In their study published in this issue, Kang and colleagues have addressed a very important question: whether copper chelators secreted by aerobic methanotrophs, which play a major role in controlling methane accumulation in the atmosphere, might serve as “public goods”, akin to siderophores and other secreted compounds [1], with important environmental implications. These would include a potential competition for copper among different types of methanotrophs, whether producing such public goods or not, and also competition between methanotrophs and other organisms requiring copper for vital functions, such as ammonia oxidizers, thus effecting biogeochemical cycling of both carbon and nitrogen. The methanotrophs require high supplies of copper as an essential cofactor of the main enzyme in the aerobic methanotrophy: particulate methane monooxygenase [2]. It has been nearly two decades since methanobactin (MB) was discovered to serve as a copper chelator (a “chalkophore”, χαλκός is Greek for copper) to supply this enzyme [3]. Such copper chelating compounds, of which very few are chemically characterized, are small peptides that are similar to siderophores, which are well-studied iron-binding peptides [2]. In alphaproteobacterial methanotrophs of the Methylocystaceae family, two classes of MBs have been characterized, with somewhat different properties and varying affinities for copper [2]. Known MBs are secreted into the environment where they bind/solubilize/reduce copper and then transport it into cells, via TonB type transporters that are encoded as parts of the gene clusters involved in MB precursor synthesis and modification [2]. These chelators seem rather promiscuous, potentially binding a variety of metals beside copper, including iron and gold, but their synthesis nevertheless is strictly regulated by the availability of copper; genes for their synthesis are only expressed under copper-limited conditions [2]. Interestingly, due to their promiscuity, MBs have been implicated in playing a role in binding and transporting methylmercury (CH3Hg+), a potent neurotoxin, thus potentially playing a role in its detoxification in certain environmental niches [4]. The mechanism for CH3Hg+ degradation by methanotrophs remains elusive, but methanol dehydrogenase, an important enzyme for methanotrophy, may be involved [4].

Despite their important role for methanotrophs, MBs are not the only mechanism used to secure copper for the essential reaction of methane oxidation. Several methane oxidizers employ a protein named MopE/MopE*, so far only characterized in Methylococcus capsulatus, which is a gammaproteobacterial methanotroph. This protein also has high affinity for copper and efficiently transports it into the cell [5], thus, it is another type of chalkophore. At this time, the mechanism for its transport has not been established experimentally. In addition, poorly characterized secreted substances have been reported for several other gammaproteobacterial methanotrophs, distinct from MBs, representing additional type(s) of chalkophores. While the machinery for MB synthesis and secretion has not been recognized in those corresponding genomes [6], the mechanism for synthesis of these poorly characterized chalkophores remains completely unknown.

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