Gene expression and enzyme activity analysis of carbohydrate digestion in Strongylocentrotus purpuratus larvae

Carbohydrates play multifaceted roles in marine ecosystems, serving as structural components in algae, energy storage molecules and vital nutrients for marine organisms. The purple sea urchin, Strongylocentrotus purpuratus, undergoes metamorphosis during ontogeny, transitioning its feeding strategy from microalgae to macroalgae as the primary food source. However, the digestive mechanisms underlying carbohydrate digestion in sea urchin larvae remain poorly understood. We investigated the carbohydrate digestion capabilities of S. purpuratus larvae, using expression-level analysis of candidate genes putatively involved in carbohydrate digestion, quantification of enzyme activity, and pH and temperature optima characterization for the digestion of starch, laminarin, cellulose, xylan and trehalose. Transcriptomic analyses revealed the expression of genes encoding putative carbohydrate-degrading enzymes during early larval development. RT-qPCR demonstrated age- and/or feeding-dependent expression patterns of glycosidase candidate genes β-1,3-glucanase (laminarinase), α-amylase, endo-β-1,4-glucanase D-like (cellulase), xylanase/β-glucanase-like and trehalase. Furthermore, enzymatic assays elucidated differential temporal patterns, and thermal and pH optima of associated carbohydrate-degrading enzymes. A comparison of the enzymatic degradation of five substrates demonstrated that laminarinase activity was five times higher than the activity of enzymes involved in digesting starch, cellulose, xylan and trehalose, leading to a hypothesis regarding the importance of laminarin for larval growth.