Abstract
The effects of the absence of nitrogen and phosphorus on AA media and three different light intensities (100% Light, 60.0± 2.7 μmol m-2 s-1, 50% Light, 30.0± 2.7 μmol m-2 s-1, and 13.5% Light, 8.1± 2.7 μmol m-2 s-1) on cell production and synthesis of heterocytes and akinetes were determined in a strain of Nostoc paludosum. In the experiment concerning the absence of nutrients, significant variations were observed between the control group and the groups with absence of nutrients, especially in cell numbers and in synthesis of heterocytes and akinetes. The absence of nitrogen boosted the formation of heterocytes and the absence of phosphorus produced the most akinetes. As for the different light conditions, the growth curves determined for each treatment showed that cell synthesis is slightly affected by the reduction of illuminance. The different light intensities are capable of reducing the maximum growth rates of Nostoc paludosum, with 13.5% light restriction being the most effective on limiting the cell growth rate and inducing the formation of akinetes. The synthesis of heterocyte does not seem directly correlated to light intensity, being better explained by nutritional factors. The data found contributes to the understanding of some of the factors involving growth and synthesis of special structures in Nostoc paludosum.
Highlights
Cyanobacteria are frequently a dominant group of primary producers in freshwaters, especially reservoirs, lakes and waters retained by dams
Our results showed that the nutritional deficiencies of nitrogen and phosphorus and the variation in light intensities are capable of altering the regular growth dynamics of Nostoc paludosum, with the nitrogen and phosphorus limitations and the severe light restriction of 13.5% being the most effective in reducing cell-growth rates
The absence of nutrients in the culture medium and the limitation of light reaching the cyanobacteria cells are capable of altering the morphology of vegetative cells, with an increase in fragmentation, affecting the special structures, akinetes and heterocytes
Summary
Cyanobacteria are frequently a dominant group of primary producers in freshwaters, especially reservoirs, lakes and waters retained by dams. In Brazil, these water bodies are generally shallow and with a long residence time, generating advantageous conditions for the development and dominance of cyanobacteria (Sant’Anna et al, 2007). It is well known that these water bodies can be directly affected by human activities, reaching a eutrophic state in which there is an increase of nutrient concentrations, especially phosphorus and nitrogen. Some cyanobacteria, such as the genera Dolichospermum, Anabaena and Nostoc, possess morphological plasticity in the vegetative cells that form the trichomes, a reflection of differential gene expression (Lyra et al, 2001), allowing them to alter cellular structures in order to form akinetes and heterocytes, which are highly specific cells. The akinetes play an important role in cyanobacterial reproduction, acting as a resistant cell that can remain dormant in the sediment when environmental conditions are unfavorable
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