13.7 Release of chlorohydrocarbons to the atmosphere by wood decay fungi

The ability of fungi, particularly white rot wood decay fungi, to metabolise chlorinated hydrocarbons is not an exotic aspect of their biochemistry. Far from it. It is a normal every-day reaction during the degradation of timber. Some of the small organic molecules (‘mediator’ compounds or co-substrates) these fungi produce contribute to the H2O2-regenerating system essential for activity of the lignin and manganese peroxidases incorporate chlorine from the wood into chloroaromatics (already mentioned in the section on Lignin degradation in Chapter 10; CLICK HERE to view the page). The synthesis of veratryl alcohol requires chloromethane (CH3Cl) which acts as a methyl donor.

When these volatile compounds are released into the atmosphere they flush the chlorine out of the substrate, but they may act as environmental pollutants. In most species, production of chloromethane is tightly coupled with its consumption so detectable quantities are not released by laboratory cultures. Excessive chloromethane production has been observed in several fungi. For example, Phellinus pomaceus (also known as P. tuberculosus), the Cushion Bracket which is weakly parasitic on many common bushes and trees, can release over 90% of the chloride ions of its substrate as chloromethane (Anke & Weber, 2006; Konuma et al., 2015; Weigold et al., 2016).

Unlike other greenhouse gases (such as chlorofluorocarbons or CFCs), chloromethane originates mainly from natural rather than human activities. About 5 million tonnes of chloromethane are released annually worldwide, making chloromethane the most abundant halocarbon gas in the outer atmosphere (stratosphere). Terrestrial sources of CH3Cl predominate; the major ones being biomass burning, wood-rotting fungi, coastal salt marshes, tropical vegetation, and decomposition of soil organic matter and reaction between plant pectin and chloride ion (which forms chloromethane abiotically at ambient temperatures in senescent leaves and leaf litter) (Keppler et al., 2005; Tsai, 2017; Misztal et al., 2018).

The fungal chloromethane contribution has been estimated at around 150,000 tonnes per annum (Watling & Harper 1998), which is about the same as world salt marshes but 60% more than is released into the atmosphere by industrial coal burning furnaces worldwide. Chloromethane is a powerful greenhouse gas and atmospheric pollutant which contributes significantly to the destruction of stratospheric ozone. Its contribution to climatic change remains to be established.

However, on a global scale, decomposition of dead wood releases billions of tons of CO2 to the atmosphere each year, a similar magnitude, in fact, to the annual CO2 emissions from fossil fuel combustion. According to the study by Rinne-Garmston et al. (2019), temperature was the main overall control on the respiration rate of dead wood in Norway spruce-dominated forest in Finland. Consequently, increased decomposition of the estimated total of 36-72 billion metric tons of carbon represented in coarse woody debris in the world’s forests might be expected as climate change increases environmental temperatures. The relationship is complex, however, as the decomposition rate of dead wood is also significantly dependent on local factors, such as the nature of the woody debris and its fungal community, nitrogen availability, and soil moisture status (Rinne-Garmston et al., 2019).

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Updated July, 2019