Some of the major harmful effects of Algae to human being are listed below:
1. Harmful to living stock:
The algae are harmful to humans in several ways. Volvocales, Chlorococcales, Myxophyceae and several others occur in such a great abundance in water, that they colour the whole water either green or blue green and cause the death of fishes.
The algae block the gills of the fishes and they respire during night and make the respiration of fishes difficult by complete depletion of oxygen.
Sometimes the algae are found in water so abundantly that they make difficult to drink water of livestock. Some blue green algae have been reported poisonous and they directly cause the death of living stock who drink this contaminated water.
2. Blocking of photosynthesis:
The epiphytic algae which are found upon other plants and trees block photosynthesis and indirectly harm the trees and plants.
3. Parasitic algae:
The well known disease ‘red rust of tea’ is not caused by any parasitic fungus but an algal form Cephaleuros virescens. This causes havoc to tea plants in Assam tea gardens. Besides, this parasitic form attacks several other plants, e.g., Mangifera, Rhododendron. Coffea, etc. The heavy losses are caused to tea and coffee by this parasitic algal form.
4. Mechanical injury:
Sometimes the filamentous forms of algae are found in such a great abundance and net-like behaviour, that many fishes and other aquatic animals may get perish in these tangles, and direct death is inflicted upon them.
5. Contamination of water supply:
Many blue-green, green and other algae contaminate the water of city reservoirs. This contamination develops a foul odour in the water and makes the water unhygienic. The algae also form some mucilaginous secretions which are the seats of harmful bacteria and other pathogens causing several human and animal diseases.
6. Fouling of ships:
Some algae are attached to the ships, and this is called fouling of ship. The fouling retards the speed of the ship. To avoid this nuisance the ships are periodically dried up and painted with copper paint.
7. Deterioration of exposed fabrics:
Commonly in rainy season, if the wet fabrics are exposed, within a few days a blue green alga appears on it and makes the cloth black spotted and weak. This was a serious problem during the Second World War. This algal growth is usually followed by bacterial infection and the fibres are completely destroyed.