Phytogenesis of halomethanes: A product of selection or a metabolic accident?

Phytoplankton (microalgae), seaweeds (macroalgae), higher plants and fungi produce halomethanes. Algae and fungi produce both methyl halides and polyhalomethanes, whereas plants are known to produce only methyl halides. Why these organisms produce halomethanes is a question frequently asked by chemists and biologists. This question implies that halomethanes have a function and have a selective value to the producing organism. Except for some fungi, the evolutionary advantage of producing halomethanes may not presently exist. Polyhalomethanes are by-products of halogenation of certain organic compounds by haloperoxidases in marine algae and perhaps some fungi, and they may be indirectly produced in aquatic environments by algal release of oxidized halogen species. A main function of this enzyme is to rid the cell of harmful oxidants such as hydrogen peroxide. Monohalomethanes (methyl halides) are products of methyltransferase activity. It has been proposed that methyl halide production may provide a mechanism to regulate chloride levels in halotolerant plants. The examination of halide cellular concentrations, halomethane production rates, and enzyme characteristics raises questions about this possible function. In algae, plants and some fungi, methyl halides may be a result of the insertion of ubiquitous halides into the active site of numerous methyltransferases. Therefore, halomethanes may be by-products or ‘accidents’ of metabolism. Abbreviations: AP = ascorbate peroxidase; BrPO = bromoperoxidase; ClPO = chloroperoxidase; CH3X = methyl halide; CHBr3 = bromoform; CH2Br2 = dibromomethane; CH2I2 = diiodomethane; CHIBr2 = iododibromomethane; CHCl2I = dichloroiodomethane; CH2ClI = chloroiodomethane; CHCl3 = chloroform; DCMU = dichlorophenyl-dimethylurea; DNP = 2-dinitrolphenol; DOM = dissolved organic matter; H2O2 = hydrogen peroxide; IPO = iodoperoxidase; Km = Michaelis-Menten constant; OTM = O-methyltransferase; ppm = part per million (dry); PSI = photosystem 1; PSII = photosystem 2; SAH = S-Adenosylhomocysteine; SAM = S-adenosyl-L-methionine; SOD = superoxide dismutase