Chloromethane formation and degradation in the fern phyllosphere.

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.