Page 171 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

methanotrophic community, at both temperature conditions. Type I methanotrophs, from the family Methylococcaceae, increased in relative abundance at both temperatures, while Type II methanotrophs were not observed in the incubations. The provided high concentrations of CH4 might have created a niche for type I methanotrophs. The dominance of type I methanotrophs was also found in the active layer of permafrost from the Lena Delta, Siberia (Wagner et al. 2005), lake sediments of Lake Qalluuraq, Alaska (He et al. 2012), soils from Ellesmere Island, Canada (Martineau, Whyte and Greer 2010), and the active layer of permafrost in the Canadian High Arctic (Yergeau et al. 2010). Interestingly, the relative abundance of the potential methylotrophs affiliated with the non-CH4 utilizing methylotrophs of the order Methylophilales increased in the enrichments. The co-occurrence of gammaproteobacterial methanotrophs and non-CH4-utilizing methylotrophs have been seen before in batch incubations with lake and Arctic sediments amended with CH4 (Beck et al. 2013; Crevecoeur et al. 2015; Hernandez et al. 2015; Oshkin et al. 2015). The applied high concentrations of CH4 and long incubation time could have led to high activity of the methane monooxygenase enzyme resulting in MeOH release, enabling cross feeding from methanotrophs to these methylotrophs (Martineau, Whyte and Greer 2010; He et al. 2012; Kalyuzhnaya et al. 2013; Tavormina et al. 2017). Stable isotope analysis have shown that carbon from CH4 does get transferred to Methylophilales (Hutchens et al. 2004; Beck et al. 2013; Kalyuzhnaya et al. 2013), but the exact mechanism is still unknown. The most abundant non-methanotrophic bacterial order was Flavobacteriales. It might be possible that the DNA extraction and 16S rRNA gene amplification is biased towards Flavobacteriales resulting in a high abundance. However, the co-occurrence of Flavobacteriales with methanotrophs has also been observed in other methanotrophic incubations (Barbier et al. 2012; Beck et al. 2013; Hernandez et al. 2015; Oshkin et al. 2015) and in thawing permafrost soils (Coolen and Orsi 2015). Ho et al., (2014) even observed a correlation between CH4 oxidation and heterotrophic richness. The role of these heterotrophs in the community is still elusive but interactions with CH4 oxidizing communities have been found with stable isotope probing (Qiu, Conrad and Lu 2009). They are likely feeding on accumulated organic compounds such as organic acids and polymeric substances excreted by methanotrophs and other community members or coming from dead and lysed cells (Wendlandt et al. 2010; Kalyuzhnaya et al. 2013; Ho et al. 2014; Yu and Chistoserdova 2017). Further work
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