Cultivo da microalga de alto valor econômico Nannochloropsis oceanica em meio alternativo utilizando melaço de cana como substrato orgânico

Abstract

Microalgae cultivation has proven to be an alternative in the production of several biochemical compounds that can be used in the industry of biodiesel, biomedicine, aquaculture, among others. Large scale production is not yet economically viable mainly due to the high production costs associated with the need to use complex culture media. Therefore, research of alternative and low-cost culture media is crucial. The objective of this research was to evaluate the growth of Nannochloropsis oceanica microalgae in autotrophic, mixotrophic and heterotrophic cultures using an alternative medium composed of agricultural fertilizers supplemented with sugarcane molasses. Three different treatments were tested, varying the concentration of sugarcane molasses (0; 0,25 g/L; 0,50 g/L and 0,75 g/L) and the light intensity (dark, 100% and 50%) over six days, during which the values of pH, cell density and dry weight were measured daily. Growth in f/2 medium (GUILLARD, 1975) was used as a control. Fertilizer medium without sugarcane molasses did not show good performance in the growth of the tested species compared to medium f/2. However, the addition of sugarcane molasses in the fertilizer medium at a low concentration of 0,25 g/L improved the autotrophic cultivation of the tested species, reaching statistically equal values to those measured in the cultures using the f/2 medium. Increasing the concentration of sugarcane molasses in the cultures did not result in an increase in microalgae growth, possibly due to a greater darkening of the culture medium and consequent reduction in the incident cell brightness and a higher growth of heterotrophic bacteria in the medium. However, in these cultures an increase in dry weight was observed, consistent with the decrease in growth after four days of cultivation, which may indicate accumulation of lipid reserves induced by suboptimal conditions, which needs to be evaluated later due to possible biotechnological application. Cultures performed in low light or darkness, representing mixotrophic and heterotrophic cultivation, were also not efficient, showing that microalgae could not use sugarcane molasses as a source of organic carbon and that, possibly, molasses acted as an alternative source of nutrients essential for the autotrophic metabolism of the tested specie, such as manganese and vitamins B1 and H. Therefore, the use of fertilizer medium with sugarcane molasses at low concentrations in the autotrophic cultivation of microalgae Nannochloropsis oceanica proved to be an option feasible when replacing the f/2 medium.