Methane oxidation

CH4 produced in water columns and sediments can either be oxidized by methane oxidizing bacteria or archaea, or emitted to the atmosphere (Bastviken et al. 2002; Kankaala et al. 2006a; Kuivila et al.

1988). CH4 oxidation has been detected not only in aerobic (reaction 4) but also anaerobic conditions (Wand et al. 2006), and due to both of these CH4 oxidation processes, the net ecosystem fluxes of CH4 are often significantly lower than the gross rates of methanogenesis (Schlesinger and Bernhardt 2013).

CH

+ 2O

→ CO

+ 2H

O (4)

The proportion of CH4 produced in lakes that is oxidized is estimated to be about 30-99 % (Bastviken et al. 2008). However, the surface waters of boreal lakes can be supersaturated with CH4 especially during spring and autumn turnover periods, and thus some part of CH4 produced may escape methanotrophic bacteria and be released to the atmosphere through ebullition (bubble flux), diffusive efflux or plant-mediated efflux (see Figure 2) (Bastviken et al. 2004). The amount of CH4 released to the atmosphere is generally the difference between the amount produced and the amount consumed by methanotrophs and anaerobic methane-oxidizing bacteria (Hanson and Hanson 1996).

Figure 2. CH4 dynamics and emission pathways in stratified lakes (Bastviken et al. 2004).

Highest CH4 oxidation rates are typically detected at oxic-anoxic sediment-water interfaces, where O2 is available as electron acceptor and CH4 as carbon and energy source (Kankaala et al. 2006a). The CH4 oxidation rates are highest in aerobic conditions, because the energy yield of this process is higher (Caldwell et al. 2008). The summer stratification period is crucial for CH4 consumption, because during the stratification approximately 3-5 times more CH4 is oxidized in the water column than emitted to the atmosphere (Kankaala et al. 2006a). In the study of Bastviken et al. (2008), most of the water column CH4 oxidation occurred in the upper hypolimnion and lower metalimnion.

However, during the summer stratification CH4 oxidation may also appear in the epilimnion (Kankaala et al. 2006a).

In lakes with well-oxygenated water column throughout the year, CH4 oxidation might be more focused on the sediment surface (Hanson and Hanson 1996). According to Duc et al. (2010), CH4

oxidation in sediments is substrate-regulated, depending on the later supply of produced CH4 into the oxygenated zone and the concentrations of oxygen. Also Hershey et al. (2015) concluded that the CH4 oxidation at the sediment-water interface was limited by CH4 availability. They also suggested that in sediments with high oxygen concentrations in overlying water, methanogenesis and CH4

oxidation may occur deeper into the sediment layers, and CH4 formed in deeper sediments can be

already oxidized before diffusing through the sediment-water interface. In the research of Bastviken et al. (2008), about 51-80 % of the CH4 produced in deep sediments was consumed by methanotrophic bacteria, while most of the CH4 produced in the surficial sediments was emitted to the atmosphere.

Thus, oxic surface sediments should also be considered as important sources of CH4 to the atmosphere (Bastviken et al. 2008).

Recent studies have also shown evidence of anaerobic methane oxidation (AOM) in sediments and stratified water columns of freshwater lakes (Borrel et al. 2011). During AOM, CH4 can be oxidized with electron acceptors such as sulphate (SO42-), nitrate (NO3-), nitrite (NO2-) and metals (Timmers et al. 2017). Sulphate-driven AOM is regulated by a consortium of anaerobic methane oxidizing archaea (ANME) and sulphate-reducing bacteria (Knittel and Boetius 2009; reaction 5). In AOM coupled to SO42- reduction, electrons are transferred from ANME to an autotrophic sulphate-reducing partner (Timmers et al. 2017). AOM associated to SO42- reduction is an important sink for CH4 in marine sediments, and this process may occur in freshwater systems as well (Schlesinger and Bernhardt 2013).

CH

+ SO

→ HCO

+ HS

+ H

O (5)

However, since SO42- concentrations are generally low in freshwater lakes, NO3- and also NO2- can be more relevant electron acceptors (Welte et al. 2016). If NO3- is available in anoxic sediments, sulphate-reduction is typically suppressed (Schlesinger and Bernhardt 2013), and CH4 can be used as electron donor by nitrate/nitrite-dependent anaerobic oxidizing bacteria (reactions 6-7) (Welte et al.

2016). This anaerobic oxidation of CH4 is coupled to denitrification (DAOM) (Deutzmann et al. 2014;

á Norði and Thamdrup 2013). á Norði and Thamdrup (2013) found that DAOM was stimulated by NO3- enrichment, but concluded that DAOM will mainly be an important sink of CH4 in oxygen-depleted, nitrate/nitrite-enriched aquatic environments.

3CH

+ 8NO

+ 8H

→ 3CO

+ 4N

+ 10H

O (6) CH

+ 4NO

→ 4NO

+ 2H

O (7)

CH4 oxidation rates are affected by weather conditions and physical factors such as turbulence (Bastviken et al. 2008; Kankaala et al. 2007). High wind speed and cooling may rapidly lead to complete mixing of the water column during autumn (Kankaala et al. 2007). In that case, a higher proportion of CH4 may be emitted to the atmosphere due to enhanced transport of CH4 across the different water layers and short residence time for CH4 in the surface water (Bastviken et al. 2008;

Kankaala et al. 2007). On the other hand, gradual mixing of the water column may also cause the hypolimnetic CH4 to be available for oxidation longer (Kankaala et al. 2007), and thus the CH4

emissions remain low.

CH4 oxidizing bacteria not only reduce atmospheric CH4 emissions, but also offer a suitable food resource for aquatic consumers (Jones and Grey 2011). Several studies (Bastviken et al. 2003; Jones et al. 1999; Kankaala et al. 2006b; Taipale et al. 2008) have examined the role of CH4 oxidizing bacteria as a carbon source for zooplankton in the stratified humic lakes. Kankaala et al. (2006b) showed that methanotrophs provided a food resource for the typical pelagic zooplankton, Daphnia.

They suggested that methane-derived carbon might be a more significant part of the lake food webs than has previously been estimated.