By Drew Lichtenstein – Updated March 24, 2022
Image credit: Seksan Mongkhonkhamsao / Moment/GettyImages
Prokaryotes are simple, single‑cell organisms that lack a true nucleus and complex organelles such as mitochondria. Their DNA is free‑floating in the cytoplasm, and they typically possess far less genetic material than eukaryotes. While most reproduce asexually, some engage in genetic exchange, and a subset can photosynthesize via chloroplast‑like structures.
Eubacteria—the traditional “bacteria” kingdom—have been studied for over a century, largely because many species are human pathogens. Thousands of eubacterial species exist, commonly classified by cell shape: rod‑shaped (bacilli), spiral (spirilla), and spherical (cocci). Beyond their medical relevance, eubacteria play a pivotal role in global biogeochemical cycles by decomposing organic matter and converting nitrogen into forms usable by plants.
Archaea constitute a distinct prokaryotic kingdom that thrives in extreme habitats, from hydrothermal vents to acidic hot springs. Their cellular biochemistry diverges markedly from eubacteria, enabling survival under conditions lethal to most life forms. Some archaea, such as Halobacterium, harness salt gradients to power proton pumps and generate energy.
Viruses are often grouped with prokaryotes in discussions of cellular life, yet they are not true organisms. They lack membrane-bound organelles, cannot reproduce independently, and rely on hijacking host cells for replication. Consequently, viruses are excluded from the kingdom classification system.
