Chemoautotrophs are unique organisms, primarily bacteria and archaea, that produce their own food without relying on sunlight. Unlike plants and algae (photoautotrophs), which convert solar energy into chemical energy, chemoautotrophs operate entirely in the dark. They synthesize all necessary organic compounds from an inorganic carbon source, like carbon dioxide, and derive the energy for this synthesis from the oxidation of inorganic chemical compounds in their environment. This energy-generating process, known as chemosynthesis, allows these microbes to form the base of food webs in the planet’s most isolated and extreme habitats.
How Chemoautotrophs Create Food
The process of chemosynthesis is a two-step metabolic strategy that first generates energy and then uses that energy to build organic matter. The initial step involves oxidizing reduced inorganic chemicals, stripping electrons from these molecules in a process called chemolithotrophy. These released electrons are passed along an electron transport chain, establishing a proton gradient across the cell membrane. The flow of protons back across the membrane then drives the enzyme ATP synthase to produce adenosine triphosphate (ATP), the universal energy currency of the cell.
The second stage of chemosynthesis is carbon fixation, where the stored energy is used to convert carbon dioxide ($\text{CO}_2$) into complex sugars and biomolecules. Many chemoautotrophs utilize a pathway similar to the Calvin cycle, which is also used by plants, to incorporate the inorganic carbon atom from $\text{CO}_2$ into an organic molecule. Other groups, particularly those found in deep-sea vents, often employ the reductive tricarboxylic acid (rTCA) cycle for this carbon assimilation. Both pathways require the newly generated ATP and another energy-carrying molecule, often $\text{NADH}$ or $\text{NADPH}$, to reduce the $\text{CO}_2$ and build the carbon backbone.
The Chemical Fuels of Life
The “chemo” part of the name refers to the diverse range of inorganic electron donors that these organisms use to power their metabolism. These chemicals are typically in a reduced state and are oxidized to release energy. The availability of these specific compounds determines where chemoautotrophs can survive. The three major groups of energy substrates are sulfur, iron, and nitrogen compounds, each supporting a distinct group of microorganisms.
Sulfur-oxidizing bacteria are highly active in environments where they can access hydrogen sulfide ($\text{H}_2\text{S}$), a compound that is toxic to most other life. They oxidize this hydrogen sulfide into elemental sulfur or sulfate ($\text{SO}_4^{2-}$), gaining energy in the process. Other species are iron oxidizers, which gain energy by converting dissolved ferrous iron ($\text{Fe}^{2+}$) into ferric iron ($\text{Fe}^{3+}$). This process is widespread due to iron’s global abundance in the Earth’s crust.
Nitrogen-oxidizing microbes, often referred to as nitrifiers, manage the stepwise oxidation of ammonia ($\text{NH}_3$) to nitrite ($\text{NO}_2^-$) and then to nitrate ($\text{NO}_3^-$). This two-step process is carried out by different groups of bacteria and archaea working in concert to complete the nitrification cycle.
Where Chemoautotrophs Thrive
Chemoautotrophs are known as extremophiles because they inhabit environments hostile to most other organisms due to the lack of sunlight. The most iconic habitats are deep-sea hydrothermal vents, which revealed thriving ecosystems entirely fueled by chemosynthesis upon their discovery in 1977. At these vents, superheated, chemical-rich fluids erupt from the seafloor, providing an abundance of hydrogen sulfide, methane, and other reduced compounds that the microbes can oxidize for energy. These microbial communities form thick mats that are consumed by or live in symbiosis with larger organisms like giant tube worms and mussels, forming a complete food web independent of surface ecology.
Another significant habitat is the deep subsurface, extending miles below the surface in continental crust and ocean sediments. Here, chemoautotrophs derive energy from the geological reactions between water and rock, such as the serpentinization process that produces hydrogen gas. Cold seeps are also biologically rich sites where methane and sulfide-rich fluids slowly percolate out of the seafloor, providing another chemical feedstock for chemosynthetic microbes.
Their Role in Earth’s Ecosystems
Chemoautotrophs are fundamental drivers of global biogeochemical cycles. In the extreme environments where they are found, these organisms function as the primary producers, converting inorganic carbon into organic biomass that supports higher trophic levels. They effectively transfer energy from geological sources into the biological realm, allowing complex communities of animals to exist in total darkness.
On a planetary scale, chemoautotrophs are indispensable agents in the cycling of elements like nitrogen and sulfur. The nitrogen cycle relies heavily on nitrifying chemoautotrophs to convert ammonia into forms that are more readily accessible to plants and other microorganisms. Similarly, sulfur-oxidizing bacteria interconvert reduced sulfur compounds, which can be toxic, into the sulfate that is incorporated into proteins by other organisms.