Waste Not; Want Not. Do Frogs Recycle Their Antifreeze?

Many advances in the biological sciences have come from what can be classified as—and funded as part of—agricultural research. Reproductive biology is one prime example but there are many others.

One advance the origins of which lie firmly in agricultural production is the recycling of urea. We were all taught at school that excess protein in the diet ends up as urea in urine with the nitrogen contained therein lost forever. But things are not that simple. Now it is known that in many animals, the urea is not simple dumped but can be broken down by bacteria in the gut and the nitrogen it contains recycled into protein.

It was the demonstration that urea, a form of non-protein nitrogen, added to the feed of farm ruminants, can be used within the rumen to make protein for growth or milk production that set off the interest in the recycling of urea under different nutritional and environmental regimes. It is not the ruminant’s tissues that are involved in breaking down the urea but the bacteria of the rumen. The bacteria then produce protein using urea as a source of nitrogen and this protein is digested and absorbed further down the alimentary canal. The history of the use of urea rather than expensive protein nitrogen sources was explained by my predecessor but one as Director of the Hannah Research Institute, James Andrew Buchan Smith (1906-2006), known throughout the world as ‘JABS’. Since those early days the recycling of urea nitrogen has been recognised in more and more organisms that produce urea as a nitrogenous waste product.

Amphibian species use high concentrations of urea in the blood for two main purposes: protection against dehydration and as an antifreeze. Some, notably the famous Crab-eating Frog (Fejervarya cancrivora) which can survive in sea water, build up high concentrations of urea in their body fluids in order to prevent the osmotic loss of water into the surrounding salty water. Other frogs that hibernate on land in sub-zero temperatures build up high concentrations of urea to act as a cryoprotectant. The high concentration lowers the freezing point of body fluids, preventing the formation of ice crystals.

In a paper published last year, a team in the USA produced preliminary evidence that the urea accumulated during hibernation in the Wood Frog, Rana sylvatica or Lithobates sylvaticus from northern North America, can be recycled into protein. Bacteria in the hind gut were found which could produce urease, an enzyme not produced by vertebrate tissues but essential for the recycling of urea nitrogen; more were present in winter than summer. Thus it is possible that as temperatures rise in spring, the bacteria become active, the urea is broken down and the products of that breakdown are converted into amino acids and hence protein.

Wood Frog
Brian Gratwicke [CC BY 2.0 (https://creativecommons.org/licenses/by/2.0)]

The findings on this frog indicate what could happen. However, there are some qualitative and quantitative pieces of evidence that would be more fully convincing of the first demonstration of urea recycling in amphibians. In mammals, specific urea transporters carry urea into the interior of the gut. Are they present in amphibians? Similarly, we do not know the magnitude and, therefore, the quantitative importance of any breakdown of urea to the overall nitrogen metabolism of these frogs at the end of hibernation. However, this could be another case of waste not, want not.  

Wiebler JM, Kohl KD, Lee RE, Costanzo JP. 2018 Urea hydrolysis by gut bacteria in a hibernating frog: evidence for urea-nitrogen recycling in Amphibia. Proceedings of the Royal Society B 285: 20180241. http://dx.doi.org/10.1098/rspb.2018.0241