Oxic water column methanogenesis as a major component of aquatic CH4 fluxes

Abstract

Methanogenesis has traditionally been assumed to occur only in anoxic environments, yet there is mounting, albeit indirect, evidence of methane (CH 4 ) production in oxic marine and freshwaters. Here we present the first direct, ecosystem-scale demonstration of methanogenesis in oxic lake waters. This methanogenesis appears to be driven by acetoclastic production, and is closely linked to algal dynamics. We show that oxic water methanogenesis is a significant component of the overall CH 4 budget in a small, shallow lake, and provide evidence that this pathway may be the main CH 4 source in large, deep lakes and open oceans. Our results challenge the current global understanding of aquatic CH 4 dynamics, and suggest a hitherto unestablished link between pelagic CH 4 emissions and surface-water primary production. This link may be particularly sensitive to widespread and increasing human influences on aquatic ecosystem primary productivity.

Microbial communities reflect temporal changes in cyanobacterial composition in a shallow ephemeral freshwater lake

“…The emergence of acI, in Yanga Lake, subsequent to the bloom (Figure 4e) is consistent with the genomic potential of these organisms to take advantage of cyanobacterial exudates, specifically cyanophycin, following the lysis of cyanobacteria (Ghylin et al, 2014). Albeit contributing a smaller proportion of the total community, methylotrophs betIV and gamI (Figures 4g and h) exhibited similar distributions consistent with observations that the heterotrophic decomposition of cyanobacterial biomass leads to an accumulation of acetate, selecting for taxa capable of acetoclastic methanogenesis under either oxic or anoxic conditions (Bogard et al, 2014). The late emergence of methylotrophs following cyanobacterial blooms is also implicated as contributing to the accumulation of methane derived from archaeal methanogenesis in surface waters .…”

Emission and oxidation of methane in a meromictic, eutrophic and temperate lake (Dendre, Belgium)

“…Such a decrease cannot be related to exchange with the atmosphere since the atmospheric d 13 C-CH 4 is close to À47‰ (Quay et al, 1999). A possible explanation would be CH 4 production in oxic conditions related to primary production by pathways that remain elusive (Tang et al, 2016) as recently reported in several lakes (Grossart et al, 2011;Bogard et al, 2014;Tang et al, 2014). Such an explanation is consistent with the eutrophic nature of the Dendre Lake and should be further investigated in future.…”

Evidence for methane production by the marine algae <i>Emiliania huxleyi</i>

“…Because the surface ocean is also saturated or slightly supersaturated with oxygen, which does not favor methanogenesis, the observed CH 4 supersaturation has been termed the oceanic methane paradox (Kiene, 1991). To explain the source of CH 4 in surface waters, it has been suggested that methanogenesis takes place in anoxic microenvironments of organic aggregates (Grossart et al, 2011;Karl and Tilbrook, 1994;Bogard et al, 2014), the guts of zooplankton or fish (de Angelis and Lee, 1994;Oremland, 1979), and inside bacterial cells (Damm et al, 2015). It has also been shown that contrary to the conventional view, some methanogens are remarkably tolerant to oxygen (Angel et al, 2011;Jarrell, 1985).…”

“…For comparison CH 4 emission rates presented so far for terrestrial plants range from 0.3 to 370 ng g −1 DW (dry weight) h −1 (Keppler et al, 2006;Wishkerman et al, 2011;Lenhart et al, 2015;Brüggemann et al, 2009). …”