Early microbial ecologists knew that hydrogen gas is oxidized in soil and that ammonia is oxidized in sewage. How did these scientists determine that microbes are responsible for these ecological reactions? Choose one: A. by removing hydrogen gas and ammonia from soil and sewage samples and observing dead microbes under the microscope. B. by adding pure cultures to sewage samples and detecting the formation of nitrate C. by applying heat and antimicrobial treatment to sewage samples and observing that nitrate was no longer formed D. by preparing pure cultures from soil and sewage samples and observing their metabolic reactions in the laboratory

The correct answer is option C. "by applying heat and antimicrobial treatment to sewage samples and observing that nitrate was no longer formed".

Explanation:

The easiest way to demonstrate that an element is responsible for a reaction, is to remove the element and see if the reaction still occurs. In this case, early microbial ecologists removed microbes from sewage samples by applying heat. When ecologists observed that nitrate was no longer formed in the samples, they concluded that microbes are responsible for hydrogen gas oxidation in soil and ammonia oxidation in sewage. Nowadays, the bacteria that perform these ecological reactions are known as nitrifying bacteria, and the reactions that they perform are well characterized.