The symbiosis between cnidarians and dinoflagellate algaeis not understood at the cell or molecular level, yet thisrelationship is responsible for the formation of thousands ofsquare kilometres of coral reefs. We have investigated thenature of crystalline material prominent within marine algalsymbionts of Aiptasia sp. anemones. This material, whichhas historically been considered to be calcium oxalate, isshown to be uric acid.We demonstrate that these abundanturic acid stores can be mobilized rapidly, thereby allowingthe algal symbionts to flourish in an otherwise N-poor environment.This is the first report of uric acid accumulation bysymbiotic marine algae. These data provide new insight andconsiderations for understanding the physiological basis ofalgal symbioses, and represent a new and previously unconsideredaspect of N metabolism in cnidarian, and a varietyof other, marine symbioses.