Diffuse ways to get rid of ammonia?

Ammonia is a problem. All animal cells produce it, but how do they get rid of it? New research by Dr Dirk Weihrauch (University of Illinois at Chicago) to be presented on Wednesday 2 April (session A2.2) suggests that the crab may have evolved a rather novel solution.

For us mammals, the key to getting rid of waste ammonia is to detoxify it into urea in the liver. Marine organisms can employ the rather simpler solution of allowing the toxic ammonia to diffuse into their environment. However, it seems that the crab’s solution is a bit more complex.

Crab’s release ammonia from their gills, but instead of simply allowing it to diffuse out, the canny crustaceans employ a novel system of molecular pumps and valves. Weihrauch’s team have found that a particular pump, normally used to move sodium and potassium ions, is employed by the crab to pump waste ammonia from its blood and into the cells of its gill. A second protein pump then tucks this ammonia away into tiny packets called vesicles which are discharged from the cell actively by exocytosis into the surrounding seawater.

There’s a problem here though. As the crab pumps ammonia into its gill cells it faces the danger of poisoning these cells with the toxic waste product. Weihrauch thinks evolution has equipped the crab with a solution though. He has discovered a particular rhesus-like protein in the crab which is capable of transporting ammonia. Amazingly, this protein has a 40% similarity with human rhesus-like proteins and may in fact be their direct ancestor. Weihrauch believes that these proteins act as a kind of ‘safety valve’ – when ammonia reaches dangerous levels in the gill cells, the protein deposits some of the tricky toxin back into the blood where it is safely buffered.

So why does the crab go to such lengths when other marine creatures simply release ammonia by diffusion? Weihrauch believes that the crab mechanism of ammonia excretion allows it to survive in places where diffusion simply won’t work, for example in the carcass of a rotting fish where external ammonia levels are high. It seems that for crabs at least, hanging around in a decaying fish carcass has its cost.