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

Abstract

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.

Keywords: Carbon assimilation; Chlorophyll fluorescence; NPQ; OJIP-curve; Photosynthesis.