Plasticidade morfológica de Cronbergia sp. (Nostocaceae), dependente do meio de cultivo e intensidade luminosa

Abstract

Some species of cyanobacteria are capable of differentiating vegetative cells into specialized cells called heterocysts, which then have the sole function of fixing atmospheric nitrogen (N2) through biological nitrogen fixation (BNF). The fixed nitrogen is converted into assimilable forms through the action of the enzyme nitrogenase and is subsequently transported to other cells. In Cronbergia, the formation of heterocysts occurs through the differentiation and division of intercalary vegetative cells, and after trichome rupture, the heterocysts are arranged terminally. For the cultivation of heterocystous cyanobacteria, the most commonly used medium is BG-11 and its variation BG-11 0 (nitrogen-free), a complex of important nutrients for the development of these microorganisms. Since they are photosynthetic, light is an important factor in the cultivation of these organisms because oxygenic photosynthesis provides the energy conservation necessary for biological processes such as BNF. Considering that there is an energy expenditure in the production of heterocysts, the objective of this work was to analyze the existence of morphological plasticity in the heterocystous cyanobacterium Cronbergia sp. when cultivated in BG-11 and BG-11 0 media, as well as under high (140 µmol/m/s) and low (60 µmol/m/s) light intensity. For this, an experiment was conducted in the laboratory, lasting 20 days. The strain Cronbergia sp. UFLA-35 belonging to the Collection of Filamentous Cyanobacteria Culture (CCF/UFLA) was inoculated in BG-11 and BG-11 0 media, with variation in light intensity, as described above. We counted heterocysts (91,044) and the number of vegetative cells (40,245), the latter as a measure of filament size. The differences between heterocyst production and vegetative cell production were interpreted as morphological plasticity. To evaluate whether there was an effect of the treatments on heterocyst production, analysis of variance (ANOVA) was performed, and the difference between pairs was evaluated by the Tukey test. Similarly, to verify if there was an effect of the treatments on filament size given by vegetative growth, the Kruskal-Wallis test was performed, and subsequently, the Dunn test was applied. Spearman correlation was also performed to check for any relationship between the obtained data. The number of heterocysts was higher in treatments with BG-11 0 medium. On the other hand, the number of vegetative cells was higher in treatments with BG-11 medium. Finally, we observed that the higher the production of heterocysts, the lower the production of vegetative cells. There was also a trade-off between the production of these cells. It is known that nutrition is an important factor in the development of cyanobacteria and that nitrogen scarcity resulted in an increase in heterocyst production. However, the presence of nitrogen in the medium led to an increase in vegetative cells. The trade-off between heterocyst and vegetative cell production indicates that the production of the former has a high cost for the strain. High and low light intensity did not seem to differentiate between them, perhaps because they did not reach a radiation level relevant to Cronbergia at the nutritional level at which they were being maintained.