Aerobic as well as anaerobic bacteria are found in marine bottom deposits. They are most abundant in the topmost few centimeters of sediment below which both types of bacteria decrease in number with depth. A statistical treatment of the data on their vertical distribution suggests that aerobes are active to a depth of only 5-10 centimeters whereas anaerobes are active to depths of 40-60 centimeters below which they seem to be slowly dying off. However, microbiological processes may continue at considerably greater depths owing to the activity of the bacterial enzymes that accumulate in the sediments. The organic content is the chief factor which influences the number and kinds of bacteria found in the sediments.

Bacteria lower the oxidation-reduction (O/R) potential of the sediments. Vertical sections reveal that the reducing intensity of the sediments increases with depth but the muds have the greatest reducing capacity near the surface. Three different types of oxygen absorption by the reduced muds are described, namely, chemical, enzymatic, and respiratory.

Bacteria that decompose or transform proteins, lipins, cellulose, starch, chitin and other organic complexes occur in marine sediments. These bacteria tend to reduce the organic matter content of the sediments to a state of composition more closely resembling petroleum although methane is the only hydrocarbon known to be produced by the bacteria. The precipitation or solution of calcium carbonate as well as certain other minerals is influenced by microbiological processes that affect the hydrogen-ion concentration. Other bacterial processes influence the sulphur cycle and the state of iron in the sediments. The possible role of bacteria in the genesis of petroleum is discussed.