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ANOXIA TOLERANCE

In the Storey lab we study a variety of animals that can live without oxygen for long periods of time. Although humans can survive for only a few minutes without oxygen delivery to the brain, many animals live in environments where oxygen is not always available or have lifestyles that allow them only intermittent access to oxygen. Below are our main model animals.

Vertebrate anoxia tolerance: Long term survival without oxygen is most highly developed in two vertebrate groups – some freshwater turtles and selected fish species. Turtles use this ability during breath-hold diving and, more importantly, to hibernate underwater during the cold winter months. The best-studied turtles are the red-eared slider (Trachemys scripta elegans) and the painted turtle (Chrysemys picta). Adults of both species can live for around 3 months in oxygen free cold water (<10°C) and do this most winters when they hibernate on the bottom of ice-covered ponds and rivers.

Carp (Carassius carassius, Cyprinus carpio) and goldfish (Carassius auratus) can also endure long term oxygen deprivation and may experience many weeks of anoxia when wintering in ice-locked ponds. Due to the respiration of plants, animals and microbes living in ice-covered ponds, oxygen can be depleted, challenging the animals living there. This can cause winter kill of many species but some like carp and goldfish have a survival advantage in small ponds because they can switch to anaerobic metabolism.

In the Storey lab we have studied turtles, carp and goldfish as models of vertebrate anoxia survival. Link to recent reviews from our lab about anoxia tolerance.

Invertebrate anoxia tolerance: Many kinds of invertebrates have excellent anoxia tolerance that helps them endure occasional, circadian or circannual exposures to oxygen deprivation. Very well studies examples include various kinds of marine intertidal organisms that can breathe with gills when the tide is high but are left out of water and unable to oxygenate their bodies at low tide. Indeed, an ability to live without oxygen for days or weeks is the reason that fish markets can offer fresh alive mussels, oysters, clams and snails for sale.

In the Storey lab we have studied anaerobiosis by a variety of marine mollusks including periwinkle snails (Littorina littorea), whelks (Busycon canaliculatum), clams (Mercenaria mercenaria), mussels (Geukensia geukensia, Mytilus edulis) and oysters (Crassostrea virginica).

Trachemys scripta elegans	Chrysemys picta marginata	C. p. marginata hatchlings	Carassius auretus
Eurosta solidaginis	Epiblema scudderiana	Littorina littorea	Geukensia demissus

Anoxia tolerance aids freezing tolerance: Animals that can survive freezing must also be able to live without oxygen because freezing stops breathing, heart beat and blood circulation. Therefore, all tissues of a frozen animal must survive without oxygen until they thaw again. Newly hatched painted turtles are an excellent example of this. They use their inborne anoxia tolerance to help them survive freezing because, unlike the adults, they spend their first winter after hatching on land in their shallow natal nests. For more information on the freeze tolerant species go to our Vertebrate Freeze Tolerance and Invertebrate Cold Hardiness pages.

Biochemical mechanisms of anoxia tolerance: Adaptations that support anoxia tolerance can include some or all of the following depending on the species and the circumstances.

1. Fuels: High levels of carbohydrate fuels are stored in tissues because carbohydrates (glycogen, glucose) can be catabolized to produce energy (ATP) without the use of oxygen via an ancient pathway called glycolysis.

2. Dealing with acidosis: The end product of glycolysis is normally lactic acid (also called lactate) and when it accumulates in high levels it causes a dangerous drop in cellular pH. Anoxia tolerant animals use mechanisms to prevent or buffer acid build-up. Turtles and mollusks release calcium carbonate from their bone and/or shell to buffer acid. Turtles also remove a lot of lactate from their blood and store it in their shell. Carp and goldfish have a unique solution – instead of letting lactate build up, they convert it to ethanol and then excrete the alcohol from their bodies across the gills.

3. Maximizing ATP yield: The energy yield from the conversion of glucose to lactate is low, a net of just 2 ATP produced per molecule of glucose processed. However, if glucose is fully burned to CO₂ and H₂O using oxygen-dependent respiration, the yield is 36 molecules of ATP per glucose. Hence, biochemical mechanisms that can increase the ATP yield from the anaerobic fermentation of glucose are very important. Marine mollusks have this mastered. Instead of producing lactate, they make a range of other products (e.g. succinate, acetate, propionate) in reactions that have additional ATP-yielding steps so that they can double or triple the ATP output compared to making lactate. These animals also use some amino acids (e.g. aspartate, glutamate) as fuels during anoxia for additional bonus ATP production.

4. Metabolic rate depression: When body oxygen levels fall below a critical point, anoxia tolerant animals strongly reduce their metabolic rate, reducing their overall energy needs to a level that can be supported by the ATP output of glycolysis. Turtles and fish typically reduce their metabolic rate by 80-90% whereas many mollusks can lower their metabolism by >95%. Hypometabolism is achieved by turning down and turning off many cellular processes in a priority manner so that just the critical life-support processes are left.

5. Antioxidant defenses: Lack of oxygen causes metabolic injuries but so does a too-rapid return of oxygen to normal levels. Indeed, much of the damage caused by blocked circulation during heart attack or stroke is caused not by the prevention of oxygen delivery to tissues but by a burst of oxygen-free radicals produced when oxygen floods back into the tissues. These reactive oxygen species can attack and damage cellular DNA, proteins and lipids. All organisms have antioxidant defenses that can destroy reactive oxygen species and repair damaged molecules but these are often overwhelmed in anoxia intolerant animals, including man. Anoxia tolerant animals correct this by maintaining extra high levels of antioxidants at all times and by triggering the synthesis of extra antioxidants whenever they are exposed to oxygen depletion.

6. Activation of anoxia responsive genes: In response to low oxygen, anoxia tolerant animals shut down the expression of most genes and the synthesis of most proteins as part of metabolic rate depression. Uniquely, however, they also increase the transcription of a few selected special genes and greatly increase the synthesis of the protein products of these genes. These protein products have special protective actions for anoxia survival and they include antioxidant enzymes, iron-binding proteins, chaperone proteins that help to protect the structures of other cell proteins, and a variety of other proteins with actions that help to extend viability in the absence of oxygen.

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