Continuous cultivation of microalgae as an efficient method to improve carbohydrate productivity and biochemical stability

Nowadays, the production of carbohydrates or lipids from microalgae with good productivities values is limited because, generally, the systems used are in batch mode operation, with two steps of nutrient starvation (mainly, nitrogen) after cultivation. In this work, a literature outlook and a comparison of the results obtained in continuous mode with Chlorella vulgaris and Scenedesmus obliquus, under nitrogen inlet in the system are presented and discussed in view of carbohydrate production. Nitrogen inlet optimization lead to a reduction of the biomass up to 25% (depending on the condition) but, as the carbohydrate fraction was increased, its productivity increased 2-3 times with respect to nitrogen replete conditions. Carbohydrate accumulation in Chlorella vulgaris appeared to be triggered by a combination of high light intensity and low nitrogen inlet (at constant residence time). The highest carbohydrate productivity obtained was 0.35 g L-1 day-1. A different behavior was observed in the case of Scenedesmus obliquus, which showed a remarkable value of carbohydrate productivity of about 0.80 g L-1 day-1, as an effect of nitrogen limitation only. Extrapolated bioethanol production showed a potentiality with these productivities to achieve between 2-5 times more ethanol per ha than the traditional crops, such sugarcane.