|
Aphanizomenon flos-aquae
Aphanizomenon flos-aquae is a diverse group of cyanobacteria with both toxic and non-toxic[1][2] strains found in brackish and freshwater environments globally, including the Baltic Sea and the Great Lakes. Recent genome sequencing efforts have identified 18 distinct varieties[3] of Aphanizomenon flos-aquae, revealing its genetic complexity. Cyanobacteria were the first organisms to achieve photosynthesis.[4] Chlorophyll and phycocyanine—two pigments contained in cyanobacteria—allow the vegetative cells to absorb light and transform it into nutrients.[4] The genus Aphanizomenon is defined as a cluster of eight morphospecies, including Aphanizomenon flos-aquae. [5] Morphology One of the main morphological characteristics of the genus Aphanizomenon is the tendency to form fascicles of trichomes containing mainly vegetative cells.[6][5] The individual vegetative cells that form Aphanizomenon flos-aquae are cylindrical and elongated. Each cell is composed of hyaline.[5] Aphanizomenon flos-aquae forms typically bent trichomes that are grouped into fascicles up to 2 cm (0.79 in) long.[6] These trichomes can also be found as single free-floating units.[5] Within these fascicles, heterocysts often appear at various intervals on the trichomes.[7] When attached to a trichome, heterocysts import carbohydrates which may act as a reducing agent and an energy source for nitrogen fixation.[8] It has been shown that heterocysts contain a nitrogenase complex which allows them to take part in nitrogen fixation. Other requirements for nitrogen fixation include ATP, low potential electrons, and an anaerobic environment.[8] Life cycleThe life cycle of Aphanizomenon flos-aquae depends on various environmental conditions such as water temperature, dissolved oxygen content, and pH. During the winter, Aphanizomenon flos-aquae persists as akinetes deep in the layers of sediment.[7] These dormant cyanobacterial cells will last all season until the water temperature rises again in the spring. During the springtime, the akinetes go through a recruitment phase as they germinate and disperse into the water column.[7] Different species of phytoplankton can provide interspecific competition for Aphanizomenon flos-aquae if they are outnumbered. Due to higher temperatures, and higher pH levels in the summer, Aphanizomenon flos-aquae begin to flourish and eventually form dense mats known as ‘blooms’ in late summer.[7] The blooms dissipate in autumn as the water temperature and pH drop again and the conditions are more favorable to akinete development.[7] Ecology Aphanizomenon flos-aquae can form dense surface aggregations in freshwater (known as "cyanobacterial blooms").[9] These blooms occur in areas of high nutrient loading, historical or current. During bloom formation, Aphanizomenon flos-aquae photosynthetically produces biomass. These accumulated mats of biomass can grow due to the concentration of nutrients available in eutrophic ecosystems accompanied by high reproductive rates and water temperatures.[10] At high concentrations, these blooms can be ecologically harmful to the aquatic species that cohabitate with the cyanobacteria. In addition to their odiferous presence, cyanobacterial blooms have been associated with lowered dissolved oxygen content, increased turbidity, and the accelerated release of nutrients from sediments.[10] Toxicity and safety Aphanizomenon flos-aquae (AFA) includes both toxic and non-toxic strains found in various global freshwater sources, with different varieties producing diverse compounds.[11][12] The toxicity of A. flos-aquae has been reported in Canada,[13] Germany,[14][15] and China.[16] Some Aphanizomenon flos-aquae varieties are known to produce endotoxins – the toxic chemicals released when cells die. Once released (lysed), and ingested, these toxins can damage liver and nerve tissues in mammals.[citation needed] In areas where water quality is not closely monitored, the World Health Organization has assessed toxic algae as a health risk, citing the production of anatoxin-a, saxitoxins, and cylindrospermopsin.[17] Dogs have been reported to have become ill or have fatal reactions after swimming in rivers and lakes containing toxic A. flos-aquae.[citation needed] The FDA recognizes wild-harvested AFA as safe for consumption as food or in dietary supplements.[18] In a 2012 study conducted in Germany, 18 samples of improperly harvested AFA used in supplement products were shown to be cross-contaminated with microcystin.[15] Ten of the eighteen samples exceeded the safety value of 1 microgram of microcystin per gram, with the authors stating that the "distribution and commercial sale of AFA products, whether pure or mixed formulations, for human consumption appear highly questionable."[15] See alsoReferences
External links
|
||||||||||||||||||||||||
Portal di Ensiklopedia Dunia
