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Biology LibreTexts

17.3A: Microorganisms and Water Quality

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  • Microorganisms from sewage can cause human disease, but can also negatively affect important ecosystems on which humans rely.

    Learning Objectives

    • Explain the relationship between microorganisms and water quality

    Key Points

    • Scientists typically measure water quality by testing for the presence of “indicator species ” of bacteria, harmless microorganisms that are found in the human gut alongside pathogenic species.
    • Typical indicator species include coliform bacteria (related to the pathogenic E. coli) and Pseudomonas aeruginosa. When levels of these bacteria are high, scientists begin testing for disease causing bacteria.
    • Ecosystems may also suffer from contaminated water. In aquatic ecosystems, sewage bacteria may cause “dead zones” when they use up the oxygen in the water while decomposing nutrients. Coral reefs may also become infected with sewage bacteria and die.

    Key Terms

    • hypoxic: Of, pertaining to, or suffering from hypoxia (lack of oxygen)

    Water Quality and Human Health

    Waterborne diseases are a infections transmitted by contaminated drinking water. Although there are many pathogens which can be transmitted through water, bacteria and protozoa are some of the most common organisms that cause disease. Monitoring for waterborne disease can be difficult because humans often shed very low numbers of pathogenic bacteria when they are infected. To test whether disease causing bacteria might be present, researchers measure the presence of indicator species, such as coliform bacteria (which are the group to which the pathogenic E. coli belongs) orPseudomonas aeruginosa. Although most coliform bacteria do not cause disease, they are commonly found in the human gut and in sewage, and their presence implies that human waste has reached the water supply. Researchers usually test water quality by sampling water and measuring the concentration of all bacteria in a sample. If the number of bacteria exceeds the limits set by water quality standards, the next step is to test for the presence of specific pathogens. Scientists can use genetic probes, or specific culture techniques to check whether harmful pathogens are present. Standards for testing may differ depending on the water source: drinking water is held to very high standards, while water quality in lakes and rivers may be held to more lax standards because recreational swimmers typically ingest very little water.

    Figure: E. coli Bacteria: Researchers monitor water quality by testing for coliform bacteria, relatives of E. coli

    Water Quality and The Environment

    Microrganisms can also have important impacts on the environment. All healthy ecosystems have their own communities of bacteria that decompose biological matter. However, contamination by sewage and human waste can disrupt the natural balance of bacteria and affect aquatic ecosystems. An influx of human pathogens can cause problems for ecosystems in several ways. First, sewage bacteria can cause hypoxic “dead zones” in aquatic ecosystems. The foreign bacteria rapidly reproduce and consume debris and nutrients in the sewage, but use up all the oxygen in the water in doing so. The de-oxygenated water is harmful to fish and other aquatic life. Coral reefs are also affected by sewage contaminated water. Coral can become infected by human gut bacteria, and this can cause “coral bleaching disease” where coral lose their normal bacterial and algae communities and die. Water quality is not just important for human health, it is important for the human communities that depend on aquatic and marine ecosystems.