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Data of Thesis

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Author: Renato José Reis Molica.

Title:Effect of light intensity on growth and microcystin production in two strains of Microcystis aeruginosa Kutz Emend. Elenkin (Cyanophyceae)

Year: 1996.    Full text (in Portuguese)

Abstract

Accelerating eutrophication of waterbodies has resulted in an exaggerated growth of blue-green algae or cyanobacteria all over the world, a phenomenon named bloom. Moreover, some of the species that form blooms are toxins-producers and are responsible for several cases of animal death and human illness. Hepatotoxins are the toxins more related to these cases and, within this group, microcystin remains the main representative. Among the several species causing toxic water blooms, Microcystis aeruginosa is the most common. During blooms, cells are submitted to high light intensities. In order to examine how microcystin production can be altered by this condition, two strains of M aeruginosa (NPLJ-4 and NPCD-1, toxic and non-toxic, respectively) were cultivated under 40µmol photons .m•2.s-l , 180µmol photons.m•2.s-l and subjected to a change in irradiance from 40 to 180µmol photons. m•2.s-l, when the cultures reached the period between the exponential and the stationary growth phases and the late exponential phase. The physiological state of the cells was accompanied by cell counting and by the analysis of some physiological parameters: chlorophyll a, carbohydrate, protein and biomass production. Also, photosynthetic light-response curves of the NPLJ-4 strain were developed when the irradiance was altered at the late exponential growth phase. Changes in light intensity did not induce toxin production by the non-toxic strain (NPCD-1). The growth phases and the different irradiance regimes influenced the microcystin synthesis by the toxic strain (NPLJ-4). The intracellular level of microcystin was highest during the period between the exponential and the stationary phases, and decreased during the stationary phase. At light intensity of 40 µmoll photons. m•2.s-l the cells contained the highest amounts of toxin and the change in irradiance to 180µmol photons.m•2.s-l caused a decrease in microcystin levels. The change in light intensity resulted in an increase in cell division. The chlorophyll, carbohydrate and protein levels tended to equalize those of the cultures that had grown at 180µmol photons.m•2.s-l. The photosynthetic light-response curves showed that cells submitted to a change in irradiance were photoinhibited and the highest values of light saturated photosynthetic rate were observed in the cultures cultivated at 180µmol photons.m•2.s-l. Our experiments did not a1low to conc1ude how light controls microcystin synthesis. However, hypothesis could be speculated: at high light intensities, most of the energy could be detected to enhance the efficiency of CO2 absorption, consequent1y, the synthesis of the enzymes responsible for microcystin synthesis diminished. The other one could be that the decrease of microcystin levels, associated with the increase of light intensity, would be a consequence of the rise in growth rate.

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