Bacteria

There are many ways to classify bacteria. One method is based on shape, of which there are three main classes. Cocci are spherical, bacilli are rod-shaped, and spirilla are spiral-shaped. Bacteria are also classified as being either gram positive or gram negative, based on whether they take up the gram stain that is commonly used to make them easier to see under the microscope. A third method of classification is whether they can live in the presence of oxygen (aerobic) or require the absence of oxygen (anaerobic).

                                         Cyanobacteria

The oldest known fossils are cyanobacteria from Archaean rocks of western Australia, dated 3.5 billion years old. This may be somewhat surprising, since the oldest rocks are only a little older: 3.8 billion years old.

Cyanobacteria are among the easiest microfossils to recognize. Morphologies in the group have remained much the same for billions of years, and they may leave chemical fossils behind as well, in the form of breakdown products from pigments. Small fossilized cyanobacteria have been extracted from Precambrian rock, and studied through the use of SEM and TEM (scanning and transmission electron microscopy).

The autotrophic (auto = "self" tropho = "nourishment", Greek) cyanobacteria were once classified as "blue green algae" because of their superficial resemblance to eukaryotic green algae. Although both groups are photosynthetic, they are only distantly related: cyanobacteria lack internal organelles, a discrete nucleus and the histone proteins associated with eukaryotic chromosomes. Like all eubacteria, their cell walls contain peptidoglycan.

Many of the most numerous and prominent forms of autotrophic bacteria are given the name cyanobacteria. Cyanobacteria are found almost always in aquatic environments. They photosynthesize like all other autotrophic bacteria and are just as efficient. Cyanobacteria are usually found as single cell organisms but can often be found in colonies. These colonies can grow up to three to four centimeters in size and are sometimes mistaken for amphibian eggs.

Cyanobacteria are named after the chemical they use to capture light which is called phycocyanin. These bacteria also contain chlorophyll a which is the same pigment found in plant cells. Cyanobacteria are also referred to as " blue-green algae". This an incorrect assumption, because although all cyanobacteria photosynthesize and some are green, they are not actually algae. In fact, some cyanobacteria can actually be red or pink. The water in the red sea retains this color because it contains a reddish species of cyanobacteria called Oscillatoria.




Ancient Fossil Bacteria : Pictured above are two kinds cyanobacteria from the Bitter Springs chert of central Australia, a site dating to the Late Proterozoic, about 850 million years old. On  the top is a colonial chroococcalean form, and below that  is the filamentous Palaeolyngbya.

Cyanobacteria are prokaryotes that photosynthesize using the pigments chlorophyll a (which is present in all photosynthesizing eukaryotes as well), phycobilins, phycoerythrin, phycoerythrocyanin, phycocyanin, some carotenoids, and allophycocyanin. Some of these pigments are not present in all cyanobacteria, but all known species contain phycocyanin, allophycocyanin, and chlorophyll a. They are usually not blue-green, despite the name, but some other color from green to yellow to even red. Color is, therefore, a poor taxonomic character for cyanobacteria. I have seen living specimens of certain species change color over time, and preserved specimens (of course) do not always retain their natural coloration. Cyanobacteria do not have flagella, cilia, pseudopodia, or any analogous locomotory organ. Many of them do move, however, by "gliding," which simply means that they (normally) will slowly and smoothly creep about without any obvious means of locomotion. Cyanobacteria are found in much the same habitats that one would expect to find eukaryotic algae, often in much greater volumes than those algae themselves.

Cyanobacteria have chlorophyll ain common with plants and certain ecological roles shared with eukaryotic algae. They could be called algae, based on these ecological roles, but not in any way that would imply actual genetic relationship. Cyanobacteria are not quite the same as bacteria, either. The only other prokaryote containing chlorophyll a is a group of 2 species (at this time) set off to itself as the order Prochlorales, which is considered very closely related to the cyanobacteria. It cannot be denied, however, that cyanobacteria are bacteria in the genetic sense.

Cyanobacteria are actually believed to be the origin of chloroplasts in plants. It is these chloroplasts which allow the plant to photosynthesize. In the late Proterozoic period, it is believed that cyanobacteria took up residence in some eukaryotic cells. The process of a bacteria taking living within another cell is called endosymbiosis. As the eukaryotic cells began to divide and form multicellular organisms, each cell contained these cyanobacteria. Eventually, a fully functioning plant developed.

Cyanobacteria also play a major role in the nitrogen cycle. They are able to convert atmospheric nitrogen into its organic form. All plants use organic nitrogen as a nutrient to promote growth. Without this source of nitrogen, the plants would die. Cyanobacteria are one of the few types of organisms that are able to make this conversion from atmospheric to organic nitrogen.
These autotrophic bacteria have a very rich fossil record. The oldest known fossils are cyanobacteria.

These fossils are dated at approximately 3.5 billion years old. An indication of how old these fossils are is the fact that the oldest rocks are estimated to be 3.8 billion years old. The idea that autotrophic bacteria are so old led many prominent scientists to believe that they were instrumental in the evolution of the world, as a whole. For example, 3.5 billion years ago, the earth's conditions were much different. The biosphere consisted of a primordial sea, ample hydrogen and ammonium gas, carbon dioxide, strong ultraviolet radiation and a limited amount of oxygen gas. The cyanobacteria were the first cells to photosynthesize and consequently produce oxygen gas as a byproduct. An atmosphere began to form as the oxygen gas continued to build up. When the atmosphere was completed, the world was now suitable for eukaryotic life, because there were large amounts of oxygen for cellular respiration and the harsh ultraviolet radiation was absorbed by the atmosphere. Essentially, without cyanobacteria, none of us would have be able to survive on planet Earth.

Studies of metabolic similarities and ribosomal RNA sequence suggest that cyanobacteria form a good, monophyletic taxon. Because motile species of cyanobacteria utilize the same mysterious gliding locomotion as the gram-negative gliding bacteria, some microbiologists suggest that cyanobacteria should be classified together as a subgroup of gliding bacteria.

Although they are truly prokaryotic, cyanobacteria have an elaborate and highly organized system of internal membranes which function in photosynthesis. Chlorophyll a and several accessory pigments (phycoerythrin and phycocyanin) are embedded in these photosynthetic lamellae, the analogs of the eukaryotic thylakoid membranes. The photosynthetic pigments impart a rainbow of possible colors: yellow, red, violet, green, deep blue and blue-green cyanobacteria are known.

Cyanobacteria may be single-celled or colonial. Depending upon the species and environmental conditions, colonies may form filaments, sheets or even hollow balls. Some filamentous colonies show the ability to differentiate into three different cell types. Vegetative cells are the normal, photosynthetic cells formed under favorable growing conditions. Climate-resistant spores may form when environmental conditions become harsh. A third type of cell, a thick-walled heterocyst, contains the enzyme nitrogenase, vital for nitrogen fixation.

It has recently been estimated that the number of bacteria on earth is  five million trillion trillion. If each bacterium were a penny, the stack would reach a trillion light years in length. This suggests that more than one-half of the living protoplasm on Earth is microbial.