Bacteria associated with eukaryotic hosts can affect host fitness, but the extent to which bacteria influence eukaryotic species interactions is mostly unknown. To determine how prokaryotes influence the strength of species interactions, we used phytoplankton, which are a classic model for evaluating species interactions. To quantify the effects of bacteria on interactions among their hosts, we rendered phytoplankton species free of bacteria (i.e. axenic phytoplankton), while also maintaining their xenic counterparts. We first confirmed that bacteria isolated from these laboratory-grown phytoplankton species affected host fitness (growth rate and carrying capacity). We then tested whether host microbiomes alter the sensitivity of its host to interspecific interactions, using the mutual invasibility criterion to perform all pairwise, reciprocal invasion experiments between four species of xenic and axenic phytoplankton. We used one species pair with consistently positive sensitivity values to calculate niche differences and relative fitness differences, which are two forces known to affect coexistence under scenarios of competition.
Results/Conclusions
We found most bacterial isolates within microbiomes altered host growth rates and carrying capacities, but the effects were algal host and bacterial symbiont-dependent. In the mutual invasibility experiments, we observed two invading species of phytoplankton that experienced competition, one invading species that experienced no interaction, and one invading species that experienced facilitation. Host microbiomes reduced the sensitivity of phytoplankton to interspecific interactions in 6 of 12 pairwise comparisons. Although microbiomes frequently changed the magnitude of interspecific interactions, bacteria rarely altered the type of ecological interaction observed (i.e., competition vs. facilitation). We found that the host microbiome increased NDs from 0.67 ± 0.03 (mean ± s.e.) within the axenic treatment to 0.78 ± 0.02 s.e. within the xenic treatment, but had no significant effect on RFDs. Our results suggest that host microbiomes tended to increase host growth rate when rare, suggesting a sizable role of host-associated bacteria in the competitive and facilitative interactions that maintain species biodiversity. These findings indicate that microbiomes modulate their host species’ interactions and have the potential to control eukaryotic species diversity and community composition.