The study of biomass yield and macromolecular content of microalgae change as a function of physiological state and nutrient supply conditions

Abstract

Recently, as the crisis of conventional fossil fuel and air pollution has become more and more serious, microalgae-based biofuel is treated as one of the most promising new energy sources to solve this issue. By knowing the relationship of algal biomass and macromolecular content, nutrient composition and physiological states, the optimal growth condition and maximum biomass and biofuel productivity can be achieved. The aim of this study was to determined how the biomass and macromolecular content varies with growth phases and cultivation parameters including pH, N:P, medium type and algal species. This experiment was based on 144 L batch-cultured green microalgae, C. kessleri and A. falcatus. pH 6.5 was more favored by C. kessleri than pH 8.5 since pH 6.5 could allow more biologically-available carbon dioxide and bicarbonate dissolved in the water as carbon resource for algae than alkaline pH. There were a decline in biomass concentration and protein content and an increase in lipid content for both species in N-limited condition, compared with N-sufficient condition. In the stationary phase, the highest lipid content of 54% of dry biomass was achieved in C. kessleri due to a severe deficiency of nitrogen in WC medium. C. kessleri removed 71% of phosphorus from WC medium with N:P = 2:1 as compared to 39% of phosphorus removal from WC medium with N:P = 50:1, indicating an excessive uptake of phosphorus of C. kessleri in N-limited condition. Meanwhile, a luxury consumption of nitrogen was found in N-sufficient condition since the N:P removal ratio from the medium was 38.5:1, which was higher than Redfield Ratio of 16:1.This excessive storage of nutrient caused a significant increase in ash content in the stationary phase in all conditions. C. kessleri performed better in biomass accumulation and lipid yield than A. falcatus in WC medium. C. kessleri showed 1.3 times higher maximum biomass concentration but about 4 times longer cultivation time in Bristol medium compared to WC medium, which was attributed to the much higher nitrogen and phosphorus concentration in Bristol medium. DOC above 5.5 mg-C/L might have a negative effect on the growth of C. kessleri because of its shading effect and combination of nutrient compounds. Overall, the maximum biomass and lipid yield was found in C. kessleri fed with N:P = 2:1 Bristol medium at pH 6.5. The findings in this study can be used as guidance for optimizing cultivation conditions to harvest maximum biomass and lipids in future field-scale experiments.