Cultivation of Haematococcus pluvialis in source-separated urine for biomass production and astaxanthin accumulation

Abstract

Source separation and decentralized resource recovery, especially from urine, are a promising method to treat domestic sewage. The use of microalgae to realize source-separated urine treatment and recovery of biomass and high value-added substances has great potential and is worth studying. In this study, the effects of urine dilution on biomass production and astaxanthin accumulation of Haematococcus pluvialis cultivated in human urine and cultivation condition optimization based on response surface methodology were investigated. Comparing the growth of H. pluvialis in 9 kinds of diluted urine (1:25–1:225, v/v), it was found that H. pluvialis grew well in urine diluted >100 times and had the highest specific growth rate (0.46 d−1) in 1:150-diluted urine. To better promote the growth of H. pluvialis and the accumulation of astaxanthin under the 1:150-diluted urine, parameter optimization was carried out by using response surface methodology for algal initial density, temperature, and light intensity. The specific growth rate of 0.34 d−1, 0.95 g·L−1 biomass, and astaxanthin yield of 59.55 ± 3.51 mg·L−1 (6.25 % DW) were harvested under the optimal conditions with initial density of 8.05 × 104 cells·mL−1, temperature of 19 °C and light intensity of 64 μmol·m−2·s−1. The decrease of temperature and the increase of light intensity had significantly positive effects on the proliferation of H. pluvialis, and increasing algal initial density had a significantly positive effect on astaxanthin yield. Furthermore, chlorophyll fluorescence parameters and increased cell size reflected the photosynthesis and environmental stress of H. pluvialis. The optimal condition led to higher Fv/Fm, which showed the higher photosynthetic potential. The increase of NPQ under higher light intensity and initial density indicated that H. pluvialis was exposed to environmental stress, then astaxanthin was accumulated and might gradually replace the NPQ protection mechanism. Additionally, the microalgal cell diameter also increased during the formation of haematocysts and astaxanthin accumulation.