Chapter 23: Microorganisms in the Environment and Effects on Human Health

Microorganisms comprise approximately half of Earth's total biomass and serve as the primary drivers of biogeochemical cycling, which sustains all life on the planet. The carbon cycle represents the most fundamental biogeochemical process, dependent on photoautotrophs and chemoautotrophs to fix atmospheric carbon dioxide into usable organic compounds. The nitrogen cycle involves a specialized group of prokaryotes employing the nitrogenase enzyme to convert atmospheric nitrogen gas into ammonia through nitrogen fixation, followed by sequential transformations including ammonification, nitrification of ammonium to nitrate, denitrification returning nitrogen to the atmosphere, and the recently characterized anammox pathway that oxidizes ammonium and nitrite directly to dinitrogen gas with applications in wastewater treatment systems. The sulfur cycle traces sulfur movement from geological deposits through dissimilatory sulfur reduction producing hydrogen sulfide and back to sulfate through oxidation pathways. Unlike other major cycles, the phosphorus cycle operates without a gaseous phase, yet remains crucial because terrestrial phosphorus runoff into aquatic ecosystems drives eutrophication, leading to algal blooms that deplete dissolved oxygen. Microbial communities in soil respond to moisture, oxygen availability, and pH conditions, while aquatic environments display distinct zonation patterns including the littoral zone with high biodiversity, the limnetic zone at the surface, the profundal zone at depth, and the benthic zone at the bottom, with marine environments containing an additional abyssal zone. The chapter shifts toward public health implications, explaining how environmental disasters including floods and tsunamis increase infectious disease risk through waterborne pathogens such as cholera, hepatitis A, and typhoid fever, alongside vector-borne diseases including malaria and dengue fever amplified by infrastructure damage and population displacement. Finally, the chapter addresses bioterrorism as a threat involving deliberate microbial agent release, categorizing the most dangerous pathogens including bacterial agents like anthrax, plague, tularemia, and botulinum toxin as well as viral threats such as smallpox and viral hemorrhagic fevers from Filoviridae and Arenaviridae families, with response requiring specialized protocols, isolation procedures, and coordination through networks like the Laboratory Response Network.