The Kingdom Monera consists of all prokaryotes, that is, unicellular organisms that lack nuclear membranes. This taxonomic kingdom consists of two phylogenetically distinct groups: eubacteria and archaebacteria. As seen in part b of figure A.1 eubacteria and archaebacteria are as genetically different from each other as they are from the eukaryotes (organisms with nuclear membranes: protists, plants, animals (invertebrate and vertebrates) and fungi).
Morphologically, archaebacteria and eubacteria differ in some key aspects. While most members of both groups have cell walls, their cell membranes are chemically different, as are their overall chemical makeups. Though there is no hard and fast distinction between the roles filled by the two types of bacteria, most archaebacteria live in extremely hostile environments, such as extremely saline waters or hot sulfur springs. Some eubacteria also live in these harsh environments, but others inhabit locations ranging from surface soils to the intestinal tracks of termites.
Though extremely small (most bacteria are significantly smaller than eukaryotic cells), bacteria fill several important roles in the natural world. We are most familiar with bacteria as the cause of diseases from strep throat to bubonic plague. However, comparatively few bacteria cause diseases. Most are beneficial to other organisms. Some basteria are photoautotrophs, producing food from inorganic material and light. In some cases, particular photoautotrophs have the ecologically important ability to fix nitrogen from the atmosphere, making it available to the roots of plants. Other bacteria are saprophytes, breaking down dead organic material. Still others live symbiotically in the digestive tracks of other organisms and aid in the digestion of diverse food materials.
The structure and reproductive cycles of the Monerans are relatively simple compared to those of the eukaryotes. They lack distinct nuclei and complex organelles. Specialized structures, such as photosynthetic machinery, take the form of internal membranes. Moneran genetic material is also relatively simple. They have small prokaryotic chromosomes and plasmids rather than the complex chromosomes found in eukaryotes. Most Monerans reproduce by binary fission.