Changes of Photosystem Stoichiometry during Cell Growth in Dunaliella <italic>salina</italic> Cultures

The photosystem stoichiometry in Dunaliella salina thylakoids was measured during cell growth in a fully contained culture. In dilute cultures, obtained after inoculation of cells into fresh growth medium, the PS II/PS I stoichiometry was about 2.2/1.0. This ratio was gradually lowered to about 1.2/1.0 in mature cultures. The decrease of the PS II/PS I ratio is discussed in terms of increasing self-shading in the culture and increasing pH in the growth medium. Changes in the pH occurred from 7.7 in young cultures to 8.9 in mature ones and caused a significant depletion of soluble CO2 from the growth medium. A correlation of the CO2/HCO3− ratio in the growth medium with the PS II/PS I ratio in the thylakoid membrane is presented.