Effects of CO2 enrichment on primary photochemistry, growth and astaxanthin accumulation in the chlorophyte Haematococcus pluvialis

The atmospheric CO2 level is limiting for growth of phototrophic organisms such as microalgae, so CO2 enrichment boosts the growth and photosynthesis of microalgal cultures. Still, excessive CO2 injection might inhibit photosynthesis of microalgae. We investigated the effect of continuous sparging of the cultures of Haematococcus pluvialis BM 1 (IPPAS H-2018) (Chlorophyceae), the richest natural source of the value-added pigment astaxanthin. H. pluvialis cultures with CO2-enriched air-gas mixtures (with CO2 level from the atmospheric to 20%) on growth and astaxanthin accumulation in the microalga. Special attention was paid to photosynthetic activity and non-photochemical excited chlorophyll states quenching in the microalgal cells, which was monitored via chlorophyll fluorescence analysis. We also report on the capability of CO2 capture by H. pluvialis derived from direct measurements of its elemental carbon content. The beneficial effect of the moderately high (5%) CO2 levels on the culture growth and astaxanthin accumulation under stress results in a higher overall astaxanthin productivity. However, increase of the CO2 level to 10% or 20% was deteriorative for growth, photosynthesis and carbon assimilation. The results support the possibility of combining a traditional two-stage H. pluvialis cultivation with CO2 bio-capture although a dilution of the flue gas before its injection is required.