Microbial Metabolism and Diversity

All life needs three things to build and run a cell: a source of energy to do work, a source of electrons (equivalently, reducing power, often supplied with hydrogen atoms) to run redox reactions, and a source of carbon to build biomass. Crucially, these three requirements are logically independent of one another, and biologists classify organisms by naming, in order, the energy source, the electron/hydrogen source, and the carbon source, then fusing the terms into a single compact word. For energy, an organism is either a chemotroph, extracting energy from chemical bonds by oxidizing a substrate, or a phototroph, capturing energy from light using pigments such as chlorophylls or bacteriorhodopsins. For electrons, an organism is either an organotroph, stripping electrons from organic molecules (sugars, fatty acids, amino acids), or a lithotroph, stripping electrons from inorganic molecules (H₂, H₂S, Fe²⁺, NH₃, S⁰). For carbon, an organism is either a heterotroph, building its cell mass from preformed organic carbon compounds, or an autotroph, building its cell mass from scratch out of CO₂ (via the Calvin cycle, the reductive TCA cycle, or other CO₂-fixation pathways). Combining one term from each axis yields the standard nutritional types. A human is a chemoorganoheterotroph: chemical energy, organic electrons, organic carbon. A cyanobacterium doing oxygenic photosynthesis is a photolithoautotroph: light energy, water as the electron donor, CO₂ as carbon source. A nitrifying soil bacterium such as Nitrosomonas is a chemolithoautotroph: it oxidizes ammonia for energy and electrons, and fixes CO₂ for carbon. Purple non-sulfur bacteria are famous for photoorganoheterotrophy, using light for energy but organic acids as both electron donor and carbon source — a strategy plants and animals never use. This vocabulary is not decoration; it is a compressed description of an organism's entire ecological niche, because it specifies exactly what resources the cell needs and what byproducts it releases.

Step 1 — Energy source: chemotrophy vs phototrophy. Every cell needs ATP (or an equivalent proton-motive force) to do mechanical, transport, and biosynthetic work. Chemotrophs generate this by transferring electrons from a reduced substrate through an electron transport chain (or via substrate-level phosphorylation in fermentation) to a terminal electron acceptor, releasing free energy that is trapped as a transmembrane ion gradient. Phototrophs instead absorb photons with light-harvesting pigments embedded in the cytoplasmic membrane or internal membrane systems, exciting an electron to a higher energy state and passing it through a photosynthetic electron transport chain that likewise builds a proton-motive force. Both routes converge on the same universal energy currency, ATP synthase turning a proton gradient into ATP; only the upstream electron source differs.

Step 2 — Electron/hydrogen source: organotrophy vs lithotrophy. This axis specifies where the electrons entering that transport chain come from. Chemoorganotrophs oxidize organic molecules like glucose, releasing electrons captured on carriers NAD⁺ and FAD. Chemolithotrophs instead oxidize inorganic substrates — hydrogen gas, hydrogen sulfide, elemental sulfur, ferrous iron, ammonia, nitrite — using specialized periplasmic or membrane-bound enzymes (hydrogenases, sulfur oxidases, ammonia monooxygenase) that feed electrons directly into the transport chain, often at different entry points than organic substrates. Lithotrophy is almost exclusively a prokaryotic (and archaeal) talent; it lets microbes colonize rock, sediment, and hydrothermal vents where organic carbon is scarce or absent.

Step 3 — Carbon source: heterotrophy vs autotrophy. Independent of where energy or electrons come from, a cell must assemble carbon skeletons for sugars, lipids, nucleotides, and amino acids. Heterotrophs take shortcuts, importing organic molecules and funneling them into central metabolism. Autotrophs instead fix CO₂ using enzymes such as RuBisCO in the Calvin-Benson-Bassham cycle (most photoautotrophs and many chemolithoautotrophs), or alternative pathways like the reductive TCA cycle (some green sulfur bacteria) or the Wood-Ljungdahl pathway (acetogens, methanogens), building six-carbon sugars from one-carbon CO₂ molecules at real metabolic cost — roughly 3 ATP and 2 NADPH per CO₂ fixed in the Calvin cycle.

Step 4 — Combining the axes and recognizing versatility. Because the three choices are independent, nine theoretical combinations exist, though a few (like photolithoheterotrophy, actually observed in some cyanobacteria and purple bacteria under mixotrophic conditions) are rarer than others. Many prokaryotes are not locked into one box: a purple non-sulfur bacterium can run as a photoorganoheterotroph in the light and switch to chemoorganoheterotrophic fermentation or respiration in the dark; E. coli can act as a facultative anaerobe, switching its terminal electron acceptor from O₂ to nitrate to fumarate as conditions change; some sulfur bacteria are mixotrophs, simultaneously fixing CO₂ and assimilating organic compounds. This flexibility — metabolic versatility — is a defining ecological advantage of the prokaryotic and archaeal world, letting the same organism persist across fluctuating light, oxygen, and nutrient regimes that would kill a metabolically rigid competitor.

The concept that a living cell could grow using only inorganic chemicals as an energy source, with no organic nutrients at all, was radical when first proposed, because 19th-century microbiology assumed all life needed organic carbon and energy sources (the near-universal metabolism of animals, fungi, and most known bacteria). This was overturned by the Russian microbiologist Sergei Winogradsky in the 1880s. Winogradsky studied colorless sulfur bacteria, notably Beggiatoa, which he observed accumulating visible sulfur granules inside their cells when grown in water containing hydrogen sulfide. He designed simple but decisive experiments: he grew these bacteria in mineral media completely free of organic carbon, supplying only H₂S, CO₂, and inorganic salts. The bacteria grew and multiplied. When he then removed the H₂S from the medium, the sulfur granules inside the cells disappeared, being further oxidized to sulfate, and this oxidation coincided with continued growth. Winogradsky reasoned, and later confirmed with nitrifying bacteria (Nitrosomonas, Nitrobacter) that oxidize ammonia and nitrite, that the energy released from oxidizing these inorganic compounds was being used to fix CO₂ into cell material — a process he named chemolithotrophy (originally chemoautotrophy). This directly falsified the assumption that organic carbon and energy sources were required for life, and it revealed for the first time that inorganic chemical energy alone could power autotrophic CO₂ fixation, decades before the biochemical machinery (Calvin cycle, electron transport chains) was understood. A second landmark came from the Dutch-American microbiologist Cornelis van Niel, working in the 1930s at Stanford's Hopkins Marine Station. Van Niel compared oxygenic photosynthesis in plants and cyanobacteria with anoxygenic photosynthesis in purple and green sulfur bacteria. Purple sulfur bacteria performed photosynthesis using H₂S as electron donor, producing sulfur granules instead of oxygen: CO₂ + 2 H₂S --(light)--> [CH₂O] + H₂O + 2 S Van Niel proposed a general equation for all photosynthesis: CO₂ + 2 H₂A --(light)--> [CH₂O] + H₂O + 2 A where H₂A is a generic electron/hydrogen donor — H₂O in plants and cyanobacteria, H₂S in purple and green sulfur bacteria, or even H₂ gas or organic compounds in other phototrophs. This unification predicted that in oxygenic photosynthesis, the O₂ released must come from the splitting of water, not from CO₂ as had been long assumed by analogy to simple combustion chemistry. This was a striking, testable prediction. It was confirmed independently in the 1940s by Samuel Ruben and Martin Kamen, who used the heavy oxygen isotope ¹⁸O to label water versus CO₂ in separate experiments with algae: when H₂¹⁸O was supplied, the evolved O₂ was ¹⁸O-labeled; when C¹⁸O₂ was supplied instead, it was not. This isotope-tracing experiment definitively proved that photosynthetic oxygen comes from water, vindicating van Niel's comparative logic across the whole spectrum of phototrophic bacteria and validating the general framework that groups phototrophs by their electron donor regardless of whether it happens to be water, sulfide, or an organic acid. Together, Winogradsky's and van Niel's work established the empirical foundation for treating energy source, electron source, and carbon source as separable, experimentally testable properties of a cell — exactly the logic this classification system depends on.

Example 1 — Assigning nutritional class from an experimental description. A soil microbiologist isolates a bacterium from a hydrothermal sediment. In culture, it grows in complete darkness in a mineral salts medium with no added organic carbon. Growth only occurs when the medium is bubbled with H₂ gas and CO₂, and is abolished if either gas is withheld. Radiolabeled ¹⁴CO₂ is incorporated into cell biomass at a rate proportional to H₂ consumption. Reasoning: No light is required, and growth stops in the dark-adapted control anyway (energy comes from a chemical reaction, so chemo-). The electron/hydrogen donor is H₂ gas, an inorganic molecule (so litho-). Carbon comes entirely from CO₂, confirmed by ¹⁴C incorporation, with no organic carbon supplement needed (so auto-). This organism is a chemolithoautotroph, mechanistically similar to hydrogen-oxidizing bacteria such as Hydrogenobacter or to hydrogenotrophic methanogens found at vents and in anoxic sediments, oxidizing H₂ via a membrane-bound hydrogenase and fixing carbon via the Calvin cycle or the Wood-Ljungdahl pathway depending on the lineage.

Example 2 — Quantifying a Winogradsky-column-style microbial food web. A Winogradsky column is a classic teaching microcosm: mud plus organic matter and sulfate, sealed in a glass cylinder and left in light. After several weeks, students observe stratified colored layers. Reasoning through the physiology: At the bottom, anaerobic organic matter decomposition by chemoorganoheterotrophic fermenters and sulfate-reducing bacteria produces H₂S. This H₂S diffuses upward. Where light penetrates but O₂ is absent (an anoxic, illuminated zone), green and purple sulfur bacteria form a colored band; these are photolithoautotrophs using H₂S as electron donor and light as energy source, precipitating sulfur granules, exactly the organisms Winogradsky and van Niel studied. Near the top, where O₂ is present, colorless sulfur-oxidizing bacteria such as Beggiatoa or Thiobacillus form a band; these are chemolithoautotrophs, oxidizing the same H₂S or S⁰ using O₂ as the terminal electron acceptor instead of light-driven electron flow. At the very surface, aerobic chemoorganoheterotrophs consume residual organic matter using O₂. If the sulfide flux from the bottom layer is measured at, say, 2 mmol H₂S per liter per day, and the purple sulfur bacteria layer oxidizes sulfide to sulfate with a stoichiometry of roughly 2 mol H₂S oxidized per mol CO₂ fixed (from van Niel's general equation), a column supporting 10 L of anoxic mud producing 20 mmol H₂S/day could in principle support fixation of roughly 10 mmol CO₂/day by that phototrophic layer alone — illustrating quantitatively how one guild's waste (sulfide from fermenters and sulfate reducers) becomes another guild's energy source (lithotrophic sulfide oxidizers), a chain of vertically stratified but metabolically linked physiologies visible as bands of color in a single glass jar.

Sergei Nikolaievich Winogradsky trained originally in plant physiology in Kyiv and St. Petersburg before turning to microbiology, driven by fascination with the colorless sulfur bacteria he observed forming visible filaments in sulfurous ponds. Working largely alone with simple perfusion and enrichment-culture techniques of his own design — precursors to what became known as "Winogradsky columns" — he built a career studying the physiology of bacteria that no one else considered biochemically interesting: sulfur oxidizers, iron bacteria, and nitrifiers. His insistence on studying microbes in conditions mimicking their natural habitat, rather than only on artificial rich media, made him a founder of environmental microbiology and soil microbiology as disciplines, and his sulfur-granule and CO₂-fixation experiments gave the world its first proof that chemistry alone, without any organic fuel, could sustain a living cell's growth.

The energy/electron/carbon framework is not academic bookkeeping; it drives real biotechnology and medicine. In bioremediation, engineers select or engineer chemolithotrophic and chemoorganotrophic bacteria to detoxify contaminated sites: sulfate-reducing bacteria precipitate toxic heavy metals as insoluble sulfides, while hydrocarbon-oxidizing chemoorganoheterotrophs are seeded onto oil spills because their metabolism uses the pollutant itself as an electron donor and carbon source. In biomining, chemolithotrophic bacteria such as Acidithiobacillus ferrooxidans oxidize ferrous iron and sulfide minerals to leach copper, gold, and uranium from low-grade ore, an industrial-scale application of exactly the lithotrophic chemistry Winogradsky first characterized. In wastewater treatment, nitrifying chemolithoautotrophs convert toxic ammonia to nitrate, while denitrifying chemoorganotrophs then reduce nitrate to harmless N₂ gas, together closing the nitrogen cycle in every municipal treatment plant. In clinical microbiology, knowing that a pathogen is an obligate chemoorganoheterotroph dependent on host-derived glucose or amino acids helps explain tissue tropism and guides nutrient-limitation or efflux-based therapeutic strategies. In biogeochemistry and astrobiology, the discovery that chemolithoautotrophy can sustain entire ecosystems (deep-sea hydrothermal vent communities built on sulfide- and hydrogen-oxidizing bacteria, with no sunlight input at all) reshaped ideas about where life could exist, on Earth's early crust or on other worlds lacking surface light, and directly motivates the search for chemosynthetic life on icy moons such as Europa and Enceladus.

Ask the tutor: How does the choice of terminal electron acceptor (O₂ vs. nitrate vs. sulfate) change the energy yield for a chemoorganoheterotroph, and how does that connect to aerobic versus anaerobic respiration?