Microbial mechanisms controlling methane-temperature hysteresis in wetlands.

Methane (CH₄)-temperature hysteresis, i.e., significantly higher CH₄ emissions in autumn compared to spring at equivalent temperatures, have been observed in wetlands globally. However, the biological basis for these seasonal changes in the effect of temperature on wetland CH₄ emissions remain unexplained.

Peat soil from four Arctic and sub-Arctic sites: Svalbard, Northern Norway, Arctic Canada, and Greenland, were collected for investigations of the mechanisms behind CH₄-temperature hysteresis. In the laboratory, under anoxic condition, the peat soils were exposed to temperature changes in weekly 2 °C increments, from 2 °C to 10 °C and back to 2 °C, simulating an Arctic spring to autumn transition. Methane accumulation rates, methanogen substrate concentrations, total microbial and archaeal RNA and DNA content, and community composition and size were monitored throughout the experiments.

Three of the four examined soils (Svalbard, Northern Norway and Greenland) expressed significantly higher CH₄ production rates during cooling compared to warming. This observation of CH₄-temperature hysteresis under anoxic condition demonstrate that CH₄-temperature hysteresis can result from anaerobic processes, while experiment replication demonstrated that CH₄-temperature hysteresis, or the lack of it, were reproducible for the respective peatlands.

The timing and extent of accumulation and depletion of methanogenic substrates and the enhanced methane production rates during cooling in the three CH₄ hysteresis-positive soils suggested that the methanogenic community itself, triggered by high substrate availability and a sufficient maximum temperature, is the major driver of CH₄-temperature hysteresis. Furthermore, the observation that both soils dominated by acetoclastic (Svalbard and Greenland) and hydrogenotrophic (Northern Norway) methanogens can express CH₄-temperature hysteresis, demonstrate that hysteresis is not restricted to one methanogenic pathway.

As only minor changes in the methanogenic community composition were observed during the experiments, CH₄-temperature hysteresis was indicated to result from physiological responses of the existing methanogenic community. In the Svalbard soil, increased methanogen population sizes, as indicated by qPCR, suggested faster methanogen growth rates during cooling, potentially explaining hysteresis, but this effect was not observed in the remaining two hysteresis positive soils. Thus, other physiological rate-increasing mechanisms are also required to explain hysteresis. Correspondingly, increased expression of genes for rate-limiting enzymes in methanogenesis, as a response to temperature and substrate increase, were demonstrated in a separate heating experiment (2 °C to 10 °C) done on Svalbard peat soil.

We propose the following CH₄-temperature hysteresis mechanism: Temperature induced imbalances between fermentation and methanogenesis at low temperatures and during heating leads to high methanogen substrate concentrations. The subsequent combination of excess substrate and reaching sufficiently high temperatures promote methanogen activity through faster growth and the buildup of rate-limiting enzyme pools for methanogenesis in the form of more new cells or larger enzyme stocks per cell. This expansion of the methane production bottleneck allows enhanced CH₄ production rates during subsequent cooling, until the depletion of substrate pools.