Chemolithotrophy

 Chemolithotrophy is the oxidation of inorganic chemicals for the generation of energy. An inorganic compound is oxidized with the electrons being passed off to carriers in the electron transport chain.  A proton motive force is generated and is used to generate ATP with the help of ATP synthase. Reducing power NADPH also is produced in the process.

Electrons donors

Chemolithotrophs use a variety of inorganic compounds as electron donors, with the most common substances being hydrogen gas, sulfur compounds (such as sulfide and sulfur), nitrogen compounds (such as ammonium and nitrite), and ferrous iron.

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  Hydrogen oxidizers – these organisms oxidize hydrogen gas (H₂) with the use of a hydrogenase enzyme. Both aerobic and anaerobic hydrogen oxidizers exist, with the aerobic organisms eventually reducing oxygen to water. Several bacterial genera (eg. Alcaligenes, Hydrogenophaga & Pseudomonas spp.) can oxidize hydrogen gas to produce energy.                                                 H₂       2H⁺ + 2^e– 

• Sulfur oxidizers – as a group these organisms are capable of oxidizing a wide variety of reduced and partially reduced sulfur compounds such as hydrogen sulfide (H₂S), elemental sulfur (S⁰), thiosulfate (S₂O₃^2-), and sulfite (SO₃^2-). Sulfate (SO₄^2-) is frequently a by-product of the oxidation. Often the oxidation occurs in a stepwise fashion with the help of the sulfite oxidase enzyme. Thiobacillus can oxidize sulfur (S⁰), hydrogen sulfide (H₂S), thiosulfate (S₂O₃^2-), and other reduced sulfur compounds to sulfuric acid; therefore they have a significant ecological impact. Some of these are extraordinarily flexible metabolically. For example, Sulfolobus brierleyi and a few other species can grow aerobically as sulfur-oxidizing bacteria; in the absence of O₂, they carry out anaerobic respiration with molecular sulfur as an electron acceptor.

• Nitrogen oxidizers – the oxidation of ammonia (NH₃) is performed as a two-step process by nitrifying microbes such as Nitrosomonas and Nitrosospira, which oxidizes ammonia to nitrite (NO₂^-) and the second group Nitrobacter and Nitrococcus oxidizes the nitrite to nitrate (NO₃^-). The entire process is known as nitrification and is performed by small groups of aerobic bacteria and archaea, often found living together in soil or in water systems.

      



• Iron oxidizers – these organisms oxidize ferrous iron (Fe²⁺) to ferric iron (Fe³⁺). Since Fe²⁺ has such a positive standard reduction potential, the bioenergetics are not extremely favourable, even using oxygen as a final electron acceptor. Also, Fe²⁺ spontaneously oxidizes to Fe³⁺ in the presence of oxygen; so, the organisms must use it before that happens.                                   (Ferrous iron is a soluble form of iron that is stable at extremely low pH or under anaerobic conditions. • Under aerobic, moderate pH conditions ferrous iron is oxidized spontaneously to the ferric (Fe3+) form and is hydrolyzed abiotically to insoluble ferric hydroxide [Fe(OH)₃].)

There are three  types of ferrous iron-oxidizing microbes.

•  The first are acidophiles, such as the bacteria Acidithiobacillus ferrooxidans and Leptospirillum ferrooxidans, as well as the archaeon Ferroplasma.  These microbes oxidize iron in environments that have a very low pH and are important in acid mine drainage.
•  The second type of microbes oxidizes ferrous iron at near-neutral pH. These micro-organisms (Gallionella ferruginea or Leptothrix ochracea) live at the oxic-anoxic interfaces and are microaerophiles.
• The third type of iron-oxidizing microbes is anaerobic photosynthetic bacteria such as Rhodopseudomonas, which use ferrous iron to produce NADH for autotrophic carbon dioxide fixation.

 

Chemolithoautotrophs vs chemolithoheterotrophs

• Most chemolithotrophs are autotrophs (chemolithoautotrophs), where they fix atmospheric carbon dioxide to assemble the organic compounds that they need. These organisms require both ATP and reducing power (i.e. NADH/NADPH) in order to ultimately convert the oxidized molecule CO₂ into a greatly reduced organic compound, like glucose.
• Some microbes are chemolithoheterotrophs, using an inorganic chemical for their energy and electron needs, but relying on organic chemicals in the environment for their carbon needs. These organisms are also called mixotrophs, since they require both inorganic and chemical compounds for their growth and reproduction.

Thus the chemolithotrophs, are autotrophs and can use CO₂ as their carbon source.  Many will grow heterotrophically also, if they are supplied with reduced organic carbon sources like glucose or amino acids.

 Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur oxidizers and reducers, nitrifiers, anammox bacteria, and thermoacidophiles.  Chemolithotrophic growth could be very fast, such as Thiomicrospira crunogena with a doubling time around one hour. 

Electron acceptors

    Chemolithotrophy can occur aerobically or anaerobically-the best electron acceptor is oxygen. Using a non-oxygen acceptor such as sulfate (SO₄²⁻), nitrate (NO₃⁻), elemental sulfur (S⁰), ferric iron (Fe³⁺) and CO₂ allows chemolithotrophs to have greater diversity and the ability to live in a wider variety of environments.

 

Amount of ATP generated

Much less energy is available from the oxidation of inorganic molecules than from the complete oxidation of glucose to CO₂. As the electron donors and acceptors vary, the amount of ATP generated also vary widely for chemotrophs. An organism makes typically 32 molecules of ATP per glucose molecule using aerobic respiration, however, chemolithotrophs do not produce that much ATP - ATP yield is low to moderate; typically 1–3 ATP per molecule oxidized. 

Because the yield of ATP is so low, chemolithotrophs must oxidize a large quantity of inorganic material to grow and reproduce. Thus, they have a significant ecological impact. 

A lithotroph is thus an organism that uses an inorganic substrate (usually of mineral origin) for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation via aerobic or anaerobic respiration. Known chemolithotrophs are exclusively microbes; no known macrofauna possesses the ability to utilize inorganic compounds as energy sources. Macrofauna and lithotrophs can form symbiotic relationships, an example of this is chemolithotrophic bacteria in deep sea worms - Giant tube worms Riftia pachyptila have an organ containing chemosynthetic bacteria instead of a gut.

Chemotrophs thus, obtain energy through the oxidation of electron donor molecules in their environments.

• These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs).
• The chemotrophs are in contrast to phototrophs, which utilize solar energy.
• Chemotrophs can be either autotrophic or heterotrophic.

Ecological impact of chemolithotrophs

Chemolithotrophs play a crucial ecological role by driving essential biogeochemical cycles, involving nitrogen, sulfur, and iron. By oxidizing inorganic compounds such as ammonia, hydrogen sulfide, ferrous iron, and hydrogen, they act as the primary producers in environments where sunlight is unavailable, such as deep-sea vents and subsurface habitats. 

They have important roles in:

1. Nutrient Cycling:
• Chemolithotrophs convert reduced inorganic compounds into oxidized forms, facilitating the recycling of nutrients like nitrogen (through nitrification), sulfur (through sulfur oxidation), and iron.
• For example, nitrifying bacteria transform ammonia into nitrate, making nitrogen available in forms usable by plants and other organisms.
2. Supporting Ecosystems in Extreme Environments:
• In habitats lacking organic carbon or light (e.g., hydrothermal vents), chemolithotrophs form the base of the food web, supporting other communities by producing organic matter through chemosynthesis.
3. Influence on Soil and Water Chemistry:
• By oxidizing iron and sulfur compounds, chemolithotrophs influence soil pH and metal availability, affecting overall soil fertility and water quality.
• Their activities can lead to acid mine drainage, impacting aquatic ecosystems negatively, but also play roles in bioremediation.
4. Environmental and Industrial Applications:
• Chemolithotrophs can be used in waste treatment, bioleaching, and biogeochemical remediation processes.

Overall, chemolithotrophs are critical in maintaining ecosystem stability and productivity, especially in nutrient-poor or extreme environments. Their metabolic activities drive elemental cycles critical for the survival of diverse life forms.