Biosynthesis and Isotopic Routing of Dietary Protein by Sea Cucumber Apostichopus japonicus (Selenka): Evidence from Compound-Specific Carbon Stable Isotope Analysis.

Abstract

Compound-specific stable isotope analysis of individual amino acids (AAs) has been widely used in studies on food webs, resource tracing, and biochemical cycling. In the present study, juvenile sea cucumbers Apostichopus japonicus were fed the microalga Cylindrotheca fusiformis (CF) or Sargassum thunbergii (ST) during a 130-day experiment. The δ13C values of individual AAs in the experimental diet and body wall of sea cucumbers were determined to calculate the variability in carbon isotopic fractionation (Δ13C) and elucidate the isotopic routing of essential AAs and biosynthesis of nonessential AAs. The results showed that the sea cucumbers fed with diet CF had higher specific growth and ingestion rates but relatively lower feed conversion efficiency compared to those fed with diet ST. The experimental diets were generally less abundant in nonessential AAs (i.e., glycine, serine, aspartic acid, and arginine) but more abundant in essential AAs (i.e., isoleucine, leucine, lysine, phenylalanine, and histidine) than body walls. The fluctuations in the δ13C values of total AAs analyzed were 19.8 ± 4.6‰ for diets and 21.3 ± 2.7‰ for body walls. Serine and threonine were 13C-enriched AAs, while leucine and phenylalanine were 13C-depleted AAs. The diet ST treatment exhibited more positive Δ13C values of nonessential AAs (e.g., glycine, alanine, aspartic acid, and proline) compared to diet CF. There were significant negative relationships between Δ13C values and differences in nonessential AA percent abundance between the experimental diets and body walls of sea cucumber (for diet CF: y = -0.79 - 0.56x, r2 = 0.47; diet ST: y = 0.75 - 0.29x, r2 = 0.51), which implied the flexibility in the routing of various dietary macronutrients (protein, lipids, and carbohydrates) by sea cucumber. This study can greatly provide a new understanding of nutrient utilization and metabolism routing during juvenile sea cucumber culturing.