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Thursday, 10 March 2011 15:50

Aquatic Plants

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Adapted from J.W.G. Lund’s article, “Algae”, “Encyclopaedia of Occupational Health and Safety,” 3rd edition.

Worldwide aquaculture production totalled 19.3 million tonnes in 1992, of which 5.4 million tonnes came from plants. In addition, much of the feed used on fish farms is water plants and algae, contributing to their growth as a part of aquaculture.

Water plants that are grown commercially include water spinach, watercress, water chestnuts, lotus stems and various seaweeds, which are grown as low-cost foods in Asia and Africa. Floating water plants that have commercial potential are duckweed and water hyacinth (FAO 1995).

Algae are a diverse group of organisms; if the cyanobacteria (blue-green algae) are included, they come in a range of sizes from bacteria (0.2 to 2 microns) to giant kelps (40 m). All algae are capable of photosynthesis and can liberate oxygen.

Algae are nearly all aquatic, but they may also live as a dual organism with fungi as lichens on drier rocks and on trees. Algae are found wherever there is moisture. Plant plankton consists almost exclusively of algae. Algae abound in lakes and rivers, and on the seashore. The slipperiness of stones and rocks, the slimes and discolourations of water usually are formed by aggregations of microscopic algae. They are found in hot springs, snowfields and Antarctic ice. On mountains they can form dark slippery streaks (Tintenstriche) that are dangerous to climbers.

There is no general agreement about algae classification, but they are commonly divided into 13 major groups whose members may differ markedly from one group to another in colour. The blue-green algae (Cyanophyta) are also considered by many microbiologists to be bacteria (Cyanobacteria) because they are procaryotes, which lack the membrane-bounded nuclei and other organelles of eukaryotic organisms. They are probably descendants of the earliest photosynthetic organisms, and their fossils have been found in rocks some 2 billion years old. Green algae (Chlorophyta), to which Chlorella belongs, has many of the characteristics of other green plants. Some are seaweeds, as are most of the red (Rhodophyta) and brown (Phaeophyta) algae. Chrysophyta, usually yellow or brownish in colour, include the diatoms, algae with walls made of polymerized silicon dioxide. Their fossil remains form industrially valuable deposits (Kieselguhr, diatomite, diatomaceous earth). Diatoms are the main basis of life in the oceans and contribute about 20 to 25% of the world’s plant production. Dinoflagellates (Dinophyta) are free-swimming algae especially common in the sea; some are toxic.

Uses

Water culture can vary greatly from the traditional 2-month to annual growing cycle of planting, then fertilizing and plant maintenance, followed by harvesting, processing, storage and sale. Sometimes the cycle is compressed to 1 day, such as in duckweed farming. Duckweed is the smallest flowering plant.

Some seaweeds are valuable commercially as sources of alginates, carrageenin and agar, which are used in industry and medicine (textiles, food additives, cosmetics, pharmaceuticals, emulsifiers and so on). Agar is the standard solid medium on which bacteria and other micro-organisms are cultivated. In the Far East, especially in Japan, a variety of seaweeds are used as human food. Seaweeds are good fertilizers, but their use is decreasing because of the labour costs and the availability of relatively cheap artificial fertilizers. Algae play an important part in tropical fish farms and in rice fields. The latter are commonly rich in Cyanophyta, some species of which can utilize nitrogen gas as their sole source of nitrogenous nutrient. As rice is the staple diet of the majority of the human race, the growth of algae in rice fields is under intensive study in countries such as India and Japan. Certain algae have been employed as a source of iodine and bromine.

The use of industrially cultivated microscopic algae has often been advocated for human food and has a potential for very high yields per unit area. However, the cost of dewatering has been a barrier.

Where there is a good climate and inexpensive land, algae can be used as part of the process of sewage purification and harvested as animal food. While a useful part of the living world of reservoirs, too much algae can seriously impede, or increase the cost of water supply. In swimming pools, algal poisons (algicides) can be used to control algal growth, but, apart from copper in low concentrations, such substances cannot be added to water or domestic supplies. Over-enrichment of water with nutrients, notably phosphorus, with consequent excessive growth of algae, is a major problem in some regions and has led to bans on the use of phosphorus-rich detergents. The best solution is to remove the excess phosphorus chemically in a sewage plant.

Duckweed and a water hyacinth are potential livestock feeds, compost input or fuel. Aquatic plants are also used as feed for noncarnivorous fish. Fish farms produce three primary commodities: finfish, shrimp and mollusc. Of the finfish portion, 85% are made up of noncarnivorous species, primarily the carp. Both the shrimp and mollusc depend upon algae (FAO 1995).

Hazards

Abundant growths of freshwater algae often contain potentially toxic blue-green algae. Such “water blooms” are unlikely to harm humans because the water is so unpleasant to drink that swallowing a large and hence dangerous amount of algae is unlikely. On the other hand, cattle may be killed, especially in hot, dry areas where no other source of water may be available to them. Paralytic shellfish poisoning is caused by algae (dinoflagellates) on which the shellfish feed and whose powerful toxin they concentrate in their bodies with no apparent harm to themselves. Humans, as well as marine animals, can be harmed or killed by the toxin.

Prymnesium (Chrysophyta) is very toxic to fish and flourishes in weakly or moderately saline water. It presented a major threat to fish farming in Israel until research provided a practical method of detecting the presence of the toxin before it reached lethal proportions. A colourless member of the green algae (Prototheca) infects humans and other mammals from time to time.

There have been a few reports of algae causing skin irritations. Oscillatoria nigroviridis are known to cause dermatitis. In freshwater, Anaebaena, Lyngbya majuscula and Schizothrix can cause contact dermatitis. Red algae are known to cause breathing distress. Diatoms contain silica, so they could pose a silicosis hazard as a dust. Drowning is a hazard when working in deeper water while cultivating and harvesting water plants and algae. The use of algicides also poses hazards, and precautions provided on the pesticide label should be followed.

 

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