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

The transfer of laboratory cultures of H. pluvialis to high irradiance outdoors caused a substantial decline in the maximum quantum yield of photosystem II (PSII), from 0.65 in the morning to 0.45 at midday, as measured by the ratio of variable to maximum fluorescence yields (Fv/Fm ), and 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 re-oxidation time constant of the reduced plastoquinone QA-, i.e. the time required for the electron transfer from primary plastoquinone acceptor of PSII QA- to secondary plastoquinone acceptor QB. Photosynthesis rates were almost constant during the day. Along with the increase in nonphotochemical quenching, a slight increase in zeaxanthin and antheraxanthin contents was observed as well as a decrease in violaxanthin showing the presence of an operative xanthophyll cycle in this microalga. A remarkable increase of secondary carotenoids was found at the end of the first day of exposure to sunlight, mainly astaxanthin monoester which 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. After 4 days of exposure to sunlight, the dry weight reached 800 mg L-1 and the concentration of secondary carotenoids (81% astaxanthin monoester) reached 4.4 % of dry weight.