Urea excretion as a strategy for survival in a fish living in a very alkaline environment

Abstract

Ammonia is toxic to all vertebrates. It can be converted to the less toxic urea, but this is a metabolically expensive process¹ found only in terrestrial vertebrates that cannot readily excrete ammonia and marine fish that use urea as an osmotic filler. Freshwater fish mostly excrete ammonia^2,3 with only a small quantity of urea^4,5. It seems the ornithine cycle for urea production has been sup-pressed in all freshwater teleosts⁶ except for some airbreathers which, when exposed to air, increase urea synthesis via the cycle⁷. Here we show that the tilapia fish Oreochromls alcalicus grahami, the only fish living in Lake Magadi, an alkaline soda lake (pH = 9.6–10) in the Kenyan Rift Valley, excretes exclusively urea and has ornithine–urea cycle enzymes in its liver. A closely related species that lives in water at pH7.1 lacks these enzymes and excretes mainly ammonia with small amounts of urea produced via uricolysis⁴. It dies within 60 min when placed in water from Lake Magadi. We suggest that urea production via the ornithine-urea cycle permits O. a. grahami to survive the very alkaline conditions in Lake Magadi.