Methane oxidation in upland soils is an aerobic microbial process. Aerobic metha-notrophs comprise several phylogeneti-cally distinct clusters of proteobacteria. There are currently 13 described genera, divided by convention into two groups: type I (Methylococcus, Methylocaldum, Methylo-microbium, Methylosphaera, Methylo-monas, Methylobacter, Methylosarcina, Methylothermus and Methylohalobius); and type II (Methylocystis, Methylosinus, Methylocella and Methylocapsa). These differ primarily in phylogenetic affiliation (type I are Gammaproteobacteria, type II are Alphaproteobacteria) and also in several biochemical characteristics, including carbon assimilation pathways (ribulose monophosphate versus serine pathway), the dominant phospholipid fatty acids (PLFAs 16:0, 14:0 and 16:1 versus 18:1) and the geometric arrangement of intracellular membranes (ICM) (Hanson and Hanson, 1996; Bowman, 2000).
Methanotrophs obtain energy via the oxidation of CH4 to CO2. The first step is the introduction of one atom of oxygen into methane. This step is catalysed by MMO, which requires three substrates: CH4, O2 and a reductant for the excess O atom from O2:
The product of MMO, methanol (CH3OH), is sequentially oxidized to formaldehyde (HCOH), formate (HCOOH) and CO2, reactions that contribute to the development of a proton-motive force (Hanson and Hanson, 1996). Two forms of MMO are known to exist: a particulate form (pMMO) for which reduced cytochrome-c is the reductant, and a soluble form (sMMO), which uses NADH + H+. The pMMO is universal to all known methanotrophic genera except Methylocella, while the soluble form is present in Methylocella and a few other type I and type II species (Bowman, 2000; Dedysh et al., 2000). All methanotrophs possessing pMMO also have an extensive ICM system, presumably in order to increase the amount of this primary metabolic enzyme. Of the two MMO forms, sMMO has a broader sub strate specificity and a lower affinity for methane. Synthesis of pMMO requires high concentrations of copper (1 |mmol/g cells) (Hanson and Hanson, 1996).
A major unanswered question is: Which particular methanotrophic species are responsible for the uptake of atmospheric methane in forest soils? If we knew more about the specific bacteria involved, we would certainly have a better understanding of the process and how to manage it properly. However, several lines of evidence indicate that the species most active in upland soils have yet to be isolated. The remainder of this chapter will outline what we know and do not know about methano-trophs in upland soils.
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