A produção de biomassa e remoção de nutrientes de águas eutrofizadas dependem do tamanho de inóculo das microalgas Chlorella vulgaris e Scenedesmus acuminatus?

Abstract

In this study, we characterized the variation of algal biomass production and nutrient removal by inoculum size of microalgae Chlorella vulgaris and Scenedesmus acuminatus in experimental cultures. Additionally, we test the hypothesis that there is an inverse relationship between algal biomass production and a) inoculum size, and b) removal of nutrients. For the experiment we cultivated the two species of microalgae, C. vulgaris and S. acuminatus. As a growing medium, we use filtered water from a eutrophicated urban pond. Different microalgae biovolumes were used as inoculums, also called treatments (0.09, 0.04, 0.02 and 0.01 mm3 ml-1). For each treatment, transparent plastic bottles containing five liters of the culture medium were used. Each treatment was performed in triplicate. The experiment lasted ten days, every two days the biovolume and the amount of orthophosphate (PO43-), ammonia (NH4+) and nitrate (NO3-) of each culture bottle were measured. The biovolume growth pattern in cultures exhibited an exponential pattern for C. vulgaris and sigmoidal for S. acuminatus. Although the removal curve patterns of the quantity of PO43-, NH4+ and NO3- were comparable between species, a difference between the respective inoculum sizes can be appreciated. The increment of C. vulgaris biovolume in the 0.02 mm3 ml-1 treatment depicted a significantly lower biovolume in respect to 0.09 mm3 ml-1 treatment, and in the 0.01 mm3 ml-1 treatment in regard to 0.09 mm3 ml-1 treatment. The increment of S. acuminatus biovolumen in the 0.01 mm3 ml-1 treatment was significantly lower in regard to 0.04 and 0.09. mm3 ml-1 treatments. The amount of PO43- for C. vulgaris in 0.02 mm3 ml-1 treatment was significantly lower in regard to 0.09 mm3 ml-1 treatment, and 0.01 mm3 ml-1 treatment in regard to 0.09 mm3 ml-1 treatment. For S. acuminatus the amount of PO43- in 0.01 mm3 ml-1 treatment was significantly lower in regard to 0.09 mm3 ml-1 treatment. The reduction in the amount of NH4+ and NO3- did not show significant differences between treatments. We conclude that chlorophyceous microalgae species cultivated in eutrophicated waters depict an intrinsic pattern in the production of biomass, nevertheless the nutrients removal patterns are comparable among species. Based on these results, we reject the initial hypothesized. The effect of inoculum size on algal biomass production and phosphorous removal is directly proportional, however the nitrogen removal is not dependent on different inoculum sizes.