Table of Contents
Chemolithoautotrophy, often called chemosynthesis, is a way of life in which organisms obtain energy from inorganic (non‑carbon) substances and use that energy to build organic molecules from carbon dioxide. This form of anabolic metabolism is especially important in environments where light is scarce or absent, such as deep oceans, underground, or inside rocks.
What “Chemolithoautotrophy” Means
The term can be broken down:
chemo-– energy comes from chemical reactions, not from light.litho-– the electron donors are inorganic (“rock‑derived”) substances such as hydrogen sulfide, elemental sulfur, ammonium, nitrite, ferrous iron, or hydrogen gas.auto-– carbon source is inorganic, mainly $CO_2$.-trophy– a way of obtaining energy and matter for growth.
So chemolithoautotrophic organisms:
- Use inorganic compounds as electron donors and energy sources.
- Use $CO_2$ as their main carbon source (they “fix” carbon).
- Produce organic compounds (sugars etc.) without using light.
The term “chemosynthesis” is often used more loosely to mean the whole process: oxidation of inorganic substances + $CO_2$ fixation to build biomass.
General Principle: Oxidizing Inorganic Compounds
Chemolithoautotrophs gain energy by oxidizing reduced inorganic compounds. In simple form, these reactions look like:
$$
\text{Inorganic electron donor} + O_2 \;(\text{or another acceptor}) \rightarrow \text{Oxidized product} + \text{energy}
$$
The released energy is then used to:
- Synthesize ATP.
- Generate “reducing power” (molecules such as NADH or NADPH).
- Drive $CO_2$ fixation into organic matter via a carbon fixation pathway (often the Calvin cycle, covered elsewhere).
A generic chemosynthetic growth reaction can be represented as:
$$
\text{Inorganic donor} + CO_2 + \text{nutrients} \longrightarrow \text{organic biomass} + \text{oxidized products}
$$
The specific donor and product depend on the type of chemolithoautotroph.
Main Types of Chemolithoautotrophs
Different groups specialize in different inorganic energy sources. Most known chemolithoautotrophs are bacteria and archaea.
Sulfur-Oxidizing Bacteria
Energy source: Reduced sulfur compounds such as:
- Hydrogen sulfide: $H_2S$
- Elemental sulfur: $S^0$
- Thiosulfate: $S_2O_3^{2-}$
- Sulfide-containing minerals
Typical oxidation reactions
For hydrogen sulfide:
$$
H_2S + 2\,O_2 \rightarrow SO_4^{2-} + 2\,H^+ + \text{energy}
$$
Here, $H_2S$ is oxidized to sulfate ($SO_4^{2-}$). The proton production ($H^+$) can also acidify the environment.
Habitats
- Deep‑sea hydrothermal vents (“black smokers”), where $H_2S$ emerges from the seafloor.
- Sulfur‑rich hot springs.
- Sulfide‑rich sediments and interfaces where oxygen meets sulfide.
Ecological role
- Primary producers in dark, sulfur‑rich environments (e.g., hydrothermal vent communities).
- Contribute to the global sulfur cycle by converting reduced sulfur to sulfate.
Nitrifying Bacteria (Chemolithoautotrophic Nitrifiers)
These are key players in the nitrogen cycle.
Energy sources: Reduced nitrogen compounds:
- Ammonia oxidizers (e.g., Nitrosomonas):
- Oxidize ammonium $NH_4^+$ (or $NH_3$) to nitrite $NO_2^-$:
$$
NH_4^+ + 1.5\,O_2 \rightarrow NO_2^- + 2\,H^+ + H_2O + \text{energy}
$$ - Nitrite oxidizers (e.g., Nitrobacter):
- Oxidize nitrite to nitrate $NO_3^-$:
$$
NO_2^- + 0.5\,O_2 \rightarrow NO_3^- + \text{energy}
$$
These groups sometimes cooperate in series: one produces nitrite, the other consumes it.
Habitats
- Soils, especially in well‑aerated layers.
- Freshwater and marine systems.
- Wastewater treatment plants (biological nitrification).
Ecological role
- Convert reduced nitrogen (ammonium) to oxidized forms (nitrite, nitrate).
- Provide nitrate that plants and other organisms can use.
- Drive crucial steps in the global nitrogen cycle.
Iron-Oxidizing Bacteria
Energy source: Ferrous iron $Fe^{2+}$ (reduced iron).
Typical reaction
Aerobic iron oxidizers (e.g., Acidithiobacillus ferrooxidans):
$$
4\,Fe^{2+} + O_2 + 4\,H^+ \rightarrow 4\,Fe^{3+} + 2\,H_2O + \text{energy}
$$
$Fe^{2+}$ is oxidized to $Fe^{3+}$.
Habitats
- Acidic mine drainage and metal‑rich waters.
- Iron‑rich groundwater seeps.
- Some neutral pH environments where $Fe^{2+}$ is abundant and oxygen is limited.
Ecological and practical significance
- Influence iron cycling and mineral formation.
- Important in biomining (bioleaching of metals from ores).
- Can contribute to environmental problems (acid mine drainage) or be used in bioremediation.
Hydrogen-Oxidizing Bacteria and Archaea
Energy source: Molecular hydrogen $H_2$.
Typical reaction
Aerobic hydrogen oxidation:
$$
2\,H_2 + O_2 \rightarrow 2\,H_2O + \text{energy}
$$
Under anaerobic conditions, other electron acceptors (e.g., nitrate, sulfate, $CO_2$) may be used.
Habitats
- Volcanic and geothermal sites, hot springs.
- Deep subsurface rocks where $H_2$ is produced by geochemical reactions (e.g., serpentinization).
- Symbioses with some animals in extreme environments.
Ecological role
- Primary producers in some subsurface and hydrothermal ecosystems.
- Link geological $H_2$ production to biological carbon fixation.
Other Inorganic Electron Donors
Some chemolithoautotrophs (especially archaea) can use:
- Reduced sulfur and metal compounds under very extreme conditions (high temperature, low pH, high salinity).
- Hydrogen and $CO_2$ in methanogenic or other anaerobic pathways (though not all such metabolisms are fully autotrophic).
These expand chemolithoautotrophy into environments where few other life forms can persist.
Energy Conservation and CO₂ Fixation
Although details are covered in other chapters, a few points are specific to chemolithoautotrophs:
- The oxidation of inorganic donors drives an electron transport chain in membranes.
- This generates a proton gradient that is used by ATP synthase to make ATP.
- Reducing equivalents (e.g., NADH, NADPH) are produced either directly or via reverse electron transport (running parts of the electron transport chain “uphill,” consuming energy).
- ATP and reducing power then drive $CO_2$ fixation (often via the Calvin cycle, though some chemolithoautotrophs use alternative $CO_2$ fixation pathways).
Because some inorganic donors yield relatively little energy per mole, many chemolithoautotrophs must process large amounts of substrate and may grow slowly.
Chemosynthesis vs. Photosynthesis
Both are forms of autotrophic, anabolic metabolism:
- Both ultimately convert $CO_2$ into organic matter.
- Both use an electron donor and an electron transport chain, generating ATP and reducing power.
Key differences:
- Energy source
- Photosynthesis: light.
- Chemolithoautotrophy: chemical energy from inorganic compounds.
- Electron donors
- Photosynthesis: often water ($H_2O$) in oxygenic photosynthesis or other donors in anoxygenic photosynthesis.
- Chemolithoautotrophy: $H_2S$, $NH_4^+$, $NO_2^-$, $Fe^{2+}$, $H_2$, and others.
- Oxygen production
- Oxygenic photosynthesis produces $O_2$.
- Most chemolithoautotrophic processes do not produce $O_2$; instead, they consume it (if aerobic) or use other acceptors (if anaerobic).
In environments without light, chemolithoautotrophs can completely replace photosynthetic organisms as primary producers.
Ecological and Global Significance
Chemolithoautotrophy has several major roles in Earth’s biosphere:
- Primary production without sunlight
- At deep‑sea hydrothermal vents, in some cave systems, and in the deep subsurface, chemolithoautotrophs form the base of entire ecosystems.
- Animals at hydrothermal vents (e.g., giant tube worms) often host symbiotic sulfur‑oxidizing or hydrogen‑oxidizing bacteria that provide them with organic nutrients.
- Driving biogeochemical cycles
Chemolithoautotrophs are key catalysts of:
- The sulfur cycle (sulfur‑oxidizing bacteria).
- The nitrogen cycle (nitrifying bacteria).
- The iron cycle (iron‑oxidizing bacteria).
- Parts of the hydrogen and carbon cycles (hydrogen oxidizers and some archaea).
- Shaping environments and geochemistry
- Oxidation of sulfides and metals can acidify water, dissolve minerals, and form new mineral deposits.
- They influence soil fertility by converting nitrogen into forms usable by plants.
- Astrobiological and evolutionary implications
- The ability to use geochemical energy sources supports the idea that early life on Earth might have been chemolithoautotrophic, especially at hydrothermal vents.
- Similar metabolisms are considered promising targets when searching for life on other planets or moons with subsurface oceans and active geology.
Summary
Chemolithoautotrophy (chemosynthesis) is a form of autotrophic metabolism where organisms:
- Oxidize inorganic compounds (e.g., $H_2S$, $NH_4^+$, $NO_2^-$, $Fe^{2+}$, $H_2$).
- Use the released energy to produce ATP and reducing power.
- Fix $CO_2$ into organic matter without relying on light.
These organisms, mainly bacteria and archaea, are essential primary producers in dark environments and play central roles in global element cycles, linking Earth’s geology and chemistry to the biosphere.