the contrary. The maximum specific CH4 oxidation rate in nitrifier cultures is lower than the minimum rate in any methanotroph (Bedard and Knowles, 1989). Extremely high nitrifier populations would be needed to account for observed methane oxidation rates in soil - much higher populations than are actually present (Jiang and Bakken, 1999; Kolb et al., 2005). Stimulation of nitrification via NH4+ fertilization usually does not stimulate methane oxidation rates (e.g. Bender and Conrad, 1994a). Soil nitrification rates and NH+ and NO- pools are often highest in surface organic soil, while the maximum methane oxidation activity occurs lower down the profile (Castro et al., 1994; Schnell and King, 1994; Kruse and Iversen, 1995). A negative relationship was observed between nitrification rates and methane oxidation rates in several soils (Reay et al., 2005).
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