The culture of Chlorella vulgaris in a recycled supernatant: Effects on biomass production and medium quality

Reusing supernatant of microalgae culture medium can have inhibitory or toxic effects on the biomass production because of the release of organic metabolites by cells in the culture medium during their growth. This work investigated the impact of Chlorella vulgaris medium recycling on culture productivity, cells quality and accumulation of excreted metabolites in the culture medium. No significant impact on the C. vulgaris growth was observed after 63 days of recycling, the productivity remained stable at around 0.55 kg m−3 day−1. Organic matters accumulated in supernatant were identified as biopolymers (BP) poor in nitrogen and with a size above 40 kDa (probably polysaccharides), and small organic molecules (SOM) richer in nitrogen with a molecular size ranging from 1 to 3 kDa. The concentration of biopolymers in the supernatant increased till to a maximum and then decreased, possibly consumed by bacteria, whereas small organic compounds accumulated in the medium.