Direct effects of ambient ammonia on the nitrogen isotope ratios of fish tissues

We tested the assumption that the nitrogen isotope ratios of fish are solely determined by their diet by exposing two species offish having contrasting nitrogen metabolic profiles to ¹⁵N ammonia. Beaugregory damselfish (Stegastes leucostictus) representing ammonotelic species and toadfish (Opsanus beta) representing ureotelic species were exposed to ¹⁵N ammonia for a period of 4 weeks, after which muscle and liver tissues were analyzed for ¹⁵N abundance and compared to their respective control group. Both species showed significant ¹⁵N enrichment when exposed to ¹⁵N ammonia, with the ammonotelic fish showing a greater enrichment compared to the ureotelic fish. We propose that the toadfish showed less enrichment in its muscle tissue because its active omithine-urea cycle (O-UC) rapidly sequesters ammonia away from the circulatory system and into liver tissue, thereby preventing any substantial exchange between ammonia and muscle tissue. We propose that no such sequestering occurs in the ammonotelic damselfish because they lack a functional O-UC. These results have important implications for studies using nitrogen isotope ratios to delineate trophic structure in aquatic ecosystems.