Anagnostidis & Komárek, 1988
Planktothrix is a genus of filamentous cyanobacteria (often called blue-green algae). P. agardhii is regarded as a type species of the genus. Like all the Oscillatoriales, Planktothrix species have no heterocyst and no akinetes, but are unique because they are planktonic, solitary trichome and have gas vacuoles. Before the work of Suda et al., some species of the taxon were grouped within the genus Oscillatoria. A tremendous body of work on Planktothrix ecology and physiology has been done by Anthony E. Walsby, and the 55.6 kb microcystin synthetase gene have been sequenced.
Like most Oscillatoriales, Planktothrix grows by cell division in a single plane to form long unbranched trichomes (also called filaments) of length up to 4 mm, but unlike other Oscillatoriales, these trichomes are phototactic and slightly motile. Typically, Planktothrix filaments do not have specialized cells such as akinetes or heterocysts, and do not produce mucilaginous envelopes, except for some rare species but only under stress conditions. Several species possess constant ratio of their two main photosynthetic pigments, i.e., phycocyanins and phycoerythrins. The production of cyanotoxins is facultative, and strains that do not produce microcystins are commonly found in nature. Apart from microcystins, they can to produce several other cyclic peptides, such as oscillapeptin J.
Both P. agardhii and P. rubescens form massive blooms in fresh water lakes and reservoirs. The green-pigmented species P. agardhii only possess phycocyanins, which gives its color. The species is commonly found at most latitudes in shallow and turbid lakes, where it can tolerate continuous mixing of the water column.
The red-pigmented species P. rubescens is regularly found in clear, deep alpine and pre-alpine lakes that are seasonally stratified. It grows at depth, in the low light conditions of the metalimnion, where it can maximize the absorption of green light with its phycoerythrin pigments. Under the action of wind-induced internal waves, P. rubescens can be moved vertically by several meters following the movements of the metalimnion, which in turn modifies rapidly (within a day) the light conditions experienced by the filaments. This was shown to significantly affects the photosynthesis rate and oxygen production in a lake, especially in lakes where the dominant organism of the phytoplankton community is P. rubescens, such as in Lake Zurich.
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