Monera

Monera is a historical taxonomic kingdom that in older classification schemes encompassed all prokaryotic organisms—organisms without a true nucleus or membrane-bound organelles. The group was used in various 20th-century systems, most notably the five-kingdom model, to contrast with Plantae, Animalia, Fungi, and Protista. Members of Monera include bacteria and archaea, as well as cyanobacteria, which are photosynthetic bacteria. They are typically unicellular or colonial. They lack a membrane nucleus; their genetic material is usually a single circular chromosome, often with plasmids. They display a wide range of metabolisms, from photosynthesis to chemosynthesis and heterotrophy; energy can be obtained aerobically or anaerobically. Reproduction is generally asexual by binary fission, budding, or fragmentation; genetic exchange occurs through transformation, transduction, or conjugation in bacteria.

In modern taxonomy, Monera is considered obsolete. The discovery of major genetic differences between bacteria and archaea led to the adoption of the three-domain system (Bacteria, Archaea, Eukarya). In this framework, prokaryotes are not placed in a single kingdom but distributed across two domains. However, some older sources and non-scientific contexts may still use Monera to refer to prokaryotes as a functional group. Ecologically, prokaryotes are diverse and play essential roles in nutrient cycling, fermentation, symbioses, and as pathogens.