Figure 3.27 Enrichment commonly leads to a switch from oxygen to (anaerobic) alternatives as a resource for respiration. (a) The proportion of microbial peptides in communities occupying the pitchers of Sarracenia purpurea that were either controls or enriched, originating from microbes with different respiratory modes. (b) The percentage of taxa that were dormant (metabolically inactive) in control and nitrogen‐enriched plots in saltmarshes over four years. Bold lines are medians, boxes represent 25–75 percentiles and whiskers show ranges, with outliers also shown.

      Source: (a) After Northrop et al. (2017). (b) After Kearns et al. (2016).

Similarly, but on a larger scale, enrichment of salt marshes in Massachusetts, USA, led to soil microbial communities in which a much increased proportion of the taxa was dormant, that is, metabolically inactive (Figure 3.27b), but among those that were active, there was a major shift from aerobic to (in this case) obligatorily anaerobic taxa, many using sulphate or sulphur rather than oxygen as their respiratory resource. Clearly the prevalence of dormancy and the presence of facultative anaerobes mean that communities can switch rapidly from a widespread reliance on oxygen to the use of alternative resources for respiration.

      APPLICATION 3.5 Permafrost, methanogenic anaerobic respiration and global warming

      As the earth warms (see Section 22.2) regions of permafrost near the poles (where the soil remains frozen, year‐round, for at least two consecutive years, see Section 1.5) are thawing. This is leading to a transition in these regions initially to ‘palsa’ habitats – mounds in the landscape supporting lichens and low shrubs – then to partly thawed bogs dominated by mosses (Sphagnum spp.), and then to fully thawed mires dominated by sedges (e.g. Eriophorum spp). This transition itself has potential implications for global warming, since it involves a shift from CO₂‐emitting palsas to mires and fens that take up CO₂ but emit methane, a more potent greenhouse gas. High‐methane emitting fen habitats contribute seven times as much greenhouse impact as palsa, per unit area (McCalley et al., 2014). Our understanding of the roles played by the microbial communities of the soils in these habitats remains poor. But this is likely to be crucial if we wish to predict the trajectory of the positive feedback loop through which warming leads to thawing, leading to methane emission, more warming, more thawing, and so on. (In Section 17.3 we discuss permafrost as an example of an ecosystem that, on thawing, can pass a ‘tipping point’, shifting it from one regime to another.)

Microbes that produce methane as a respiratory by‐product are Archaea, not bacteria. Most are hydrogenotrophic, using hydrogen as an electron acceptor. However, there is another smaller, but important group that are acetoclastic, cleaving acetate into methane and CO₂, and the methane produced by the two groups can be distinguished by characteristic isotopic signatures. Over a natural gradient of thawing in northern Sweden, methane emissions were greater from fully thawed mires than from partly thawed bogs, but were also more dominated by acetoclastic methanogens (Figure 3.28). Crucially, this shifting balance was associated in turn with variation in the ratio of methane‐to‐CO₂ production from anaerobic respiration (much higher from mires than from bogs) with consequences in turn for the models currently being used to predict future climate change, which typically assume the fraction of anaerobically metabolised carbon that becomes methane to be fixed (McCalley et al., 2014). Results like those in Figure 3.28 therefore throw doubt on the validity of this simplifying assumption and press the case for further work on the dynamics of anaerobic resource use in these rapidly changing systems.