Nitrogen routing, metabolic state, and δ15N in heterotrophic vs. photosymbiotic animals. a., Prey or particulate organic matter (POM) are ingested by the coral host. During catabolism, deamination and excretion preferentially remove 14N as ammonium, enriching retained body N in 15N and elevating δ15Nhost. In heterotrophic corals, as in heterotrophs in general, these are the dominant dynamics in setting
their δ15N relative to their feeding sources. b., In photosymbiotic corals, N sources include both feeding and the possibility of assimilating dissolved inorganic N (DIN, dominantly nitrate and ammonium). The
photosymbionts acquire metabolic N waste from the host or the DIN assimilated from the environment, using it in their photosynthetic growth. Photosynthate C and N is transferred to the host, and a tight internal N recycling loop returns the 14N-rich waste to the photosymbionts. Under anabolism (growth, stable photosymbiosis), recycling suppresses net 14N loss, keeping δ15Nhost closer to the baseline N source (righthand down arrow). Increased catabolism weakens N retention, increasing 14N-rich excretion, raising
δ15Nhost. Arrows show assimilation, ingestion, symbiont-to-host transfer, excretion, and recycling; box sizes are schematic. Overall, the δ15N of corals relative to their environmental N sources reflects the balance between coral growth and other biosynthetic activities (anabolism) and excretion (catabolism), with photosymbiosis biasing the system toward N recycling and, thus, N retention for growth.
Sort of wild this works: using nitrogen isotopes in one organism (coralline algae) to infer the ecological state of the entire reef and on the symbiont resiliance in a completely different one (corals). Won't work in deep time, but powerful monitoring tool. 🧪🌊
Link: www.nature.com/articles/s43...
