Identifying the Drivers and Consequences ofHost-Microbiome Composition and Function - PROJECT SUMMARY Microbiomes are crucial to the physiological functioning of host organisms. Understanding what regulates microbiome assembly and function can therefore have critical implications for host health. The central hypothesis of this proposal is that host phylogenetics is a key driver of microbiome assembly and function, which in turn, has physiological implications for the host. Specifically, I aim to 1) assess how host phylogenetics and host traits regulate microbiome assembly and function, and 2) assess the effects of host microbiomes on host cell physiology and consumer fitness. Unicellular algae are an ideal model system to address these questions because these microbes harbor an experimentally tractable exterior microbiome. Further, algae span an unusually broad phylogenetic distance across two domains of life, allowing us to test for the implications of over a billion years of host evolutionary divergence. Lastly, our prior work has shown that algal microbiomes are species-specific, but whether this specificity corresponds with host evolutionary history or has implications for host health is unknown. Therefore, I use this model system to test my hypothesis that microbiome composition and function will be most similar among host species that are closely related, and more divergent among host species that are distantly related. To test this, I submerged 31 strains of axenic algae in a diverse community of freshwater bacteria to allow each strain to assemble a microbiome. I am using samples from my study to assess bacterial abundance, community composition, and function of the microbiomes via amplicon and shotgun sequencing. To test whether host specificity has implications for host health, I will carry out reciprocal transplant experiments of whole microbiome communities among nine host strains that span phylogenetic distances from hosts within the same species to those from different domains of life. I will pair these microbiome swaps with measurements of host cellular stress, including profiles of nutrients and lipids. Considering that microbiomes can facilitate host acquisition of limiting nutrients, including nitrogen and phosphorus, I hypothesize that hosts with an assembled microbiome will have higher cellular %N and %P compared to hosts without a microbiome. Further, under the expectation that microbiome composition will correspond with host phylogenetics, I hypothesize that the magnitude of change between metrics of host cellular stress will correlate with phylogenetic divergence between hosts. Further, I will test whether these varied algal microbiome treatments affect consumer fitness using a zooplankton model system, thereby clarifying how the microbiomes of the foods we eat may affect human health. Overall, I aim to unravel the mechanisms behind microbiome assembly and the consequential implications of host-specific microbiome assembly on host health. Using the unparalleled phylogenetic breadth of hosts available in an algal model system, this study will clarify how hosts and their microbiome have co- evolved over evolutionary history and the implications of such host-specificity on host and consumer health.
