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Anaerobic is relating to, involving, or requiring an absence of oxygen. In laymen’s: something that needs a lack of oxygen to survive. Some organisms may die if exposed to oxygen – weird notion, huh? Anaerobic is the opposite of aerobic. Aerobic means that it needs free oxygen to survive and will die if oxygen is not present. Aerobic is usually the type of life we’re used to thinking about – humans, cats, dogs, and fish.

Most of the time when we talk anaerobic or aerobic in aquariums, we’re talking about bacteria. In aquariums, they live in compact substrates like fine sand, mud, and clay. These substrates have minimal oxygen – and in some spots – no oxygen at all. Anaerobic bacteria, much like aerobic bacteria, act as beneficial bacteria and are capable of oxidizing ammonia and nitrite. The issue is anaerobic pockets (pockets with a lack of oxygen), can be deadly to all tank inhabitants if popped. 

Lake Tanganyika is well-known for the anaerobic pockets. Most diving activity is restricted to shallow parts of the lake – if you’re brave enough to dive at all – since diving fins can rupture a pocket. If one of those pockets burst it could potentially wipe out all life in Lake Tanganyika. But, since the topic of anaerobic pockets is worthy of its own section, I’ll leave that out of here.

Some fungi can be anaerobic as well, though they’re unlikely to harm your tank inhabitants as most are obligate anaerobes (more on that below.) 

Examples Of Aerobic Bacteria

There are several sub-divisions of anaerobic bacteria:

Obligate anaerobes:

These guys are poisoned by oxygen and will die if exposed to it in (relatively) small concentrations. They will gather and reproduce in areas where oxygen is lowest. In your tank, this is likely the bottom-most layer of dense substrates.

Facultative anaerobes:

Facultative anaerobes can live with or without oxygen. They can switch between metabolizing anaerobically or aerobically. This switch is a phenomenon known as the Pasteur effect. In the absence of oxygen, these bacteria will undergo a fermentation process. 

Aerotolerant organisms:

Like you probably surmised by the name, these organisms can tolerate oxygen. However, they can’t use it, but they don’t die from it, either. Unlike facultative anaerobes, they can’t pick and chose which metabolism function suits them and the Pasteur effect is the defining difference between the two.