Methane dynamics regulated by microbial community response to permafrost thaw

Abstract

Methane dynamics regulated by microbial community response to permafrost thaw. 4,5,16 . The net effect is that the high methane-emitting fen contributes 7 55 times the greenhouse impact per unit area as the palsa. This thaw progression is also associated 56 with an increase in overall organic matter lability, including a decrease in C:N and an increase in 57 humification rates 9 . We hypothesized, consistent with previous studies of in situ bog and fen 58 systems [17][18][19] , that thaw progression also facilitates a shift from hydrogenotrophic to acetoclastic 59 CH 4 production. 60We used the distinct isotopic signatures of different microbial CH 4 production and 61 consumption pathways to directly relate changes in CH 4 dynamics across the thaw gradient to 62 underlying changes in the microbial community. Methane produced by hydrogenotrophic 63 methanogens generally has lower  13 C and higher D ( 13 C = -110 to -60‰ and D = -250 to -64 170‰) relative to that produced by acetoclastic methanogens ( 13 C = -60 to -50‰ and D = -400 65 to -250‰) 19,20 . If methanotrophic microbes then oxidize CH 4 , lighter molecules are 66 preferentially consumed, leaving the remaining CH 4 13 C-and D-enriched relative to the original 67 CH 4 pool (see expected patterns in Extended Data Fig 1) 19 . Greater fractionation is associated with hydrogenotrophic methanogenesis, and was 85 found in the thawing Sphagnum site (average  C = 1.053 ± 0.002). Significantly less 86 fractionation (p=0.002) associated with more acetoclastic production or with consumption by 87 oxidation was found in the fully thawed Eriophorum porewater (average  C = 1.046 ± 0.001). 88Here, increases in acetoclastic production, not oxidation, best explain isotopic shifts because 89 lower  C and higher  13 C-CH 4 are accompanied by significantly lower D-CH 4 (Extended Data 90 Fig. 1, p< 0.001) 19 . This is consistent with the pattern of isotopes in CH 4 emissions as well as 91 incubations of Stordalen peat 9 and studies showing bog-to-fen shifts from hydrogenotrophic to 92 acetoclastic methanogenesis [17][18][19] . 93The CH 4 flux and isotope results provide compelling but indirect evidence for changes in 94 CH 4 -cycling microbial communities with permafrost thaw. These microbiological changes could 95 be shifts in activity of particular community members or changes in community composition. We 96 examined the role of community composition through 16S rRNA gene amplicon sequencing. All 97 known methanogens belong to a small number of archaeal lineages within the Euryarchaeota 23 . 98As expected, the shift from CH 4 -neutral intact permafrost palsa to CH 4 -emitting wetland 99 corresponded to a substantial increase in the relative abundance of methanogenic archaeal 100 lineages (Fig. 1c, Extended Data Table 2,3). In the aerobic palsa and surface Sphagnum habitats, 101 methanogens were found in low relative abundance (average <0.6%), while the anaerobic 102 environments of the Eriophorum and deeper (below the water table) Sphagnum habitats harbored 10...

The global methane budget 2000–2012

“…Laser-based absorption spectrometers and isotope ratio mass spectrometry techniques have recently been developed to increase sampling frequency and allow in situ operation (McManus et al, 2010;Santoni et al, 2012). Measurements of δ 13 CH 4 can help to partition the different methanogenic processes of methane: biogenic (−70 to −55 ‰), thermogenic (−55 to −25 ‰) or pyrogenic (−25 to −15 ‰) sources (Quay et al, 1991;Miller et al, 2002;Fisher et al, 2011) or even the methanogenic pathway (McCalley et al, 2014). δD(CH 4 ) provides valuable information on the oxidation by the OH radicals due to a fractionation of about 300 ‰.…”

“…Indirect methane emissions are probably more important. They rely on the following: (1) methanogenesis induced when the organic matter contained in thawing permafrost is released; (2) the associated changes in land surface hydrology possibly enhancing methane production (McCalley et al, 2014); and (3) the formation of more thermokarst lakes from erosion and soil collapsing. Such methane production is probably already significant today and could be more important in the future associated with a strong positive feedback to climate change.…”

Environmental correlates of peatland carbon fluxes in a thawing landscape: do transitional thaw stages matter?

“…In addition to magnitude, the dominant pathway of CH 4 production is also altered after thaw, shifting from CO 2 reduction (hydrogenotrophic) to acetate cleavage . (acetoclastic; Hodgkins et al, 2014;McCalley et al, 2014). The change in pathway is likely related to shifts in vegetation, for example, a decrease in Sphagnum abundance could lead to an increase in pH and related increase in acetoclastic methanogens (Hines et al, 2008;Ye et al, 2012).…”

The geochemistry of methane isotopologues

“…Carbon ( 13 C/ 12 C) and hydrogen (D/H) isotope ratios of methane are widely applied for distinguishing microbial from thermogenic methane in the environment (Welhan and Lupton, 1987;Whiticar, 1990;Sherwood Lollar et al, 2002;Flores et al, 2008;Sherwood Lollar et al, 2008;Pohlman et al, 2009;Baldassare et al, 2014) as well as for apportioning pathways of microbial methane production (Whiticar et al, 1986;Burke et al, 1988;McCalley et al, 2014). This bulk isotope approach, however, is largely based on empirical observations, and different origins of methane often yield overlapping characteristic isotope signals (Schoell, 1988;Whiticar, 1990; Whiticar, 1999;Pohlman et al, 2009;Etiope and Sherwood Lollar, 2013).…”