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.
