Organism Tolerance for Atmospheric Conditions

Instructor: Stephanie Gorski

Steph has a PhD in Entomology and teaches college biology and ecology.

In this lesson, we'll talk about the different 'comfort levels' that different organisms have for atmospheric conditions. In particular, we'll focus on oxygen levels, but we'll also discuss temperature and other conditions that affect an organism's survival.

Organism Diversity

Close your eyes and imagine your ideal place - somewhere you could just enjoy the weather and the scenery forever. Maybe you're imagining a sunny, beautiful beach with perfect azure waves. Maybe you're imagining a hidden garden, deep in a mountain forest. However, if you were Colwellia polaris, a bacterium that lives in Antarctica, or Beggiatoa spp., a bacteria that live in deep-sea vents, you would probably be imagining something very different!

Organisms come in a wide range of tolerances for different atmospheric conditions. However, all of the ones that you can see need at least one thing: oxygen! Yes, even plants need oxygen. Plants respire just like animals do, and respiration requires oxygen. What makes plants different is that they make oxygen as a byproduct of photosynthesis, so they're contributing to the atmosphere's oxygen supply as well as using it. Way to leave things better than you found them!

However, there are some organisms that don't need oxygen, or that will even die in the presence of oxygen. These organisms are called anaerobes. Almost all anaerobes are single-celled organisms, either bacteria or another single-celled domain of life called archaea.


How can something live without oxygen? We think of oxygen as necessary because we need it to turn stored energy (food) into energy molecules our body can use (ATP). But, there are many different ways that an organism could turn food into ATP, and organisms that live in oxygen-poor environments have evolved to do so. When life began, Earth's atmosphere didn't have much free oxygen. Only after organisms began photosynthesizing and releasing oxygen as a waste product did organism start evolving ways to use it. So, the first life on Earth was anaerobic!

Why would oxygen be poisonous to some organisms? Well, oxygen is actually very reactive - that means that it can be pretty destructive because it will cause all sorts of reactions that we don't want! If you've ever seen a health publication talking about the dangers of 'free radicals,' they're usually talking about unpaired oxygen atoms that bounce around causing biochemical reactions that can sometimes be damaging - but free radicals are a whole different story!

Anaerobic bacteria, Borrelia burgdorferi
Anaerobic bacteria, Borrelia burgdorferi

The main categories of anaerobes are:

  1. Facultative anaerobes, which will use oxygen if it's present, but if there isn't any oxygen, they're fine without it.
  2. Obligate anaerobes, which must live in an oxygen-free environment. If oxygen is introduced, they'll die.
  3. Aerotolerant organisms, which, if you don't mind the anthropomorphism, simply don't care if oxygen is present or not; they can't use it, but it doesn't hurt them either.
  4. Microaerophilic aerobes, which are another related category, do need oxygen, but only a little bit of it. In addition, they require low concentrations of oxygen because they're very sensitive to oxygen-derived free radicals.

Until recently, we believed that the only organisms that didn't need oxygen were single-celled and very simple, either bacteria or archaea. However, in 2010, Italian scientists found anaerobic animals living on the sea floor in the Mediterranean Sea. The animals are less than a millimeter long, and look like little jellyfish. But, this discovery begs all sorts of exciting questions about where else such life might be found. If we can have multi-cellular animals without oxygen, could there be similar aliens on other planets in our very own solar system!?!

Other Atmospheric Conditions

There are also organisms that have adapted to a range of other conditions. Xerophiles are well-adapted to dry climates. Barophiles are well-adapted to high-pressure environments. Toxitolerant organisms are able to withstand high doses of chemicals that would damage other organisms, such as nuclear waste.

If you live in the Yukon, you might be awestruck that anyone could possibly handle living in Texas in August. If you live in Texas, you might be baffled as to why anyone would actually drive during a snowstorm instead of just staying home. Similarly, some organisms are better adapted to cold temperatures, while other organisms are better adapted to hot temperatures. For example, psychrophiles are cold-loving organisms and can grow in freezing weather, whereas thermophiles are hot-loving organisms and have an optimal growth pattern at around 60 degrees Celsius. There are even some archaea that are hyperthermophiles, and can have a growth optimum of 80 degrees Celsius or higher!

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