Interplay between photochemical activities and pigment composition in an outdoor culture of Haematococcus pluvialis during the shift from the green to red stage

The aim of the work was to investigate the interplay between photochemical activities and pigment composition in the green alga Haematococcus pluvialis exposed to high irradiance outdoors in photobioreactors. Photochemical changes during adaptation to stress were monitored by photosynthetic oxygen evolution and chlorophyll fluorescence measurements. The transfer of laboratory cultures of H. pluvialis to high irradiance outdoors caused a substantial decline in the maximum quantum yield of PSII as measured by the Fv/Fm ratio (from 0.65 in the morning to 0.45 at midday), and in the electron transfer rate (ETR). These changes were accompanied by a steep rise in nonphotochemical quenching (NPQ). Chlorophyll fluorescence induction curves of morning samples showed a clear I step reflecting a certain PSII heterogeneity. Single turnover flash measurements carried out on samples taken from the outdoor photobioreactor in the middle of day revealed an increase in the QA- re-oxidation time constant, i.e. the time required for the electron transfer from QA- to QB. Photosynthesis and respiration rates slightly increased during the day. Respiration rate was about 30 % of net oxygen evolution. Along with the increase in nonphotochemical quenching, a slight increase in zeaxanthin and anteraxanthin contents was observed as well as a decrease in violaxanthin showing the presence of an operative xanthophyll cycle in this microalga. However, the contribution of xanthophyll cycle in NPQ rise has remained debatable since the formation of zeaxanthin was only partially inhibited even with high doses of DTT. While traces of secondary carotenoids were already present at the beginning of the experiment, a remarkable increase was observed at the end of the first day of exposure to sunlight; at that time the amount of secondary carotenoids (mainly astaxanthin monoester) reached 15.5% of the total carotenoid content. Though cells turned to a reddish colour during the second day, the decline in the fluorescence parameters Fv/Fm in the middle of day resulted lower than that observed during the first day, and the value of NPQ did not increase further. Similar behaviour was observed during the third day when the culture was fully red. This can indicate that synthesis of astaxanthin efficiently protected PSII from degradation. After 4 days of exposure to sunlight, the dry weight reached 800 mg L-1 and the amount of secondary carotenoid (81% astaxanthin monoester) reached 4.4 % of dry weight. Neither further exposure to light nor dilution of the culture increased the astaxanthin content.