Diurnal patterns of photosynthesis were studied in July and April populations of Cladophora glomerata (L.) Kutz. from open and from shaded sites. Summer samples exposed to full sunlight showed decreased efficiency of open photosystem II at noon, and only slight differences were found between samples that had grown at open or at shaded sites. Electron transport rate was limited at highest fluence rates in shade plants, and non-photochemical quenching (NPQ) revealed faster regulation in samples from open sites. Daily course of de-epoxidation was not linearly correlated with the course of NPQ. The comparison of samples from open and from shaded sites revealed a higher capacity of thermal energy dissipation and an increase in the total amount of xanthophyll-cycle pigments (21%) in samples from open sites. In April, down-regulation of the efficiency of open photosystem II was related to lower water temperature, and hence, increased excitation pressure. In April the pool size of xanthophyll-cycle pigments was increased by 21% in comparison with summer and suggested higher levels of thermal energy dissipation via de-epoxidized xanthophylls. In both, summer and spring the amount of xanthophyll-cycle pigments was 20% higher in samples from open sites. Acclimation of C. glomerata to growth light conditions was further shown by experimental induction of NPQ, indicating NPQ increases of 23%, and increases of 77% in the reversible component of NPQ in open site samples. The effect of temperature on photosynthetic rate was non-linear, and different optimum temperatures of electron transport rate and oxygen evolution were exhibited. [References: 31

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The influence of mercury ions (Hg2+, 10 μM) on photosynthesis was investigated in flagellates and aplanospores of Haematococcus lacustris. Hg2+ stress resulted in a fast decrease of chlorophyll fluorescence yield. This was initially caused by an increase in reversible non‐photochemical quenching of chlorophyll fluorescence. During further exposure to Hg2+, an increasing contribution of pH independent non‐photochemical quenching and a parallel rise in the content of the xanthophyll cycle pigment zeaxanthin was detected. An increase of the initial chlorophyll fluorescence as a final sign of Hg2+ induced adverse effects on photosynthesis supports our hypothesis that mercury ions predispose to non‐reversible, “chronic” photoinhibition.

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