Functional enzymatic characterisation and microbiome analysis of the digestive tract of the green sea urchin Strongylocentrotus droebachiensis

The digestive system of sea urchins is well-described, but the biochemical properties of the individual organs are only insufficiently characterized. The digestive tract begins in the buccal cavity, surrounded by a biting apparatus called Aristoteles' lantern. It then forms a tubular structure, which runs two times around the body wall, ending at the aboral side of the animal. This main part of the digestive tube has been described histologically as the stomach (first loop, anticlockwise) and the intestine (second loop, clockwise). We characterized the enzymatic profile as well as the microbial composition of the stomach and the intestine of the green sea urchin Strongylocentrotus droebachiensis, an ecological and economical important arctic-boreal grazer. Using qualitative and quantitative enzyme activity assays on tissue of the stomach and intestine, we identified differences in the biochemical processes, primarily concerning the protein and lipid metabolism. The stomach expressed higher esterase and esterase/lipase activity, while the intestine showed predominantly high exopeptidase activity. Low carbohydrate degrading enzyme activity suggests that polysaccharide digestion, despite their abundance in the species' diet, may primarily occur extracellularly in the lumen of the digestive tube. Our enzymatic findings are complimented by metabarcoding of the microbial community and analysis of functional properties: We found microbial taxa possibly associated with nitrogen fixation and carbohydrate degradation. Nevertheless, the composition and potential functional genes of the microbial community were similar between the two organs, suggesting that they do not play an organ-specific role in digestive processes.