Prokaryotes are single-celled microorganisms that lack a true nucleus and other membrane-bound internal compartments. Metabolism is the set of chemical reactions that occur within a living organism to sustain life, involving energy conversion and the synthesis and breakdown of organic molecules. Prokaryotes possess incredibly diverse metabolic capabilities that are fundamental to their survival and the global cycling of elements on Earth. This repertoire allows them to thrive in virtually every environment, showcasing a complexity far greater than their simple cellular structure might suggest.
The Cellular Basis of Prokaryotic Metabolism
Prokaryotic cells (Bacteria and Archaea) manage chemical processes without specialized organelles like mitochondria. The cytoplasm serves as the primary location for metabolic activity, including initial energy-releasing steps like glycolysis. All necessary enzymes are readily available within this single, continuous space.
The plasma membrane, which encloses the cell, takes on the highly important role in energy generation that is performed by internal membranes in eukaryotes. The electron transport chain, which generates the cell’s primary energy currency, adenosine triphosphate (ATP), is embedded directly within this membrane. Chemiosmosis, where a proton gradient is established across the membrane to power ATP synthase, is localized at the cell surface.
The small size of prokaryotes gives them a high surface-area-to-volume ratio, influencing their metabolic efficiency. This ratio allows for rapid uptake of nutrients from the environment across the plasma membrane. Consequently, prokaryotes exhibit high metabolic rates and rapid growth, as the limited distance supports quick diffusion of resources and waste removal.
The Vast Spectrum of Energy and Carbon Sources
The metabolic diversity of prokaryotes is classified based on how they obtain two fundamental resources: energy and carbon. Organisms are categorized as phototrophs (using light energy) or chemotrophs (deriving energy from chemical compounds). They are then classified as autotrophs (using carbon dioxide (\(\text{CO}_2\)) as the sole carbon source) or heterotrophs (using organic compounds from other organisms).
Chemoheterotrophs
This group obtains both energy and carbon from organic molecules, similar to humans and animals. They include bacteria that function as decomposers, breaking down dead organic matter and waste products. They recycle nutrients by consuming complex molecules and releasing simpler compounds back into the environment.
Photoautotrophs
Photoautotrophs use light as their energy source and fix atmospheric \(\text{CO}_2\) to build organic molecules, performing photosynthesis. Cyanobacteria are a prominent example, responsible for producing the oxygen that accumulated in Earth’s atmosphere. These organisms use water as an electron donor and are primary producers in many ecosystems.
Chemoautotrophs
Chemoautotrophs utilize energy derived from the oxidation of inorganic chemicals, using \(\text{CO}_2\) as their carbon source. They live in environments devoid of light, such as deep-sea hydrothermal vents, where they oxidize compounds like hydrogen sulfide (\(\text{H}_2\text{S}\)). This capability forms the basis of complex ecosystems in the dark ocean.
Photoheterotrophs
Photoheterotrophs use light for energy while requiring organic compounds for their carbon source. These organisms cannot fix \(\text{CO}_2\) but harvest energy from the sun. An example is the purple non-sulfur bacteria, often found in stagnant water and mud where light is available but oxygen levels are low.
Unique Roles in Global Biogeochemical Cycles
The specialized metabolic pathways of prokaryotes enable chemical transformations unavailable to eukaryotes, making them the primary drivers of global biogeochemical cycles. One significant process is nitrogen fixation, where certain bacteria and archaea convert inert atmospheric nitrogen gas (\(\text{N}_2\)) into biologically usable ammonia. This is carried out by organisms such as Rhizobium, which live symbiotically in legume root nodules, and free-living bacteria like Azotobacter.
Fixed nitrogen is then transformed through other prokaryotic processes. Nitrification converts ammonia into nitrites and nitrates, and denitrification returns nitrogen gas back to the atmosphere. Without these metabolic steps, the nitrogen necessary for proteins and nucleic acids would become unavailable to most life forms.
Prokaryotes also play a central role in the sulfur cycle, oxidizing sulfur compounds like hydrogen sulfide to elemental sulfur or sulfate, and reducing sulfate back to sulfide. This transformation is carried out by different groups of bacteria and archaea. In the carbon cycle, specialized Archaea called methanogens produce methane (\(\text{CH}_4\)) as a metabolic byproduct in anoxic environments. Other prokaryotes, known as methanotrophs, metabolize this methane, using it as a source of energy and carbon, thereby regulating the atmospheric concentration of this greenhouse gas.