Dominance of eutrophic generalist species after microbial community coalescence events

Sara Beier, Leibniz Institute for Baltic Sea Research and Guilherme P. de Moraes, Department of Botany, Universidade Federal de São Carlos, São Carlos, Brazil

Background/Question/Methods
Microbial coalescence events, defined as the mixing of entire communities, occur frequently in nature. However, little is known about how properties and traits of the members in the original communities influence microbial interactions and, consequently, the functional properties of the hybrid community. In this study, communities from two contrasting aquatic environments concerning both temporal heterogeneity and trophic status were mixed and incubated for 5 days under two different DOM regimes. These DOM regimes represented, based on their stoichiometry, more oligotroph or more eutrophic conditions. Additionally, unmixed communities were grown under both DOM regimes as control incubations. We assumed that members of the oligotroph site would dominate the mixed community if incubated under the oligotroph DOM regime and vice versa.
Results/Conclusions
In contrast to our expectations, members of the eutrophic sampling site always dominated the mixed communities. Indeed, after 5 days of incubation, community compositions of all treatments that contained members of the eutrophic community were highly similar to the original community of the eutrophic site. As a consequence, parameters describing co-occurrence network architecture in the mixed communities, as well as the measured functional parameters, mostly resembled that of the eutrophic community and little impact of interactions with the oligotrophic community members was detectable. We inferred that the eutrophic site, which was simultaneously characterized by high temporal environmental heterogeneity, selected for generalists and r-strategists with short lag phases and high growth rates that were able to outcompete the community members of the oligotrophic sample site regardless of the provided DOM substrate. The prevalence of highly competitive generalist taxa in the eutrophic site was supported by the finding of a high average genome size and an elevated fraction of transcription factors in the community members, which are genomic traits that have been discussed as possible indicators for high metabolic versatility. We furthermore detected high codon usage biases and increased 16s rRNA gene copy numbers in the eutrophic community members, which are genomic traits that have earlier been linked to competitive advantage during the colonization of environments. Beyond the characteristics of individual community members, high species diversity may also protect communities from invading species. However, since we detected higher species diversity in the community from the oligotrophic site, it seems that species characteristics were, in our case, more important to predict the fate of community members in the hybrid communities than the species diversity levels of the initial unmixed communities.