Difficulty: Introductory | Prerequisites: Basic cell biology (prokaryotic vs. eukaryotic cell structure, Chapter on Prokaryotic Cellular Structure)
Prokaryotic organisms are the most metabolically diverse group of life on Earth, yet introductory biology courses often treat them as simple. This topic sits at the intersection of cell biology and ecology: you need to understand how prokaryotes obtain energy and carbon before you can make sense of biogeochemical cycles, symbiosis, or microbial pathogenesis. If you have covered basic cell structure and know the difference between prokaryotic and eukaryotic cells, you are ready for this material. Everything here builds toward understanding how bacteria and archaea shape the environments they inhabit, from deep-sea vents to the human gut.
Prokaryotes are classified by two axes: where they get energy (light or chemicals) and where they get carbon (CO₂ or organic molecules). Combining these gives four metabolic modes: photoautotroph, photoheterotroph, chemoautotroph, and chemoheterotroph. Prokaryotes also vary in how they respond to oxygen, ranging from strict aerobes to organisms that oxygen kills outright.
Autotroph
An organism that fixes inorganic carbon (CO₂) into organic compounds, producing its own food. Think of it as a self-feeder: it builds complex molecules from simple ones.
Heterotroph
An organism that obtains carbon from pre-formed organic molecules made by other organisms. In simple terms, it eats what something else already built.
Phototroph
An organism that uses light as its primary energy source. Think of it as solar-powered.
Chemotroph
An organism that obtains energy by oxidising chemical compounds, either organic or inorganic. In simple terms, it runs on chemical fuel rather than sunlight.
Photoautotroph
An organism that uses light energy and fixes CO₂ for carbon. Cyanobacteria and plants are the classic examples. Think of it as: sunlight in, CO₂ in, organic molecules out.
Photoheterotroph
An organism that uses light for energy but relies on organic compounds for carbon. These are relatively rare; purple non-sulphur bacteria are the textbook example.
Chemoautotroph (chemolithoautotroph)
An organism that gets energy from oxidising inorganic molecules (such as H₂S, NH₃, or Fe²⁺) and fixes CO₂ for carbon. Think of it as: no sunlight needed, no organic food needed. Found at hydrothermal vents and in soil nitrification.
Chemoheterotroph
An organism that gets both energy and carbon from organic compounds. Most animals, fungi, and the majority of bacteria you encounter in everyday life fall here. In simple terms, this is the "eat organic food, burn organic food" category.
Obligate aerobe
A bacterium that requires molecular oxygen (O₂) for growth. It uses aerobic respiration and cannot survive without oxygen.
Obligate anaerobe
A bacterium that cannot tolerate oxygen at all; O₂ is toxic to it. It relies on anaerobic metabolic pathways such as fermentation or anaerobic respiration.
Facultative anaerobe
A bacterium that prefers oxygen when available but can switch to anaerobic metabolism when oxygen is absent. Think of it as metabolically flexible: it uses the best option at hand.
Aerotolerant anaerobe
A bacterium that does not use oxygen for metabolism but is not harmed by its presence. It carries on with fermentation regardless of whether O₂ is around.
Microaerophile
A bacterium that requires oxygen but only at concentrations lower than atmospheric levels. Too much O₂ is harmful; too little is also a problem.
The first axis of metabolic classification asks where an organism gets its energy.
Phototrophs harvest light energy using pigments (e.g. bacteriochlorophyll in purple bacteria, chlorophyll in cyanobacteria).
Chemotrophs obtain energy by oxidising chemical compounds. If the compound is inorganic (H₂S, Fe²⁺, NH₃), the organism is a chemolithotroph. If organic (glucose, amino acids), it is a chemoorganotroph.
The second axis asks where carbon comes from.
Autotrophs fix CO₂ into organic molecules, typically via the Calvin cycle or alternative carbon-fixation pathways.
Heterotrophs take in pre-made organic carbon from their environment.
Combining the two axes yields four categories:
Photoautotroph: energy from light, carbon from CO₂. Examples: cyanobacteria, algae, plants. These organisms drive most of Earth's primary production.
Photoheterotroph: energy from light, carbon from organic molecules. Examples: purple non-sulphur bacteria, green non-sulphur bacteria. Relatively uncommon but important in certain aquatic environments.
Chemoautotroph: energy from inorganic chemical reactions, carbon from CO₂. Examples: nitrifying bacteria (Nitrosomonas, Nitrobacter), sulphur-oxidising bacteria at hydrothermal vents, iron-oxidising bacteria. These organisms are the primary producers in ecosystems without sunlight.
Chemoheterotroph: energy and carbon both from organic molecules. Examples: most bacteria you encounter in lab and clinical settings, all fungi, all animals. This is the most common metabolic mode across life.
Prokaryotes also differ in their relationship with oxygen, and this is independent of the energy/carbon classification:
Obligate aerobes require O₂ for aerobic respiration. O₂ serves as the terminal electron acceptor in their electron transport chain. Example: Mycobacterium tuberculosis.
Obligate anaerobes are killed by O₂. Oxygen generates toxic reactive oxygen species (superoxide, peroxide) that these organisms cannot neutralise because they lack the enzymes superoxide dismutase and catalase. Example: Clostridium botulinum.
Facultative anaerobes use aerobic respiration when O₂ is present (higher ATP yield) and switch to fermentation or anaerobic respiration when it is not. Example: Escherichia coli.
Aerotolerant anaerobes do not use O₂ but tolerate its presence. They rely exclusively on fermentation. Example: Streptococcus pyogenes.
Microaerophiles need O₂ but at lower-than-atmospheric concentrations. They have limited defences against reactive oxygen species. Example: Helicobacter pylori.
A classic lab tool for distinguishing these categories is the thioglycollate broth tube: a test tube with a gradient from oxygenated at the top to anaerobic at the bottom. Each type of organism grows at a characteristic depth in the tube.
Chemoautotrophic bacteria at deep-sea hydrothermal vents form the base of entire ecosystems where sunlight never reaches. The same principle is why astrobiologists study chemoautotrophy when looking for possible life on other planets or moons (e.g. Europa, Enceladus): if life can run on chemical energy in total darkness on Earth, it might do so elsewhere.
The thioglycollate tube principle is used in clinical microbiology to identify pathogens. Knowing whether a wound infection is caused by an obligate anaerobe (like Clostridium) versus a facultative anaerobe (like E. coli) directly affects treatment decisions, including whether to debride tissue and which antibiotics to choose.
Students often assume that "autotroph" means "uses photosynthesis." It does not. Chemoautotrophs fix CO₂ without any light at all. The "auto" refers to carbon source, not energy source.
Students frequently confuse "facultative anaerobe" with "aerotolerant anaerobe." The key difference: a facultative anaerobe actively uses oxygen when it is available (switching metabolic gears for higher ATP yield), whereas an aerotolerant anaerobe ignores oxygen entirely and sticks with fermentation regardless.
A common mistake is thinking that obligate anaerobes simply "prefer" no oxygen. In fact, oxygen is actively toxic to them because they lack the enzymes (superoxide dismutase, catalase) needed to neutralise reactive oxygen species.
Students sometimes treat the four metabolic modes as four separate, unrelated categories. They are not. They are two independent binary choices (energy source × carbon source) combined into a 2×2 grid. Once you see the grid, classification becomes straightforward.
⚠️ Expect to be given a description of an organism's energy and carbon sources and asked to classify it into one of the four metabolic modes. Practice the 2×2 grid until it is automatic.
⚠️ The thioglycollate tube question is a perennial exam favourite. Know where each oxygen-tolerance category grows in the tube: obligate aerobes at the top, obligate anaerobes at the bottom, facultative anaerobes concentrated at the top but present throughout, aerotolerant anaerobes evenly distributed, microaerophiles in a band just below the top.
⚠️ Be able to name the two key enzymes that protect against reactive oxygen species: superoxide dismutase (converts superoxide to H₂O₂) and catalase (converts H₂O₂ to water and O₂). Obligate anaerobes lack both.
⚠️ Do not confuse energy source with carbon source. "Photo-" and "chemo-" describe energy. "-autotroph" and "-heterotroph" describe carbon. These are tested separately and together.
True or false: All autotrophs use photosynthesis.
False. Chemoautotrophs fix CO₂ using energy from inorganic chemical reactions, with no light involved.
Fill in the blank: An organism that uses light for energy and organic compounds for carbon is called a ________.
Photoheterotroph.
True or false: Facultative anaerobes die in the presence of oxygen.
False. Facultative anaerobes thrive with oxygen and switch to anaerobic metabolism without it. It is obligate anaerobes that are killed by oxygen.
Fill in the blank: The two enzymes that protect cells from reactive oxygen species are _______ and _______.
Superoxide dismutase and catalase.
True or false: Chemoheterotrophs get their energy from light.
False. They get both energy and carbon from organic compounds. The "chemo-" prefix indicates chemical energy, not light.
Q: A bacterium is discovered living near a hydrothermal vent on the ocean floor. It oxidises hydrogen sulphide (H₂S) and uses CO₂ as its sole carbon source. What is its metabolic classification?
A: Chemoautotroph. It derives energy from an inorganic chemical (H₂S) and fixes CO₂ for carbon.
Q: You inoculate a thioglycollate broth tube and observe bacterial growth evenly distributed throughout the tube. What oxygen classification does this organism most likely have?
A: Aerotolerant anaerobe. Even distribution means the organism does not respond to the oxygen gradient; it uses fermentation regardless of oxygen levels but is not harmed by O₂.
Q: E. coli can grow in both aerobic and anaerobic conditions but grows faster with oxygen. Classify its oxygen requirement and explain why growth is faster with O₂.
A: Facultative anaerobe. Aerobic respiration produces more ATP per glucose molecule (approximately 38 ATP) than fermentation (2 ATP), so the organism grows faster when it can use oxygen as the terminal electron acceptor.
Q: Why is oxygen toxic to obligate anaerobes?
A: Oxygen generates reactive oxygen species such as superoxide radicals (O₂⁻) and hydrogen peroxide (H₂O₂). Obligate anaerobes lack the protective enzymes superoxide dismutase and catalase, so these toxic byproducts accumulate and damage cellular components.
Q: A student says "cyanobacteria are chemotrophs because they carry out chemical reactions to make energy." What is wrong with this statement?
A: Cyanobacteria are phototrophs, not chemotrophs. Although photosynthesis involves chemical reactions, the primary energy input is light. The "photo-" vs. "chemo-" distinction refers to the original energy source, not to whether chemical reactions occur (all metabolism involves chemical reactions).
This material connects directly to photosynthesis and cellular respiration: photoautotrophs use photosynthesis, and the aerobe/anaerobe distinction depends on how organisms handle the electron transport chain and terminal electron acceptors. When you reach ecology and biogeochemical cycles, the nitrogen cycle and carbon cycle both depend on the metabolic diversity covered here, particularly chemoautotrophy and nitrogen fixation (covered in the companion notes on Prokaryotic Symbiosis). Understanding metabolic classification also sets up the pathogenesis section: knowing that Clostridium is an obligate anaerobe explains why deep wounds are at risk for tetanus and botulism.
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