Chloromethane sinks

2.2.1. Abiotic degradation of chloromethane

Different abiotic sinks of chloromethane, which vary in their ability to eliminate chloromethane, have been identified (Tableau 1.4). Even though an abiotic degradation of VOCs has been observed in soils, abiotic degradation of chloromethane has not been described (Miller et al., 2004; Insam and Seewald, 2010).

Table 1.4. Estimation of atmospheric chloromethane sinks

Sink type Estimation

(Gg. year^-1)^a

Low/high values (Gg. year^-1)^b

Tropospheric reaction with OH -3,180 -3800/ -4,100

Stratospheric loss -200 -100/ -300

Reaction with Cl^- in marine boundary layer -370 -18/ -550 Microbial degradation in soil < -1,000 -100/ -1,600

Cold ocean water loss -75 -93/-145

Total sinks < -4,875 -4273/ -6,695

a Data from Keppler et al., 2005

b Data from Clerbaux et al., 2007

2.2.2. Biotic degradation of chloromethane

Biotic degradation of chloromethane is mainly due to microorganisms activity, although its estimation is variable (from 180 to 1,600 Gg.an^-1) (Keppler et al., 2005).

− Degradation by bacteria in marine environments

Marine environments play a role in the chloromethane degradation. Bacterial strains able to utilize chloromethane have been isolated from marine environments, such as Leisingera methylohalidovorans MB2, the first class of Rhodobacteracea shown to be able to degrade chloromethane (Goodwin et al., 1997; Schaefer; 2002, Tableau 1.5). Three other chloromethane-degrading strains of this family have been isolated from marine environments: Roseovarius sp. 179, Roseovarius sp. 217 and Ruegeria sp. 198 (Schäfer et al., 2005; Tableau 1.5).

Table 1.5. Chloromethane-utilizing bacterial isolated from contrasting environments

Bacterial strain Origin Gram

type Metabolism Trophic type Presence

of cmuA^a Reference Acetobacterium dehalogenans MC Activated sludge Positive Anaerobic Homoacetogenic nd Traunecker et al., 1991

Aminobacter ciceronei IMB1 Fumigated strawberry field Negative Aerobic Facultative methylotroph Yes Hancock et al., 1998 Aminobacter lissarensis CC495 Beech woodland soil Negative Aerobic Facultative methylotroph Yes Coulter et al., 1999

Hyphomicrobium sp. AT1 Phyllosphere Negative Aerobic Facultative methylotroph Yes Nadalig et al., 2011 Hyphomicrobium sp. AT2 Phyllosphere Negative Aerobic Facultative methylotroph Yes Nadalig et al., 2011 Hyphomicrobium sp. AT3 Phyllosphere Negative Aerobic Facultative methylotroph Yes Nadalig et al., 2011 Hyphomicrobium sp. AT4 Phyllosphere Negative Aerobic Facultative methylotroph Yes Nadalig et al., 2011 Hyphomicrobium sp. MC1 Industrial sewage plant Negative Aerobic Facultative methylotroph Yes Hartmans et al., 1986 Hyphomicrobium sp. MC2 Soil from a petrochemical factory Negative Aerobic Facultative methylotroph Yes Doronina et al., 1996 Leisingera methylohalidovorans MB2 Marine tide pool Negative Aerobic Facultative methylotroph No Schaefer, 2002

Methylomicrobium album BG8 Fresh water Negative Aerobic Obligatory methylotroph nd Han and Semrau, 2000 Methylobacterium extorquens CM4 Soil from a petrochemical factory Negative Aerobic Facultative methylotroph Yes Doronina et al., 1996

Pseudomonas aeruginosa NB1 Activated sludge Negative Anaerobic Facultative methylotroph nd Freedman et al., 2004 Roseovarius sp. strain 179 Coastal seawater Negative Aerobic Facultative methylotroph Yes Schäfer et al., 2005 Roseovarius sp. strain 198 Coastal seawater Negative Aerobic Facultative methylotroph Yes Schäfer et al., 2005 Roseovarius sp. strain 217 Seawater Negative Aerobic Facultative methylotroph Yes Schäfer et al., 2005

a nd; not determined

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− Bacterial degradation in the phyllosphere

The aerial parts of plants constitute the phyllosphere, which is home for many microorganisms. The total leaf surface (bottom and upper side) corresponds to twice the surface of Earth, with a bacterial density of 10⁷.cm^-2 (Vorholt, 2012). Few studies have studied so far the role of phyllospheric microorganisms in the degradation of plant-emitted chloromethane. Some bacteria able to utilize chloromethane have been isolated from the surface of leaves of A. thaliana (Nadalig et al., 2011). Nevertheless, the potential role of these epiphytic bacteria as biotic filters for chloromethane has so far not been taken into account.

− Bacterial degradation in soils

Soil contains a large variety of compounds, such as halogenated compounds. Soil has been shown to be major sink for some of halogenated compounds, and for instance 70% of bromomethane (CH3Br) is consumed by soils (Shorter et al., 1995). Although chloromethane degradation in soils is misevaluated, it is estimated to exceed 1,000 Gg. year^-1 (Tableau 1.4).

Chloromethane consumption in soil has been correlated to bacterial and fungal degradation mainly in the O horizon with was found to be very weak in lower horizons (Redeker and Kalin, 2012).

The exchange of chloromethane between the terrestrial ecosystems and the atmosphere are modulated by bacteria living in soil (Miller et al., 2004; Borodina et al., 2005; Keppler et al., 2005; Clerbaux et al., 2007; Schäfer et al., 2007; Rhew et al., 2010). A variety of bacteria affiliated to Alpha- or Beta-proteobacteria that are able to utilize chloromethane have isolated from forest soils, but their contribution to the chloromethane budget remains to be evaluated (Miller et al., 2004; Borodina et al., 2005).

Chloromethane consumption has also been demonstrated in fungi via reactions with produced secondary metabolites including more than 200 halogenated metabolites (Gribble, 2003) such as brominated, fluorinated, iodinated, and chlorinated compounds (Anke et Weber, 2006). Among chlorinated metabolites, chloromethane can be released during methylation reactions in fungi (Figure 1.8). Methylation is a common reaction of fungal metabolism (Anke and Weber, 2006).

Figure 1.8. Chloromethane degradation reactions by fungi (Modified from Anke and Weber, 2006)

Methylation in fungi involves SAM, as a donor of the methyl group, as found for chloromethane synthesis in plants. In fungi, the utilization of chloromethane is preferred over SAM, since the reaction needs less energy (Harper, 2000). When chloromethane serves as a methyl group donor to an hydroxyl (-OH) or carboxyl (-COOH) group, products such as veratryl alcohol or methyl benzoate are produced (Figure 1.8). Veratryl alcohol is a secondary metabolite involved in the synthesis of enzymes that are necessary for lignin degradation (Dekker et al., 2001). Methyl benzoate is an olfactive compound synthesized to attract pollinators, and acts in inhibition of A. thaliana root growth (Horiuchi et al., 2007).

Chloromethane balance in fungi is difficult to determine since they are able to simultaneously produced and degraded chloromethane (Anke and Weber, 2006).

Although the role of soil as a chloromethane sink does not need to be further evidenced, uncertainties persist about global chloromethane fluxes in soils (see section below).