Chapter 27: Environmental Microbiology

Environmental Microbiology explains how microbes contribute to biogeochemical cycles—including the carbon, nitrogen, sulfur, and phosphorus cycles—by breaking down organic material, fixing atmospheric gases, and transforming chemical compounds essential for life. In the carbon cycle, microbes decompose organic matter, releasing carbon dioxide (CO₂) through respiration and fermentation, while methanogens produce methane (CH₄) in anaerobic conditions. In the nitrogen cycle, nitrifying bacteria convert ammonia to nitrates, denitrifying bacteria return nitrogen gas to the atmosphere, and nitrogen-fixing bacteria like Rhizobium and cyanobacteria convert atmospheric nitrogen into usable forms for plants. Similar microbial processes play key roles in the sulfur and phosphorus cycles, converting these elements between oxidized and reduced states to support life. The chapter also explores aquatic microbiology, distinguishing between freshwater and marine environments. In lakes, rivers, oceans, and estuaries, microbes form biofilms, break down organic matter, and regulate nutrient availability. Special attention is given to deep-sea and hydrothermal vent communities, where chemoautotrophic bacteria form the base of the food web by oxidizing hydrogen sulfide. Extremophiles, including thermophiles, halophiles, and acidophiles, are discussed as models for understanding life in harsh environments and for potential applications in biotechnology. Soil microbiology is another key focus, examining how microbes influence plant growth, soil fertility, and decomposition. The chapter covers mycorrhizal fungi, rhizobacteria, and actinomycetes, which form symbiotic relationships with plants or break down tough organic material like cellulose and lignin. The final section discusses bioremediation—the use of microbes to clean up pollutants like oil spills, pesticides, and heavy metals—and wastewater treatment, including primary, secondary (biological), and tertiary processes. Topics such as sewage sludge digestion, trickling filters, and activated sludge systems are explained, along with the role of indicator organisms (like E. coli) in monitoring water safety. The chapter concludes with a look at how environmental microbiology is essential for sustainable ecosystems, pollution control, and public health.