Characterization of protein, carbohydrate and lipid digestion in larvae of the sea urchin Strongylocentrotus purpuratus
The dissertation investigates the digestive physiology of the larvae of the sea urchin Strongylocentrotus purpuratus, in particular the degradation of proteins, carbohydrates and lipids. Extracellular digestion is considered a crucial step in the evolution of multicellularity and heterotrophy. While digestion is well described in adult sea urchins, it has been little studied in planktonic larvae.
Molecular, biochemical and physiological methods were used to investigate the activity and gene expression of digestive enzymes under different feeding conditions (high vs. low food supply). The results show that larvae already have a fully developed digestive machinery at an early stage (from day 7). Gene expression was age-dependent and responded to food availability. The expression of protease genes was particularly high, followed by lipases and significantly lower levels of glycosidases.
Enzymatic analyses showed that proteases responded rapidly to feeding - even with a low diet - while lipases were only activated with a high diet. Glycosidases, on the other hand, showed a slower, continuous activation.
The pH optima of the enzymes correspond to the conditions in the larval intestine: Proteases and lipases worked best under alkaline conditions (pH 9.0-9.5), whereas glycosidases worked best at neutral to slightly acidic pH (pH 5.0-6.0), indicating a possible localisation outside the midgut.
Overall, the work provides new insights into the functional development and adaptability of the digestive system of sea urchin larvae. It forms a basis for further studies on dietary changes during metamorphosis and the reaction of these organisms to environmental changes.
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