Chloromethane formation and degradation in the fern phyllosphere

Abstract

Chloromethane (CHCl) is the most abundant halogenated trace gas in the atmosphere. It plays an important role in natural stratospheric ozone destruction. Current estimates of the global CHCl budget are approximate. The strength of the CHCl global sink by microbial degradation in soils and plants is under discussion. Some plants, particularly ferns, have been identified as substantial emitters of CHCl. Their ability to degrade CHCl remains uncertain. In this study, we investigated the potential of leaves from 3 abundant ferns (Osmunda regalis, Cyathea cooperi, Dryopteris filix-mas) to produce and degrade CHCl by measuring their production and consumption rates and their stable carbon and hydrogen isotope signatures. Investigated ferns are able to degrade CHCl at rates from 2.1 to 17 and 0.3 to 0.9μggday for C. cooperi and D. filix-mas respectively, depending on CHCl supplementation and temperature. The stable carbon isotope enrichment factor of remaining CHCl was -39±13‰, whereas negligible isotope fractionation was observed for hydrogen (-8±19‰). In contrast, O. regalis did not consume CHCl, but produced it at rates ranging from 0.6 to 128μggday, with stable isotope values of -97±8‰ for carbon and -202±10‰ for hydrogen, respectively. Even though the 3 ferns showed clearly different formation and consumption patterns, their leaf-associated bacterial diversity was not notably different. Moreover, we did not detect genes associated with the only known chloromethane utilization pathway "cmu" in the microbial phyllosphere of the investigated ferns. Our study suggests that still unknown CHCl biodegradation processes on plants play an important role in global cycling of atmospheric CHCl.

Response to referee#2

“…b Greule et al 2012; please note that all values provided for CH 3 Cl released from dried plants at elevated temperatures have been corrected by −23 ‰ due to recalibration of the reference gas. c Jaeger et al (2018b). d Li et al (2017).…”

“…However, current estimates of the CH 3 Cl global budget and the apportionment between sources and sinks are still highly uncertain. Known natural sources of CH 3 Cl include tropical plants (Yokouchi et al, 2002(Yokouchi et al, , 2007Umezawa et al, 2015), wood-rotting fungi (Harper, 1985), oceans (Moore et al, 1996;Kolusu et al, 2017), plants of salt marshes (Rhew et al, 2000(Rhew et al, , 2003, aerated and flooded soil (Redeker et al, 2000;Keppler et al, 2000), senescent leaves and leaf litter Derendorp et al, 2011) and wildfires. Anthropogenic CH 3 Cl release to the atmosphere comes from the combustion of coal and biomass with minor emissions from cattle (Williams et al, 1999) and humans (Keppler et al, 2017).…”

Methyl sulfates as methoxy isotopic reference materials for δ13C and δ2H measurements

“…For example, the information gained from the isotope signatures of plant methoxy groups can be applied for climate reconstructions of various climate archives such as tree rings, sediments and peat cores on time scales from the Anthropocene until the Eocene . Furthermore, they are useful in food science for studies on the authenticity of vanillin, in atmospheric sciences to investigate the origin and fate of C 1 volatile organic compounds such as methanol, chloromethane and bromomethane in the atmosphere and in biochemical investigations to better understand bio‐methylation processes …”

“…For example, the information gained from the isotope signatures of plant methoxy groups can be applied for climate reconstructions of various climate archives such as tree rings, sediments and peat cores on time scales from the Anthropocene until the Eocene. [18][19][20][21][22][23] Furthermore, they are useful in food science for studies on the authenticity of vanillin, 17,[24][25][26] in atmospheric sciences to investigate the origin and fate of C 1 volatile organic compounds such as methanol, chloromethane and bromomethane in the atmosphere 3,6,8,27,28 and in biochemical investigations to better understand bio-methylation processes. 29,30 Stable carbon and hydrogen isotope ratios of methoxy groups from various plant origins have been determined by site-specific natural isotope fractionation nuclear magnetic resonance (SNIF-NMR) and isotope ratio mass spectrometry (IRMS).…”

Isotopic Characterization (2H, 13C, 37Cl, 81Br) of the Abiotic Sinks of Methyl Bromide and Methyl Chloride in Water and Implications for Future Studies.

“…For methyl chloride and methyl bromide no multi-element isotopic studies are available yet. Apart from carbon isotope analysis, hydrogen isotope 10 measurements have been demonstrated for CH3Cl (Nadalig et al, 2013;Keppler et al, 2018;Jaeger et al, 2018). For CH3Br bromine isotope analysis was presented in two studies (Horst et al, 2013;Horst et al, 2014).…”