Investigating the consequences of host-microbiomne feedback versus host innate selection on phytoplankton micrbiome assemblages

Background/Question/Methods

Phytoplankton microbiomes influence the diversity and the performance of primary producers that affect many ecosystem functions in the natural aquatic system. Phytoplankton microbiome assemblages can be shaped by the composition of their host’s dissolved organic matter (DOM) that is host species and growth phase-specific (referred to as host-innate selection based on DOM produced in absence of a microbiome). The microbiome also influences host physiological state and thus changes the produced DOM composition (the host-microbiome feedback). Here, we set out to deconvolute the effects of both forces on the microbiome assemblage of the green alga Chlorella sorokiniana. We used varying ratios of initial host-microbiome feedback versus initial host-innate selection effects (here after, initial feedback-to-host ratio) by inoculating algal microbiomes into six ratios (0%-to-100%, 5%-to-95%, 25%-to-75%, 50%-to-50%, 75%-to-25%, and 100%-to-0%) of bacterial-free algae culture but without initial algal DOM (representing the host-microbiome feedback) versus bacterial-free algae-produced DOM that without host (representing the host innate selection). Five percent of cultures were transferred to the fresh medium with the same level of initial feedback-to-host ratio every other day for 12 days. The rest of the 95% cultures were collected and followed by 16S rRNA gene sequencing processes and analysis for revealing algal microbiome composition.

Results/Conclusions

We hypothesized that host-microbiome feedback would select for bacterial species that are able to interact with the host and trigger algal DOM composition change (e.g. algal mutualistic partners), while host-innate selection would favor the strong innate algal DOM competitors. Therefore, we expected these two effects would select for different bacterial species (Hypothesis I) and, as a result, lead to the highest diversity under intermediate initial Feedback-to-Host ratio (Hypothesis II). Our results showed a continuous algal microbiome composition change along with the initial feedback-to-host ratio gradient, and a largest composition difference between 0%-to-100% and 100%-to-0%; consistent with Hypothesis I. In addition, we found the highest algal microbiome diversity (for richness, Shannon, and Simpson diversities) under 25%-to-75% initial feedback-to-host ratio on the final day. These results indicated the presence of both host-microbiome feedback and host innate selection can together maintain higher phytoplankton microbiome diversity (consistent with Hypothesis II).