Microbiome Sharing Between Reef-Building Corals and Associated Epibiotic Gastropods in French Polynesia

Field: Agricultural and Biological Sciences (Life Sciences)

Every macroorganism has a unique association of microbes, also known as a microbiome. This is a complex association in reef-building corals, and research has shown that these microbes are a functional extension of the coral animal itself. Together these components of host, symbiotic zooxanthellae, protists, bacteria, algae, archaea, viruses, and fungi, form what is now known as the coral holobiont. Recent studies have shown that the microbes found in the surface mucus layer of corals play a key role in holobiont ecology. Microbial interactions in the coral surface mucus layer are especially important when considering the increasing prevalence of coral disease. While increased research continues to elucidate surface mucus microbe functions, far less is known about the microbial communities of the epibiotic invertebrates feeding on or living in close association with these corals. It is known that microbes may be exchanged in the coral surface mucus layer, but the extent of exchange is not well studied.\nWorking in Moorea, French Polynesia, I collected surface mucus samples from three species of coral, Porites lobata, Porites rus, and Pocillopora damicornis, and five species of epibiotic gastropods found on these corals, Drupella cornus, Coralliophila violacea, Coralliophila monodonta, Drupa ricinus, and Drupa grossularia, to assess potential sharing of the microbiomes when these invertebrates live in close association with each other. Nextiv generation sequencing methods were used to identify microbe taxa in each microbiome sample using the 16S rDNA marker, following the protocols of the Earth Microbiome Project. Operational Taxonomic Units (OTUs) were scored for their relative abundance in each sample and were characterized into family-level and functional groups based on taxonomic relationships and previous literature of the types of biological processes in which these microbial taxa were involved. OTUs were assigned based on the role or properties they may exhibit in the coral surface mucus layer (nutrient cycling, antimicrobial, potential pathogens, or commensal), or were unassigned.\nResults indicated that coral and epibiotic snail microbiomes differed significantly from one another. When compared at the species level, the difference in microbiomes of snails varied across all coral hosts and gastropod species, with some corals host microbiomes having greater similarity to their epibionts than others in addition to high variability among conspecific snail microbiomes. All microbiomes of gastropods found on P. lobata and P. rus were different from each other when found on the same coral, and therefore did not exhibit explicit microbiome sharing. The only exception to this was the microbiomes of D. cornus and D. grossularia when they co-occurred on P. lobata, and their microbiomes did not differ. In contrast, almost all gastropod microbiomes on P. damicornis did not significantly differ from each other, and demonstrated greater similarity to the microbiome of their coral host than other coral hosts to their respective epibionts.\nWhen OTUs were grouped by function, snail and coral communities differed, however the functional similarity ranged from 75-80%. Functional differences between snail and coral microbiomes occurred due to higher abundances of OTUs exhibiting antimicrobial properties in corals and higher unassigned taxa in snails. Additionally, I discovered that snail v species found on P. lobata harbor far fewer antimicrobial OTUs than snails living on P. damicornis, potentially due to the high antimicrobial properties of P. damicornis’s own surface mucus layer. Snails that feed on coral tissue (corallivores) and snails that do not (noncorallivores) were also different in their mucus microbial community functions, with corallivores harboring a higher number of potential pathogens than non-corallivores regardless of their coral hosts. These results suggest that corallivory may play a role in how opportunistic or pathogenic microbes are spread, as well as identify functional groups playing an important role in both corallivorous and non-corallivorous snails.\nOverall, coral and snail surface microbiomes are distinct from one another even when living in close association. Microbiome sharing does occur between the two invertebrate groups and is most prominent on host coral P. damicornis. However, microbiome sharing is not host-dependent across all snail species. Corallivorous epibiotic gastropods cannot be ruled out for their increased disease vector potential. Even though the coral surface mucus layer harbors higher abundances of microbes with antimicrobial properties than epibiotic snails, coral disease researchers will benefit from further investigation into the vector potential of other epibiotic invertebrates and mobile corallivores in coral reef ecosystems.