Photosynthetic light reactions take place in the thylakoid membranes, where lightdriven electron transfer occurs from water to carbon dioxide through two photosystems (PSII and PSI), between of which is an electron-transferring agent called plastoquinone (PQ). PQ is present in the thylakoids as a pool (PQ-pool) that consists of its reduced and oxidized forms. The ratio of the two forms of PQ has long been known to control the movement of the Light-harvesting complex II trimers between PSII and PSI, a phenomenon termed as state transitions. Most of our understanding on algal photosynthesis originates from plant studies. However, as our knowledge of the algal system has increased, it is now clear that deeper understanding, e.g. about regulation of algal light reactions, is required.

This thesis set out to examine how the green alga Chlamydomonas reinhardtii withstands strong light that damages the photosynthetic machinery. The results demonstrate how mutant strain cells resistant against photoinhibition of PSII perform better under high light and how they divert the resources saved from less active PSII repair to biomass production. The photoinhibition-tolerance of the mutant is hypothesized to be partly due to changes in the redox potentials inside PSII.

However, it is also shown that even a wild-type population of Chlamydomonas can survive under extreme light intensities via modifications to the photosynthetic machinery. This variation-dependent acclimation enables the cells to cope with the increased photon flux. The demonstrated acclimation contains numerous changes in the properties of PSII, such as significantly reduced rate of singlet oxygen production. However, the acclimated cells exhibit signs of severe photoinhibition.

Lastly, this work highlights the dynamics of the PQ-pool and its relation to state transitions in Chlamydomonas. The PQ-pool of Chlamydomonas is shown to be of similar size as in cyanobacteria and plants. It is highly reduced under almost all light conditions applied, possibly due to fairly similar wavelength dependence profiles of algal PSII and PSI. The non-photochemical reduction and oxidation of PQ are very active in the used alga. It is also shown that state transitions do not correlate with the PQ-pool redox state in Chlamydomonas as in plants, and that light states respond more to the intensity, rather than the quality of light in autotrophic Chlamydomonas.