Why is oxygen so deadly?
Oxidative Stress and Living in Extreme Places
Imagine living inside a hot, salty halite rock lying around on the surface of the driest desert on Earth, the Atacama Desert. This is a place similar to the red plains of Mars but with an important difference, there is oxygen in the atmosphere, and this is not all good news for an extremophile microbe. Some microbes are aerobic, giving them a great energy source as long as there is food available for aerobic respiration, but to many it is a source of what are called reactive oxygen species (ROS), but probably more familiar as free radicals. We hear a lot about antioxidants, much of it hype when referring to human diet, but actually the difference between life and death for any aerobic cell on Earth. Free radicals cause “oxidative stress” to cells and antioxidants fight it.
There are many extremophile microbes living and growing in extreme conditions, at least from a human point of view, of heat, up to about 131ºC, and cold at -20ºC; salinity, in saturated salt solutions; pH, both in extreme acids and alkalis, pH2 and pH11; under extreme levels of ultraviolet radiation, near radioactive rocks and in desiccating aridity. Some places on Earth combine several of these such as the high-altitude lakes of the Andes which can sometimes dry out, have high salinity and ultraviolet, may be acidic, hot and cold and sometimes contain arsenic! Only polyextremophiles can survive this punishment. A common thread running through these microbes is that these conditions all cause oxidative stress because they all make the cells produce free radicals. Many of these belong to the great microbial domain, the archaea, but some are bacteria.
What are free radicals and why are they dangerous?
We are particularly interested in hydrogen peroxide (H2O2), superoxide (O2-) and hydroxyl (OH-). These all have an unpaired electron which makes them particularly reactive. They try to react with any molecule in the vicinity such as proteins, DNA, lipids and carbohydrates. When they attack, they can cause breaks in a molecule or changes in the molecule, such as base changes in DNA which disrupt cell metabolism and can cause mutations which lead to cancer or cell death. Once the gene for a protein has been changed the cell can continue to produce more of the same, damaged, kind. How are free radicals produced?
Making free radicals
We just can’t help it as these reactive oxygen species are produced as a result of our normal aerobic cell respiration reactions. The mechanism remains to be clarified but as the electrons pass down the respiration electron chain they normally combine with a H+ proton and oxygen to form water, for some reason, in a few cases, the oxygen picks up the electron too soon forming the super oxide which can then produce hydrogen peroxide with protons.
O2 + e− → •O−2
2 H+ + •O−2 + •O−2 → H2O2 + O2
This occurs in mitochondria in our cells as well as in microbe cells. If there is too much damage done to the mitochondria, they destroy themselves. Other molecules can also for m free radicals when conditions are right.
Free radicals are also produced in cells when they are bombarded by ultra violet rays, we can see this directly in darker spots on our skin and skin cancers. There are also other sources such as chemicals in tobacco smoke, heavy metals and atmospheric pollutants.
Extremophile microbes are able to withstand very stressful environments, these adaptations are the reason that they are able to live under arid, salty, acidic, etc environments.
Controlling free radicals
If a cell is to survive it must remove these ROS as quickly as possible, there are several ways to do this as it is so important for the survival of living cells:
1. Enzymes, superoxide dismutase converts superoxide into hydrogen peroxide and oxygen, then the catalases convert the hydrogen peroxide into oxygen and water. When you pour hydrogen peroxide onto a bleeding cut, to sterilise it, it fizzes furiously as the catalase in your blood breaks it down and forms oxygen bubbles. It sounds a bit round about, but the final products are oxygen and water which can be used in your body.
2. Antioxidants such as carotenes (used to make vitamin A) and rhodopsin can help mop up free radicals. Many extremophiles are pigmented such as the pink Deinococcus radiodurans(the name means radiation resisting pink berry) which is found in hi altitude lakes in the Andes as well as in being to tolerate extreme doses of gamma rays.
3. Some microbes have been seen to change to anaerobic respiration under high levels of ROS, this will relieve the pressure by stopping further free radical production.
4. In 2018 Diego Rivera Gelsinger and his team investigate the importance of RNA in resisting attack by the free radicals. They investigate Messenger RNA is used to take information from the chromosome to the ribosome to which then forms proteins, transfer RNA carries amino acids around the cell and brings them in when the code demands it. They found that there was a great deal more RNA, much smaller molecules with no known use, they investigated an archaean Haloferax volcanii. (the fertile salt organism named after Volcani, who first discovered life in the Dead Sea”).
They found that these small RNAs probably protected the DNA and stopped messenger RNA from forming more damaged proteins
All of this was caused by the great oxygenation events of early Earth. All the first microbes were anaerobic then along came the cyanobacteria with their fancy new kind of photosynthesis which used chlorophyll to harvest light energy to split water and fix carbon. All plants on the planet depend on chloroplasts, which are symbiotic cyanobacteria. They were so successful in those early days, that they filled the atmosphere with oxygen. This would have been totally catastrophic for all the other microbes; most would simply have died. Others developed adaptations for using aerobic respiration (some of these became our mitochondria). As today the rest would only have been able to survive in anaerobic environments. The archaea were particularly successful in adapting to the extreme environments where they continue to flourish to this day.
Let’s give the last word to a scientist in this fieldDiego Rivera Gelsinger,
“These microbes flourish in very salty environments such as in the small pores of salt rocks from the Atacama Desert in Chile or in the salt beds of the Dead Sea. They endure the heights of solar radiation, salinity, and dryness that cause massive oxidative stress and kill most life forms.
Oxidative stress is the underlying cause of several human conditions from neurodegenerative and cardiovascular diseases to cancer and even the aging process. Understanding the causes of this stress-resistance unique to Haloarchaea could help researchers learn what other species, like humans, need to tackle the damage caused by oxidative stress.”