Sea urchin microbiome

Kelp deforestation by sea urchin grazing is a widespread phenomenon, yet the ability of sea urchins to survive on a kelp diet of poor nutritional quality is not fully understood. Kelp biomass is rich in complex polysaccharides and has a high C:N ratio. As animals rarely produce enzymes capable of digesting complex structural molecules such as alginate, or enzymes fixing nitrogen, bacterial communities in the sea urchin gut may play an important role in the digestion process. We hypothesized that the diversity and composition of bacteria within sea urchin intestines changes depending on diet, and that bacteria capable of degrading different structural components of algae and of fixing nitrogen are a prominent component of the sea urchin intestinal microbiome. In order to test this, a no-choice feeding experiment was conducted with the green sea urchin Strongylocentrotus droebachiensis, offering three different algae as diet treatments; the kelp Saccharina latissima, the wrack Fucus serratus and the red alga Palmaria palmata. Starved sea urchins served as an experimental control and field-captured sea urchins as a natural baseline comparison. Amplicons of the 16S rRNA gene of prokaryotes were analyzed from fecal pellet samples and 614 amplicon sequence variants (ASVs) were identified. One ASV (related to Psychromonas marina) accounted for 44 % of the total sequence reads and was present in all samples, but especially abundant in the sea urchins held on algal diets, suggesting that this is an important nutritional symbiont in the sea urchins. The control (starved) and field captured sea urchins, had a significantly higher ASV diversity (richness and evenness) than the sea urchins given algae, regardless of the algal diet. This higher diversity was also linked to a higher predicted abundance of genes involved in nitrogen fixation. Cloning and sequencing of the NifH gene revealed nitrogen fixers belonging to the Gammaproteobacteria, Flavobacteria and Verrucomicrobia in the sea urchin microbiome. These findings demonstrate that the sea urchin intestinal microbiome is dynamic, responds to the food ingested and has the capacity for nitrogen fixation. However, studies of the sea urchin microbiome during ongoing kelp forest grazing events are needed in order to gain a better understanding of how sea urchins are sometimes able to thrive on high-carbon kelp biomass and thereby turn productive kelp forests into barren grounds. If interactions between kelp, the sea urchin and its microbiome can be understood, microbiome monitoring may provide a way to predict catastrophic kelp forest grazing events by sea urchins.