Which of the following organisms is least important in reducing sewage to "soil," liquids, and gases?

The least important organisms in the reduction of sewage to "soil," liquids, and gases are anaerobic bacteria. While anaerobic bacteria play a role in the initial breakdown of organic matter in the absence of oxygen, their activity typically occurs in specific environments such as landfills or deep sediments where decomposition occurs. In these processes, anaerobic bacteria primarily produce methane and less stable intermediate products rather than fully breaking down the organic matter into stable soil-like substances.

In contrast, aerobic bacteria are highly effective in the decomposition process, as they utilize oxygen to break down organic materials further and convert them into more stable forms. Earthworms contribute significantly to soil formation through the consumption of organic matter and their role in aerating and mixing soil layers. Protozoa also play a role in the food web of decomposer organisms, helping to mineralize nutrients and further breakdown waste products.

Thus, while anaerobic bacteria are active in the sewage treatment process, their contribution to transforming sewage into stable soil and liquid forms is less significant compared to aerobic bacteria, earthworms, and protozoa.