Cyanobacteria – Definition, Structure, Importance and Examples
Cyanobacteria are Gram negative photosynthetic prokaryotes, being the most primitive organisms to have oxygenic photosynthesis. They added oxygen to the atmosphere, which is indispensible for the existence of aerobic forms of living organisms. They are also known as BGA (Blue green algae) and are classified variously under cyanophyceae or myxophyceae.
Occurrence of Cyanobacteria
They are mainly fresh water forms, though few are marine. Red sea is named so because of abundant ocurrence of a cyanobacterium Trichodesmium erythraeum, which imparts red colouration to water. They occur in symbiotic association with almost every group of eukaryotes i.e. green algae, fungi, bryophytes like mosses and Anthoceros, ferns, gymnosperms, angiosperms, sponge, shrimps, mammals etc. Anabaena azollae is associated with Azolla, an aquatic fern. Anabaena cycadeae is asociated with coralloid roots of Cycas. In many lichens (symbiotic association of algae and fungi), the algal partner may be a cyanobacterium. When they live endozoically in protozoans they are called cyanelle.
Structural Organization of Cyanobacteria
These may be unicelled, filamentous and colonial. Filamentous form consists of one or more cellular strands, called trichomes, surrounded by mucilagenous sheath. Cyanobacteria are characterised by the absence of flagellum throughout life cycle.
Cell Structure of Cyanobacteria
The cell structure in cyanobacteria is typically prokaryotic. The cell lacks a well defined nucleus and the chromatin material is centrally located, resembling the bacterial chromosome. The cell wall is 4 layered and is invariably covered by mucilagenous sheath, composed largely of mucopeptides.
Protoplasm in cyanobacterial cell can be distinctly divided into two parts the centroplasm and chromoplasm. The central colourless centroplasm contains the chromatin material. The peripheral protoplasm is coloured or pigmented because of the presence of thylakoids, called as chromoplasm. The protoplast lacks membrane-bound organelles like endoplasmic reticulum, golgi bodies, mitochondria, lysosomes, plastids and contains 70S ribosomes. Similar to the mesosome of bacteria, a group or coiled membrane called lamellasome is found which connects nudleoid to the cell membrane. It helps in respiration and replication of DNA. The cell membrane lack sterols. The sap vacuoles are absent. Instead, the cell may contain gas filled vacuoles which help to regulate the buoyancy of the organism in water. The characteristic feature of cyanobacterium cell is the presence of a system of photosynthetic lamellae called thylakoids. The characteristic photosynthetic pigments present in the thylakoids are chlorophyll a and phycobilins i.e., phycocyanin (blue coloured), phycoerythrin (red coloured) and allophycocyanin (light blue coloured).
The cyanobacterial cell contains reserve food material in the fllowing forms
- Cyanophycean granules (Protein)
- B-granule (Fat droplets)
- Cyano-or myxophycean starcn or d-granuie (Smilar to gycogen but negative to iodine test)
- Volutin body (Reserve phosphate)
- Polyhedral body (Rubisco rich)
They are the most self-dependent organisms, because most of these are capable of converting atmospheric nitrogen into ammonium compounds besides utilizing atmospheric CO, for synthesis of organic food during photosynthesis. Biological nitrogen fixation is an anaerobic process as nitrogenase enzyme required tor the process acts efficiently in the absence of oxygen. Nitrogen fixation under anaerobic conditions occurs mainly in large, specialized cells called heterocysts as in Nostoc. Heterocyst has terminal pores which at maturity develops a polar granule. Thickened cell wall of these cells is impermeable to oxygen so this creates anaerobic environment in the cell even under aerobic conditions. Besides this, heterocysts also lack PS Il activities and CO, fixation is done only by vegetative cells. Oxygen is not evolved due to absence of PSll. However, PSI remains active in heterocyst which generates ATP required to fix nitrogen.
Besides N2 fixation heterocyst promotes fragmentation. Because of this property of nitrogen fixation, most of the BGA enrich the soil by releasing nitrogenous compounds in the surroundings.
Reproduction of Cyanobacteria
Cyanobacteria reproduce asexually. Typical sexual reproduction is absent.
Asexual reproduction occurs by following methods
(i) Binary fission : It occurs in unicellular forms. The daughter cells formed by amitotic division separate immediately after the division.
(ii) Fragmentation : it occurs in filamentous forms. The filament breaks up into short pieces or fragments which grow to fom new filaments.
(iii) Heterocysts: Under special conditions, the heterocysts germinate to form new filaments.
(iv) Hormogonia : Due to the formation of biconcave, mucilage filled dead cells called necridia, in between living cells of trichome, the filament breaks into hormogonia.
(v) Akinetes: Vegetative cells are transformed into thick walled akinetes due to the deposition of food material followed by the thickening of wall. On the arrival of favourable conditions, they germinate to form new filaments.
Importance of Cyanobacteria
They are the most ancient organisms having oxygenic photosynthes is and thus, played a significant role in the evolution of aerobic forms of life.
They convert atmospheric nitrogen into ammonium compounds and excess of these compounds is excreted out, enriching the soil. The death and decay of these also increase the soil fertility, particularly the nitrogen content of the soil. Tolypothrix and Aulosira fix N, non-symbiotically in rice fields.
Cyanobacteria like Nostoc and Anabaena have been used for reclaiming usar soils. As they can live in damp or aquatic habitat, they enrich the root environment in any wetland condition as in rice fields.
Cyanobacteria are associated in symbiotic relationship with almost every group of plants. They benefit the parther by providing nitrogenous compounds because of their capability of nitrogen fixation.
Some cyanobacteria serve as food to several aquatic animals. Spirulina is edible, non-toxic, fast growing cyanobacterium. It is cultivated in tanks as source of protein rich animal food (SCP).
Extract of Lyngbya is used for the manufacture of antibiotic.
Some cyanobacteria like Microcystis aeruginosa, Anabaena flos-aquae, Aphanizomenon flos-aquae are known to cause algal blooms in water bodies. These also secrete toxins into the surroundings, which are harmful to aquatic animals and even to human beings. Water from such sources is harmful and may even prove fatal for organisms drinking it. They also deplete the oxygern trom the water reservoir and thereby, cause large scale death of the fishes and other aquatic animals.