Dissecting host-microbiome interaction effects on phytoplankton microbiome composition and diversity

Phytoplankton and their associated microbiomes of heterotrophic bacteria are foundational to primary production, energy transfer, and biogeochemical cycling in aquatic systems. While it is known that these microbiomes are shaped by host-released dissolved organic matter (DOM), the extent to which dynamic phytoplankton-bacteria interactions shape bacterial community assembly remains to be examined. Here, we investigated the effects of two mechanisms in host-microbiome interactions on phytoplankton bacterial microbiome formation: (i) innate host selection and (ii) host-microbiome feedback. For the former, phytoplankton-produced DOM composition is based solely on the host's properties (species or physiological state); for the latter, the presence of the microbiome modifies host DOM production. The microbiome of Chlorella sorokiniana was extracted and exposed to six ratios of the two effects. We found that microbiome composition changed along with the six host-microbiome feedback versus innate host selection ratios, with the highest compositional distance between communities under the strongest and the weakest ratio of the two effects. This indicates that each mechanism selects for different bacterial species. In addition, our findings showed that when both selective forces were applied, it led to a higher community richness, while host-microbiome feedback alone reduces community evenness due to its strong species-specific selection.