Anoxic Environment: Definition & Conditions

Instructor: Danielle Haak

Danielle has a PhD in Natural Resource Sciences and a MSc in Biological Sciences

Anoxic environments are those where oxygen is completely absent. Surprisingly, certain critters have evolved to live in these conditions. This lesson will describe some different anoxic environments and the organisms that live there.

What Is Anoxia?

Anoxia is the absence of oxygen, so an anoxic environment is one that has no oxygen available. When we talk about anoxic environments, we are often referring to an aquatic environment with no dissolved oxygen or an underground environment (like soil or rock deep beneath the surface) without oxygen. However, the term can also be used to describe medical conditions where body tissues aren't receiving oxygen (causing cell death).

Sometimes the term 'anoxic' is inappropriately used interchangeably with the term 'hypoxic.' While an anoxic environment is one with no oxygen, a hypoxic environment is one with very little oxygen. It may have too little oxygen to support life, but it still has some available oxygen, so there is a difference between the two descriptors.

Anoxic Environments

The most common type of environment that may be anoxic is a body of water. Swamps or hypereutrophic water bodies are commonly devoid of dissolved oxygen. A hypereutrophic water body is one that is extremely rich in nutrients like phosphorus or nitrogen, creating an explosion of plant life. Because plants undergo photosynthesis to produce oxygen, we might first think that water bodies with a lot of plants would be oxygen-rich, but this isn't the case, especially if the primary plant matter is algae.

Algal blooms like this can lead to anoxic conditions.
algal bloom that can cause anoxia

You see, when a pond or swamp experiences an algal bloom, there might be an initial surge of dissolved oxygen. However, the algae will eventually die and sink to the bottom, where it's decomposed by bacteria and other critters. All of this food enables the population of decomposers to increase rapidly, and these organisms undergo respiration, meaning they are using up all available oxygen. Eventually, the oxygen is entirely depleted, creating an anoxic environment. In stagnant water bodies, there is little activity to introduce new oxygen to the system.

Other environments that may be anoxic are lakes, streams, or groundwater, and anoxic conditions commonly exist in the substrates at the bottom of a water body. For example, if you were to dive to the bottom of the lake and insert a probe to measure oxygen one foot into the sand or muck at the bottom, it's likely that it would be completely anoxic.

Organisms in Anoxic Environments

We usually assume that an environment without oxygen is an environment without life, but that isn't exactly true. Scientists have known for awhile that many unicellular organisms (like bacteria, protozoa, and viruses) are capable of living in extreme environments like anoxic ones. But more recently, scientists discovered a few types of multicellular animals living in anoxic conditions. (Some multicellular organisms have been known to live in anoxic conditions for short periods, but until this research, none were believed to live entirely in anoxic conditions.)

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