Striped catfish (Pangasianodon hypophthalmus) exploit food sources across anaerobic decomposition- and primary photosynthetic production-based food chains.

Dietary information from aquatic organisms is instrumental in predicting biological interactions and understanding ecosystem functionality. In freshwater habitats, generalist fish species can access a diverse array of food sources from multiple food chains. These may include primary photosynthetic production and detritus derived from both oxic and anoxic decomposition. However, the exploitation of anoxic decomposition products by fish remains insufficiently explored. This study examines feeding habits of striped catfish (Pangasianodon hypophthalmus) at both adult and juvenile stages within a tropical reservoir, using stable carbon, nitrogen, and sulfur isotope ratios (δ13C, δ15N, and δ34S, respectively) and fatty acid (FA) analyses. The adult catfish exhibited higher δ15N values compared to primary consumers that feed on primary photosynthetic producers, which suggests ingestion of food sources originating from primary photosynthetic production-based food chains. On the other hand, juvenile catfish demonstrated lower δ15N values than primary consumers, correlating with low δ34S value and large proportions of bacterial FA but contained small proportions of polyunsaturated FA. This implies that juveniles utilize food sources from both anoxic decomposition and primary photosynthetic production-based food chains. Our results indicate that food chains based on anoxic decomposition can indeed contribute to the dietary sources of tropical fish species.

D/H exchange in C-H bonds of fatty acids: implication for geographical discrimination of food materials.

The stable isotope analysis of non-exchangeable hydrogen in food materials has been believed as a powerful tool for tracing geographical origins and delivery of the materials. However, little information is available for the D/H exchanges even in hydrophobic molecules, which leads to potential uncertainty on the observed results. To further evaluate the reliability of hydrogen isotopes of organic molecules, we examined the isotope exchangeability in fatty acids within phospholipid bilayers exposed to D(2)O for 12h. The results indicate that the isotope ratio is clearly increased during all examines (e.g., 93-328 ‰ in 50% D(2)O), in which its magnitude depends on type of fatty acids and state of bilayers. However, these results also indicate very clearly that the observed exchange rate is negligible if samples are exposed to natural abundance of deuterium (0.0156%) in natural and laboratory environments before/during analysis.