In contrast to the vast majority of vertebrates, which usually die within minutes when deprived of oxygen, freshwater turtles are extremely resistant to oxygen deprivation, or anoxia. However, not all freshwater turtles are equally capable of surviving anoxia. For instance, the champion survivor, the Western painted turtle, can recover from up to 100 days of anoxia at winter temperatures (3-5°C), whereas the red-eared slider turtle succumbs after around 45 days of anoxia at the same temperature. Why such differences in anoxia survival time exist between turtle species remains a mystery, although Donald Jackson's team at Brown University has been working feverishly on elucidating what factors may account for the differences in anoxia survival times.
In their recent paper, the team surmises that, since all freshwater turtles are resistant to anoxia per se, differences in anoxia survival time among turtle species must be due to how effectively a turtle can extend the time before the changes in physiology that accompany anoxia become irreversible and lethal. For example, turtles' fuel reserves, such as liver glycogen, become depleted and the harmful acidic end products of anaerobic metabolism, lactate and H+, accumulate. Therefore, the greater anoxia tolerance of some turtles compared with others could be due to more anoxia-tolerant species exhibiting traits that prolong the time until glycogen reserves are exhausted and/or a critically acidic pH is reached.
Innovatively, a turtle's shell is not just protective armour but also reduces the harmful effects of H+ accumulation during anoxia exposure. Specifically, shells release calcium, magnesium and sodium carbonates into the extracellular fluid in exchange for lactate to supplement extracellular buffering of H+. Thus, Jackson's team hypothesized that some of the difference in anoxia tolerance between turtle species could be due to differences in buffering characteristics of their shells.
To test this, the team made a number of measurements to test how well the shells of different turtle species could contribute to acid buffering. First,they compared shell mineral composition and total shell CO2concentration, which is related to the amount of carbonate in the shell, to tell them how many ions were potentially available for buffering. They found that shells from the more anoxia-tolerant painted and snapping turtles had more mineralised shells and more shell CO2 than shells from the less anoxia-tolerant map, musk and red-eared slider turtles, suggesting that anoxia-tolerant turtles had more ions available for buffering.
Next, the team tested if the amount of buffer chemicals released from the shell differed among species. They measured the amount of acid they needed to add to powdered shell samples before the solution pH could no longer be maintained constant at pH 7 and found that more acid was needed for shell samples from more anoxia-tolerant species, indicating that these shells could release more buffer. Finally, the team tested whether there were differences in the shells' ability to accumulate lactate. They incubated pieces of shell in a standard lactate solution for a set time period, finding that shells from anoxia-tolerant turtles accumulated more lactate.
Because shells of more anoxia-tolerant turtle species contain more minerals and CO2, release more buffer chemicals and accumulate more lactate than those of less anoxia-tolerant turtles, the team concludes that differences in shell composition and buffering properties likely account for some of the reported differences in anoxia tolerance among freshwater turtles. The superior shell-buffering mechanisms of the more anoxia-tolerant turtles probably slow the development of highly acidic conditions during anoxia,prolonging survival time.
