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

## TL;DR

This study explores how purple sea urchin larvae digest carbohydrates, finding they are especially good at breaking down laminarin, a type of sugar found in algae.

## Contribution

The study identifies and quantifies larval sea urchin carbohydrate digestion mechanisms, including enzyme activity and gene expression patterns.

## Key findings

- Transcriptomic analysis shows expression of genes for carbohydrate-degrading enzymes in early larval development.
- Laminarinase activity is five times higher than other carbohydrate-digesting enzymes in sea urchin larvae.
- Enzymatic assays reveal distinct pH and temperature optima for different carbohydrate substrates.

## Abstract

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.

Summary: Investigation of the carbohydrate digestion capabilities of Strongylocentrotus purpuratus larvae reveals comparably high digestive capacity for laminarin, suggesting it may be an important carbon source in benthic and planktonic communities