Trophic dynamics and metabolic pathways in host parasite interactions revealed by nitrogen isotope analysis of amino acids in multiple tissues

Trophic interactions are crucial for understanding food web complexity and energy flow in ecosystems. Including parasites in these analyses can alter dynamics and challenge ecological assumptions. Compound-specific isotope analysis offers greater precision than bulk stable isotope analysis by analyzing individual amino acids, providing deeper insights into nutrient exchange and metabolism in host-parasite systems. In this study, we conducted a 120-day controlled feeding experiment to investigate parasite metabolic pathways, host-parasite trophic dynamics, and trophic fractionation between infected and uninfected sticklebacks. Nitrogen isotope composition (δ¹⁵N) was measured in host Gasterosteus aculeatus liver and muscle tissues, and its 'cestode' parasite (Schistocephalus solidus). Parasite serine δ¹⁵N values were higher by 4.4 ± 2.4‰ compared to host liver, indicating a strong metabolic link. Lower proline concentrations and increased alanine δ¹⁵N values (5‰) indicate support of parasite growth through conversion to hydroxyproline and increased gluconeogenesis. The trophic position difference between parasite and host tissues was < 0.5, suggesting direct assimilation of host-derived products. Infected host tissues exhibited ~ 5‰ increase in glycine δ¹⁵N over time compared to control tissues, likely reflecting the host's increased metabolic demand for immune support during infection. This study highlights the complex metabolic interactions in host-parasite systems.