ABSTRACT
This chapter broadly describes the role of microorganisms (or microbes) in some key ecological processes. As suggested by the word itself, microbes are organisms of very small size; that is, they typically are microscopic, meaning that man can see them only by using a microscope. Microbes are mostly unicellular organisms ubiquitously found in all the earth’s ecosystems and include both prokaryotes and eukaryotes. Examples include bacteria, protozoa, microalgae, archaea, and fungi, therefore, both autotroph (Chapter 13) and heterotroph microorganisms exist (mixotrophic microbes also exist, which exploit both organic and inorganic carbon sources). Microorganisms even exist inside the body of other organisms (e.g., in the digestive system of man, i.e., the intestinal flora, or cows). Although viruses are generally not considered as living beings and cannot be classified as microorganisms, they are studied in the field of microbiology due to the size and the significant influence they exert on the dynamics of bacterial populations. In aquatic ecosystems, viruses can represent a major source of bacterial mortality.
