Cyanobacteria(Blue-green algae) are both useful and harmful to mankind. . Their economic importance are as fallows.
1) Algal Nuisance in Water Supplies
The extensive growth of blue green algae create serious problems in the maintenance of water supplies, particularly in low land water reservoirs. They implode the routine filtration procedure essential in the provision potable water. They may rapidly clog not only the fine sand filters but even the fast filters. In addition to this, these also produce bad smell and secrete toxic substances to drinking water. The mass appearance of blue green algae has also a distinctive effect upon recreational activities like swimming, sailing or fishing in lakes and reservoirs.
2) Toxic Blue green algae
Several examples of poisoning of farm animals due to the ingestion by drinking animals of lethal doses of toxic blue greens like Microcystis aeruginosa. Some toxic substances like microcystin, anatoxin, saxitoxin etc. have been isolated and identified. Microcystin causes enlargement and congesting of the liver followed by necrosis, hemorrhage and neurotoxic activity.
3) The nutritive value of Blue Greens(SCP)
Blue green algae have recently been considered to be potentially excellent source of food for animals and even for mans. Spirulina lakes are now commercially exploited for the production of nutritious health food. Its protein contains all essential amino acids with exception to low quantities of Sulphur containing Cysteine and methionine. The nutritive value of Spirulina was estimated to be around 50%. Now, it is commonly referred to as single cell protein. Spirulina tablets are now being sold in the market and medically prescribed by physicians for protein deficient patients in America and India.
4) The use of blue green algae in Agriculture
i) In view of world food crisis, attention has been turned to traditional farming practices and to more recent inventions which enable the more extensive and efficient utilization of biological nitrogen fertilizers. The fertility of paddy soil is maintained by the activities of heterocystous blue green algae which grows spontaneously and often luxuriantly in the water logged fields. The nitrogen fixing forms of Indian paddy fields are Aulosira fertilissima, Tolypothrix tenuis etc.
ii) In India, blue green algae are also used for the reclamation of alkaline and saline waste lands. The waste land is enclosed by an earth embarkment about 0.5m high which promotes water logging during the rainy period. The rapid growth of blue greens in the water-logged plots decreases soil alkalinity and increases the nitrogen and organic contents of the soil. In a few years' time, the waste land can be converted to fertile fields.
iii) Anabaena-Azolla association has been found an efficient fertilizer of paddy fields. The cultivation of Azolla has been introduced in China, Thailand, Indonesia and India to enhance rice production. Estimates of the annual quantities of Nitrogen fixed by Azolla range between 120 and 312 kg nitrogen per hectare. Trial studies revealed that Azolla can accumulate over 60kg nitrogen per hectare within 45 days. The application of Azolla can increases rice yield by up to 38 percent.
5. Solar Energy Conversion Through Bio photolysis.
Heterocyst containing blue green algae have the unique ability to simultaneously evolve oxygen in photosynthesis and hydrogen molecule by nitrogenase catalyzed electron transfer to H+ ions. Efforts are now being made to exploit this potential through the development of a bio photolytic system for solar energy conversion.