Phytoplankton is widely recognized as being regulated mainly by resources (nutrients and light) and predation by higher trophic levels. In reservoirs, these controls also can be modulated by hydrology, for example through the influence of flow pulses generated by the operation of the dam. In this study, we tested the influence of light, nutrients, and zooplankton grazing pressure, and also hydrology (as water residence time) on the phytoplankton biomass in eight tropical hydroelectric reservoirs, which differ in size, morphometry, location, trophic state, and water residence time. Our hypothesis was that, as these reservoirs are used for hydroelectric purposes, the control that would otherwise be exerted on phytoplankton biomass primarily by resource availability and grazing will also be modulated by hydrology. Low phytoplankton biomass (range of system medians = 12-299 lg C l -1 ) occurred in most systems, except for one highly eutrophic reservoir (median = 1331 lg C l -1 ). Our data showed that phosphorus was more often likely to be the limiting nutrient in these systems, as assessed through nutrient limitation indexes (nitrogen and phosphorus), based on concentrations and ratios. For most reservoirs, excluding the eutrophic system with high cyanobacteria biomass, seasonal water residence time was the variable that best explained phytoplankton variation among the several environmental variables analyzed in this study (P \ 0.0001; adjusted r 2 = 0.38). Hydrology was an important and additional factor modulating phytoplankton in these tropical reservoirs, directly removing phytoplankton populations and their potential zooplankton grazers by washout, and also affecting nutrient availability.

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Biodiesel production is increasing worldwide to supply a demand for renewable, environmentally-friendly fuels. In this sense, microbial oils appeared as potential feedstock for biodiesel production with scientific and commercial interest. While yeast oils have been studied for decades, recent years showed literature suggesting optimization of cultivation processes as a good strategy to improve lipid accumulation in oleaginous yeasts. Some factors, like carbon nitrogen (C/N) ratio, pH and temperature are considered as the main parameters that affect the production of microbial oil. This study aimed to improve the optimization of cultivation process through multivariate analysis, increase the lipid accumulation and transform the condition non-oleaginous to an oleaginous condition in Candida zeylanoides QU 33. For optimization, response surface methodology was applied, using the levels of the variables temperature (20-35ºC), pH (2.0 to 6.0), and glucose concentration (10-40 g/100mL). The results showed two adjusted models to improve biomass production and lipid yield in C. zeylanoides QU 33, where temperatures lower than 28ºC, and glucose concentrations greater than 25% are favorable for the accumulation of lipids. The concentrations of glucose lower than 15% were unfavorable. The best conditions were observed in the experiment at 27.5 °C, pH 6 and 25% (w/v) glucose, with a lipid yield of 0.2 g/L and lipid content of 12.42% (g/g). Besides the unsatisfactory effort to change the oleaginicity condition for C. zeylanoides QU 33, our results confirmed the hypothesis to use this strain as a model of lipid metabolism for non-oleaginous yeast.