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|September 20, 2007||
The majority of blue-greens are aerobic photoautotrophs: their life processes require only oxygen, light and inorganic substances. A species of Oscillatoria that is found in mud at the bottom of the Thames, are able to live anaerobically. They can live in extremes of temperatures -60°C to 85°C, and a few species are halophilic or salt tolerant (as high as 27%, for comparison, conc. of salt in seawater is 3%). Blue-greens can grow in full sunlight and in almost complete darkness. hey are often the first plants to colonize bare areas of rock and soil, as an example subsequent to cataclysmic volcanic explosion (at Krakatoa, Indonesia in 1883). Unlike more advanced organisms, these need no substances that have been preformed by other organisms.
At the onset of nitrogen limitation during bloom conditions, certain cells in Anabaena and Aphanizomenon evolve into heterocysts, which convert nitrogen gas into ammonium, which is then distributed to the neighboring cells of a filament. In addition, blue-greens that form symbiotic (mutually beneficial) relationships with a wide range of other life forms, can convert nitrogen gas into ammonium.
Finally, at the onset of adverse environmental conditions, some blue-greens can develop a modified cell, called an akinete. Akinetes contain large reserves of carbohydrates, and owing to their density and lack of gas vesicles, eventually settle to the lake bottom. They can tolerate adverse conditions such as the complete drying of a pond or the cold winter temperatures, and, as a consequence, akinetes serve as "seeds" for the growth of juvenile filaments when favorable conditions return. Heterocysts and akinetes are unique to the blue-greens.
The epilithic community displays a clearly discernable zonation in lakes. Members of the genera Pleurocapsa, Gloeocapsa and Phormidium often dominate the dark blue-black community of the spray zone. Scytonema and Nostoc species form olive-green coatings and are more frequent about the water line, whilst the brownish Tolypothrix and Calothrix species are more typical components of the subsurface littoral community.
The epiphytic flora of lakes is usually dominated by diatoms and green algae, and blue-greens are of less importance in this community. Species of the genera Nostoc, Lyngbya, Chamaesiphon and Gloeotrichia have been occasionally encrusting submerged plants.
The epipelic community commonly includes blue-greens like Aphanothece and Nostoc particularly in the more eutrophic lakes. Benthic blue-greens growing over the littoral sediments and on submerged plants may be responsible for the occasional high rates of N2-fixation measured in oligotrophic lakes.
|The formation of water blooms results from the redistribution and often rapid accumulation of buoyant planktonic populations. When such populations are subjected to suboptimal conditions, they respond by increasing their buoyancy and move upward nearer to the water surface. Water turbulence usually prevents them reaching the surface. If, however, turbulence suddenly weakens on a calm summer day, the buoyant population may 'over-float' and may become lodged right at the water surface. There the cells are exposed to most unfavourable and dangerous conditions, like O2 supersaturation, rapidly diminishing CO2 concentrations and intense solar radiation, which are inhibitory to photosynthesis and N2-fixation, causing photo-oxidation of pigments and inflicting irreversible damage to the genetic constitution of cells. A frequent outcome of surface bloom formation is massive cell lysis and rapid disintegration of large planktonic populations. his is closely followed by an equally rapid increase in bacterial numbers, and in turn by a fast deoxygenation of surface waters which could be detrimental to animal populations within the lake. Water blooms are objectionable for recreational activities, and more importantly, create great nuisance in the management of water reservoirs.
Most of these conditions are produced by just three blue-greens, informally referred to as Annie (Anabaena flos-aquae), Fannie (Aphanizomenon flos-aquae) and Mike (Microcystis aeroginosa). An oversupply of nutrients, especially phosphorus and possibly nitrogen, will often result in excessive growth of blue-greens because they possess certain adaptations that enable them to outcompete true algae. Perhaps the most important adaptation is their positive buoyancy, which is regulated by their gas vesicles which are absent in true algae.
Humans also consume Spirulina. It contains all of the amino acids essential for humans, and its protein content is high (± 60%). It is a staple food in parts of Africa and Mexico. In China, Taiwan and Japan, several blue-greens are served as a side dish and a delicacy. Several areas in North America culture and commercially process certain blue-greens for various food and medicinal products such as vitamins, drug compounds, and growth factors.
Heterocystous blue-greens possess the unique ability to simultaneously evolve O2 in photosynthesis (in vegetative cells) and H2 by nitrogenase catalyzed electron transfer to H+-ions (in heterocysts), in the absence of N2 or other substrates of nitrogenase. This is the basis for the attempts of several workers to exploit the potential through the development of a `biophotolytic system' for solar energy conversion, even though to date the thermodynamic efficiency has been disappointingly low.
Nevertheless, the utilization of blue-greens in food production and in solar energy conversion may hold immense potential for the future, and could be exploited for man's economy. Progress in the study of the genetics of blue-greens may enable us to manipulate the N2-fixation (nif) and associated genes, and produce strains which fix N2, evolve H2 or release ammonia with great efficiency.
|Poisonous blue-greens occur in ponds and lakes throughout the world. In Canada, they primarily occur in the prairie provinces. Poisoning has caused the death of cows, dogs, and other animals. Although humans ordinarily avoid drinking water that displays a blue-green bloom or scum, they may be affected by toxic strains when they swim or ski in recreational water bodies during a bloom. Typical symptoms include redness of the skin and itching around the eyes; sore, red throat; headache; diarrhea; vomiting; and nausea. The frequently occurring `swimmers itch' is attributed to contact with Lyngbya majuscula, Schizothrix calcicola and Oscillatoria nigroviridis, which are commonly found in tropical and subtropical seawaters. The toxins responsible are lipid-soluble phenolic compounds. Since the same or similar symptoms can be produced by bacteria or viruses, one should not necessarily conclude that blue-greens are responsible for a human illness simply because the sick individual recently swam in a lake or pond that has suffered a bloom. Human death has not been documented. Reported cases affecting humans list Anabaena as the main culprit.
Most of the recorded toxic blooms are caused by Microcystis aeruginosa, which manufactures "microcystin", which yields 7 (or 14) amino acids upon hydrolysis. It causes enlargement and congestion of the liver followed by necrosis and haemorrhage, and may also exhibit neurotoxic activity.
Alkaloid toxins (anatoxins, aphantoxins) act on the nervous system, leading to paralysis of muscles needed for breathing.
Two other genera, Oscillatoria and Nodularia are also known to produce toxic populations. Whether the animal survives the poisoning depends primarily upon the concentration of toxin ingested. Blue-green toxins may act on zooplankton and might be an effective mechanism of protection against grazing pressures.
Little is known about the percent of blooms that are toxic (upto 25% quoted in literature), and also why a toxic population is produced. A complicating factor is that part of a bloom can be toxic and another part nontoxic within the same lake. It has been suggested that toxic strains may develop only under a particular set of environmental conditions, or that toxin production may be associated with plasmid-mediated gene transfer.
The blue-greens are microscopic life forms that exhibit several different types of organization. Some grow as single cells enclosed in a sheath of slime-like material, or mucilage. The cells of others aggregate into colonies that are either flattened, cubed, rounded, or elongated into filaments. Actual identification of cyanobacteria (blue-greens) requires microscopic examination of cells, colonies, or filaments, although experienced aquatic biologists can usually recognize Microcystis (colonies look like tiny grey-green clumps) and Aphanizomenon (green, fingernail-like or grass-like clippings).
Biological control is in principle possible, though not always practical and as effective. Invertebrates like cladocerans, copepods, ostracods and snails are known to graze on green algae and diatoms. Daphnia pulex has been reported to feed on Aphanizomenon flos-aquae while present in the form of single filaments or small colonies but avoid large raft-like colonies. The copepod Diaptomus has been implicated in the grazing of Anabaena populations in Severson Lake, Minnesota.
Micro-organisms (fungi, bacteria and viruses) appear to play an important part in regulating growth of blue-greens in freshwaters. Certain chytrids (fungal pathogens) specifically infest akinetes, other heterocysts. Bacterial pathogens belonging to the group of Myxobacteriales can effect rapid lysis of a wide range of unicellular and filamentous blue-greens, though heterocysts and akinetes remain generally unaffected. Viral pathogens belonging to the group of cyanophages exhibit some degree of host specifity. Phage AR-1 attacks Anabaenopsis, phages SM-1 and AS-1 are effective against the unicellular forms, Synechococcus and Microcystis, Phage C-1 lyses Cylindropermum, and the LPP-1 virus is effective against strains of Lyngbya, Phormidium and Plectonema.
The long-term approach is no doubt the systematic removal of major nutrients.
References (cf. also Select References in Limnology)
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