Most of the algae are aquatic and are found in fresh water or marine habitat. Therefore, they are related with different aspects of fresh-water or marine pollution.
Algae as water pollutants
During the favourable season of algal growth, water bodies like lakes, ponds and rivers show so much growth of various species of blue-green algae, green algae and golden brown algae that the water becomes turbid, cloudy and yellowish/greenish in colour. Filamentous planktonic algae form thick floating mats on the water surface. Such excessive algal growth is called water bloom. This bloom cuts off the light to deeper layers of water body and thus inhibits decomposition of organic matter in that water body. The algae further add a large amount of organic matter after death and decay to the water body which is not decomposed quickly due to prevailing conditions in that water body. This causes serious water pollution.
Many algae like Microcystis, Aphanizomenon produce toxic substances that are harmful to fishes and aquatic animals. These toxins also harm the land animals drinking this polluted water.
In general, water blooms make water oily, unpleasant in smell, fishy in taste and unfit for drinking.
Many species of blue-green and green algae that form excessive growths also choke the water tanks, pipe lines and other associated installations causing undesirable problems.
Algae as pollution indicators
Analysis of the composition and growth pattern of the algal flora in a water body can be used to identify the type and level of water pollution. Such studies of algal flora have been used in identification of following types of water pollution problems:
1. Water acidity: Increase in the acidity of water initially causes general increase in filamentous algae. However, high levels of water acidity due to pollution by acid forming chemicals or acid rains results in decrease in planktonic algae in the water body. Most algae and diatoms disappear completely in water below the pH 5.8.
2. Diatoms are highly sensitive to pH and different species of diatoms are found at different pH values of water body. Thus changes in the species composition of diatoms very accurately indicates the pH level of the water body.
3. Sewage, organic matter and chemical fertilizers: Increase in the organic matter or chemical fertilizers that are washed off into the water body results in increased nutrient supply for algal growth. Such a condition of increased nutrient supply is termed eutrophication and results in water blooms of various types of algae. The water blooms of planktonic algae like Microcystis, Scendensmus, Hydrodictyon and Chlorella indicate pollution of water body due to excessive addition of organic matter, nitrates or phosphates.
4. Heavy metals: Some algae like Cladophora and Stigeoclonium absorb and accumulate many heavy metals from the water. Thus the excessive growth of these algae in the water indicates pollution due to heavy metals.
5. Oil pollution: Excessive growth of algae like Duniella tertiolacta, Skeletonema costatum, Cricosphaera carterae, Amphidium carterae, Cyclotella cryptica and Pavlova lutheri indicate oil pollution of water bodies.
Algae in pollution control
Algae like Chlorella, Chlamydomonas, Scendensmus and Spirullina are grown in sewage treatment plants along with suitable bacteria (algal-bacterial systems). Organic sewage degraded by bacteria is used up by algae in their photosynthesis and growth. The abundant algal growth the treatment plant is periodically removed and used as animal feed or source of protein. The polluted water is thus cleaned by the combined action of bacteria and algae.
For the treatment of water containing metals as pollutants, algae like Chlorella are cultured in the polluted water. These algae absorb the metals from the water. The algal growth is periodically removed and destroyed.