Gut Microbial Diversity and Genome-Wide Variation of the Cape Sea Urchin (Parechinus angulosus) Across a Thermal Gradient

Marine bacteria are the dominant biomass in the oceans. Diverse microbial communities have colonised different organs in various life forms, and their genetic diversity, and biochemical functions they perform, play a critical role in an organism's fitness and ecosystem status. The study of gut microbiota in marine organisms has gained increasing attention due to the critical role of gut bacteria in host digestion, nutrient metabolism and immune function. The Cape sea urchin, Parechinus angulosus, is the most widely distributed echinoid along southern Africa's coastline and inhabits coastal regions with contrasting oceanic physico-chemical features. The diversity of the gut-associated bacteria, spatial compositional variation across its distribution range, and the connection between host genomic diversity and gut microbiota are currently unknown. This study used a combination of 16S rRNA metabarcoding and ezRAD sequencing of host genomes to describe the diversity in Cape sea urchin and its associated gut microbiota. Bacterial taxa belonging to 20 phyla, 39 classes, 89 orders, 128 families and 191 genera were identified. While α diversity did not vary significantly within the bioregions inhabited by the sea urchin (west coast and south coast), β diversity indices revealed significant differences in bacterial community composition between individuals collected from the two bioregions. A distance decay analysis indicated a statistically significant correlation between geographical distance and dissimilarity in bacterial assemblage throughout the distributional range. Genomic analysis revealed no statistically significant population structure throughout the species' range. Similarly, after taking geographical distances into consideration, no statistically significant correlation between genomic distance and dissimilarity of bacterial assemblage was found, and the topologies of the genomic tree and clustering tree of microbial diversity were not concordant, showing that the factors that affect genomic structure in the host are not directly affecting their gut-associated microbiota. This study serves as a first stepping stone towards a better understanding of the role of gut-associated microbiota in Cape sea urchins and will help to enhance our understanding of the intricate relationships between marine organisms and their associated microbial communities.