Prokaryotes are essential for life on Earth, playing crucial roles in nutrient cycling and energy acquisition. These tiny organisms have diverse metabolic strategies, allowing them to thrive in various environments and contribute to global ecological processes.

From photosynthetic cyanobacteria to chemoheterotrophic decomposers, prokaryotes drive key biogeochemical cycles. They transform essential elements like carbon, nitrogen, and sulfur, making them available to other organisms and maintaining the delicate balance of ecosystems worldwide.

Prokaryotic Macronutrients and Metabolism

Essential macronutrients for prokaryotes

• Carbon
  • Builds organic molecules (proteins, nucleic acids, lipids)
  • Obtained from organic compounds or fixed from inorganic CO2
• Nitrogen
  • Essential for amino acids, nucleic acids, other biomolecules
  • Obtained from organic compounds or fixed from inorganic N2
• Phosphorus
  • Synthesizes nucleic acids (DNA, RNA), phospholipids
  • Transfers energy (ATP, GTP)
• Sulfur
  • Component of amino acids (cysteine, methionine), cofactors
  • Obtained from organic compounds or inorganic sulfate
• Other essential elements in smaller quantities
  • Potassium, magnesium, calcium, iron
  • Enable enzyme function, cell structure, various metabolic processes

Energy acquisition in prokaryotes

• Phototrophy obtains energy from light
  • Photosynthetic prokaryotes (cyanobacteria) use light to fix carbon
• Chemotrophy obtains energy from chemical compounds
  • Chemoautotrophs fix inorganic CO2 using energy from inorganic compounds
  • Chemoheterotrophs obtain carbon and energy from organic compounds
• Lithoautotrophy derives energy from inorganic compounds
  • Sulfur-oxidizing bacteria, hydrogen-oxidizing bacteria, ammonia-oxidizing archaea
• Organoheterotrophy obtains energy and carbon from organic compounds
  • Includes fermentation and aerobic or anaerobic respiration

Prokaryotic Energy Metabolism

• Glycolysis: Initial glucose breakdown pathway common to most prokaryotes
• Aerobic respiration: Uses oxygen as final electron acceptor
  • Electron transport chain generates proton gradient
  • Oxidative phosphorylation produces ATP using proton gradient
• Anaerobic respiration: Uses alternative electron acceptors (e.g., nitrate, sulfate)
• Fermentation: ATP production without external electron acceptors

Prokaryotes in global nutrient cycles

• Carbon cycle
  1. Photosynthetic prokaryotes (cyanobacteria) fix atmospheric CO2, contributing to primary production
  2. Chemoheterotrophic prokaryotes decompose organic matter, releasing CO2
  3. Methanogenic archaea produce methane (CH4) during anaerobic respiration, a potent greenhouse gas
• Nitrogen cycle
  1. Nitrogen fixation: Prokaryotes (Rhizobium) convert atmospheric N2 into ammonia (NH3), making it bioavailable
  2. Nitrification: Ammonia-oxidizing bacteria and archaea convert NH3 to nitrite (NO2-), nitrite-oxidizing bacteria convert NO2- to nitrate (NO3-)
  3. Denitrification: Prokaryotes reduce NO3- back to atmospheric N2 under anaerobic conditions
  4. Ammonification: Prokaryotic decomposition of organic matter releases NH3, used by plants or undergoes nitrification
• Prokaryotes make essential elements bioavailable, maintain ecosystem balance through nutrient cycling