4.3: Representative Groups

Scientists have studied prokaryotes for centuries, but it wasn’t until 1966 that scientist Thomas Brock (1926–) discovered that certain bacteria can live in boiling water. This led many to wonder whether prokaryotes may also live in other extreme environments, such as at the bottom of the ocean, at high altitudes, or inside volcanoes, or even on other planets. Prokaryotes have an important role in changing, shaping, and sustaining the entire biosphere. They can produce proteins and other substances used by molecular biologists in basic research and in medicine and industry. For example, the bacterium Shewanella lives in the deep sea, where oxygen is scarce. It grows long appendages, which have special sensors used to seek the limited oxygen in its environment. It can also digest toxic waste and generate electricity. Other species of prokaryotes can produce more oxygen than the entire Amazon rainforest, while still others supply plants, animals, and humans with usable forms of nitrogen; and inhabit our body, protecting us from harmful microorganisms and producing some vitally important substances. This chapter will examine the diversity, structure, and function of prokaryotes.

• 4.3.1: Prokaryote Habitats and Ecology

  This page explores prokaryotes, specifically Archaea and Bacteria, emphasizing their ubiquitous presence and crucial ecological roles in soil formation, nutrient cycling, and gas exchanges impacting life. They thrive in extreme conditions with metabolic flexibility. While many prokaryotes are beneficial, some can cause diseases and spoilage and contribute to climate change by emitting greenhouse gases from melting permafrost.

• 4.3.2: Metabolic Lifestyles

  This page discusses the classification of organisms based on carbon and energy sources. Autotrophs produce organic compounds from inorganic CO2, while heterotrophs rely on organic compounds, including pathogens. Energy sources are divided into phototrophs (light) and chemotrophs (chemical), with the latter further split into organotrophs and lithotrophs. Most organisms are chemoheterotrophs, using organic molecules for carbon and energy. A summary table categorizes these distinctions.

• 4.3.3: Proteobacteria

  This page covers the phylum Proteobacteria, detailing its five classes: Alphaproteobacteria, Betaproteobacteria, Gammaproteobacteria, Deltaproteobacteria, and Epsilonproteobacteria. It highlights their Gram-negative characteristics and diverse metabolic functions, including obligate intracellular pathogens like Rickettsia in Alphaproteobacteria and numerous human pathogens like Pseudomonas and Escherichia coli in Gammaproteobacteria.

• 4.3.4: Cyanobacteria

  This page focuses on nonproteobacteria gram-negative bacteria, particularly the Cyanobacteria phylum, known for their role in oxygenic photosynthesis and ecological contributions. It highlights their adaptability, nitrogen-fixing abilities, and the potential for harmful blooms. Additionally, it covers other gram-negative groups like spirochetes and the CFB group, detailing their characteristics and ecological significance, underscoring the vital role of phototrophic bacteria in ecosystems.

• 4.3.5: Gram-positive Bacteria

  This page explores the classification of gram-positive bacteria into high and low G+C groups, detailing notable genera such as Actinobacteria and Bacilli. It covers important pathogens like Mycobacterium linked to tuberculosis, Clostridium toxins, and Staphylococcus aureus related to skin infections. The implications for diagnostics and treatment of infections are highlighted, alongside ethical considerations in bioprospecting.

• 4.3.6: Deeply Branching Bacteria

  This page examines deeply branching bacteria, some of the oldest life forms on Earth, linked to the last universal common ancestor (LUCA). It highlights examples like Acetothermus and members of the classes Aquificae and Thermotogae, which thrive in extreme conditions. The study of these bacteria offers insights into early life's characteristics and functions, while their resilience to harsh environments underscores their evolutionary importance and potential industrial applications.

• 4.3.7: Archaea

  This page discusses the unique characteristics of Archaea, including their distinct membranes, complex genomes, and survival in extreme environments as extremophiles. Notable phyla like Crenarchaeota and Euryarchaeota are highlighted, along with the role of methanogens. The page also references academic contributions regarding their implications in human health, particularly relating to gum disease, and emphasizes ongoing research into their ecological roles and significance in microbiology.

Thumbnail: A cladogram linking all major groups of living organisms to the LUCA (the black trunk at the bottom), based on ribosomal RNA sequence data.