Chloromethane formation and degradation in the fern phyllosphere

Highlights: • Ferns both produce and degrade atmospheric CH₃Cl with large individual variations. • Ferns degrade CH₃Cl at rates ranging from 0.3 to 17 μg·g_{(dry weight)}⁻¹ day⁻¹. • CH₃Cl degradation was correlated to a large ε_C and almost no ε_H isotope effect. • Involvement of the bacterial cmu pathway in CH₃Cl degradation was not detected. • Still unknown CH₃Cl biodegradation processes in plants contribute to the CH₃Cl cycle. Chloromethane (CH₃Cl) 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 CH₃Cl budget are approximate. The strength of the CH₃Cl global sink by microbial degradation in soils and plants is under discussion. Some plants, particularly ferns, have been identified as substantial emitters of CH₃Cl. Their ability to degrade CH₃Cl 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 CH₃Cl by measuring their production and consumption rates and their stable carbon and hydrogen isotope signatures. Investigated ferns are able to degrade CH₃Cl at rates from 2.1 to 17 and 0.3 to 0.9 μg g_dw⁻¹ day⁻¹ for C. cooperi and D. filix-mas respectively, depending on CH₃Cl supplementation and temperature. The stable carbon isotope enrichment factor of remaining CH₃Cl was −39 ± 13‰, whereas negligible isotope fractionation was observed for hydrogen (−8 ± 19‰). In contrast, O. regalis did not consume CH₃Cl, but produced it at rates ranging from 0.6 to 128 μg g_dw⁻¹ day⁻¹, 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 CH₃Cl biodegradation processes on plants play an important role in global cycling of atmospheric CH₃Cl.