Introduction to Cyanobacteria

The cyanobacteria are true bacteria (singular bacterium). They are prokaryotes and do not possess a true nucleus or membrane-bound organelles such as mitochondria or plastids. Like other prokaryotes they have 70S ribosomes. Although there are other bacteria which can photosynthesis, the cyanobacteria are unique in possessing the pigment chlorophyll a. This pigment is also present in algae and plants and is responsible for the evolution of oxygen during photosynthesis. The photosynthetic bacteria posses a different kind of pigment, bacteriochlorophyll which does not permit oxygen evolution during bacterial photosynthesis.

The term “cyan” in cyanobacteria refers to the color, blue. Cyanobacteria possess certain accessory pigments such as phycocyanin and phycoerythrin. The presence of these pigments and chlorophyll a together impart characteristic color to these organisms. It is for this reason (hat the cyanobacteria are commonly known as blue- green algae. Like true algae they also evolve oxygen during photosynthesis and often occupy habitats where algae occur, in fresh, marine and brackish water bodies and on moist soil surface. However, true algae are eukaryotic and the two are not immediately related.

Since the affinities of the cyanobacteria are with the other bacteria we must briefly examine these organisms for a more complete picture of the position of cyanobacteria in the world of living things. About 4,000 species of bacteria have been described so far. These include about 1,700 species of cyanobacteria. Although small in number of species, bacteria are the most abundant of all organisms. They are also the most ancient. (Not the amoeba, which is a eukaryote of later origin). Bacteria are known in the fossil record as far back as 3.5 billion years ago. Bacteria are morphologically and anatomically the simplest of organisms. Yet, metabolically they are very diverse. Many bacteria are identified not by the morphology of the individuals but by their characteristics in culture.

Bacteria are very small, ranging in size between I to few 11m. A most unusual discovery was made in 1993 of a bacterium living in the intestinal tracs of a surgeonfish that is 600 am in length! Bacteria vary in shape. Some are rod-shaped, others spherical and yet others spiral or even comma-shaped. Tiny as they are, bacteria are responsible for activities that strongly affect our lives. Many are agents of serious diseases of human beings, animals and plants. Others ferment food and are thus useful in making varied products such as curd or ‘idli’ as well as many industrial chemicals. Some are the . source of life-saving antibiotics.

Bergey’s Manual of Determinative Bacteriology is the standard reference for the classification of bacteria. Since sufficient information is not available to place all bacteria into a hierarchical system of classification, the Bergey’s Manual recognises 19 major groups such as the spirochetes, Gram-positive cocci, gliding bacteria, mycoplasma and actinomyeetes. Cyanobacteria is included in one such group. The classification of bacteria is an active area of research. In recent years molecular biologists have analyzed the structure of ribosomal RNA (rRNA) and the sequence of rRNA nucleotides in bacteria and other organisms. Such analysis has revealed fundamental differences among two major bacterial groups, the ARCHASBACTERIA and EUBACTERIA. Differences have also been noted in the chemical composition of the cell membranes of these two bacterial groups and the eukaryotes.

The American scientist Carl Woese considers that the differences between the archaebacteria and the eubacteria are as fundamental as between these groups and the eukaryotes. Thus, life on this planet is considered to comprise of three ancient and primary lineages. The three ancient domains are shown in Fig. 2.1. The cyanobacteria arc members of the true bacterial lineage. The archaebacteria include members that live in most unusual environments such as very hot and acidic pools or in waters with extremely high salt contents. Some members of this group live in deep sea vents several kilometers below the ocean surface. The bacteria which produce methane gas are called methanogens.

Cyanobacteria are of great evolutionary interest according to the endosymbiont theory some ancestral cyanobacterial cells became the plastids of different algal groups. The plastids of red algae resemble (he cells of 4anobacteria and both possess chlorophyll a and biliproteins. The green algae and plants possess both chlorophylls a and b. Although most cyanobacteria possess only chlorophyll a at least three organisms are known (o contain both the chlorophylls. Prochloron didevni live as symbionts in (he gut walls of sea squirts. Prochlorothrix hollandica was recently discovered in lakes in Holland. More recently Prochlorococcus was discovered as a free-floating form in open seas. All these organisms possess chlorophylls a and b, and their cells resemble die chloroplast of green algae and plants. For this reason, some authors describe them as prochlorophytes and include (he three genera in a separate division nr class. Some ancestral prochlofophyte was perhaps the endosymbiont that evolved into the green plant chloroplast.