Discussion
Here, we attempted to determine the role of very rapid (co)evolution of taxa within a complex methanogenic community in shaping community methane production. Methane production was increased by the addition of a pre-adapted community, and this increase could not be explained by direct changes in community composition.
Taken together, our findings data strongly suggests that only rapid evolution, i.e within taxon variation, can explain the observed increase in gas production, either directly, or indirectly through the resultant changes in community composition. We emphasise that taxa here are defined as 16S rRNA Amplicon Sequence Variants with <99% similarity in the 254 bp V4 region of the gene. Within-ASV variation, on which selection acted, may have been standing variation or have arisende novo by mutation or horizontal gene transfer, including infection by mobile genetic elements. Unfortunately, current sequencing and analytical approaches preclude identification of anything but a tiny subset of genetic changes associated with microevolution within such complex communities in the absence of target genes or multiple reference genomes of each taxon , so we are unable to directly quantify the extent of molecular evolution of individual community members.
It is unclear how far our results are generalisable to different systems. Rapid evolution could potentially play an even more important role in high nutrient, aerobic environments, where microbial population growth rates are much faster because of greater energy availability . That said, we used this experimental system because we anticipated rapid evolution would play a particularly important role in altered community functions. Methanogenic communities have a linear biochemistry, funneling all the substrates to methane, and as a consequence selection on many taxa likely acts in the same direction to ultimately increase methane production, especially when a community is introduced to a novel environment .
Irrespective of whether our findings are limited to methanogenic communities, our results have relevance for understanding how methane production may change in the face of environmental change. In addition, from a more practical point, our novel approach could be used to enhance industrial biogas production by, for example, pre-adapting industrial communities to a feed shift. If our findings can be confirmed in other non–AD systems, it opens doors for further applications, including pre-adaption of gut microbiomes to help coping with gut-related illnesses, rather than recruiting already adapted species by random processes like fecal transplant , as a tool in harnessing rhizosphere microbes to improve agricultural yields or to study the impact of rapid evolution on adaptation to the warming planet .