Water, the home of most bacteria, contains oxygen in
two forms. The first form, free oxygen, is the most readily available
form. Free oxygen is basically the same as dissolved oxygen - oxygen
from the atmosphere which has become dissolved in water.
Aerobic bacteria require free oxygen in order to survive.
Oxygen can also be found in the water in another form. Food and even water itself contain oxygen, but this oxygen is tightly bound to the food and water. As you can see in the picture above, each molecule of water contains one oxygen molecule (O) and two hydrogen molecules (H). The oxygen can be ripped out of the water molecule by anaerobic bacteria, but it takes much more energy to break apart food and water in search of oxygen than it does to simply use free oxygen. Since anaerobic bacteria use so much of their time and energy scrounging for oxygen, they take longer to digest organic matter in water.
The third type of microorganisms, those which are facultative, have properties of both aerobic and anaerobic organisms. They can live with or without free oxygen. When the oxygen content of water is high, facultative bacteria consume food very quickly using the free oxygen in the water. In low oxygen concentrations, facultative bacteria are still able to consume organic material, although they do so much more slowly.