The compounds of chloroplastic and mitochondrial electron transport chain play an important role in the regulation of cell redox homeostasis and signalling, but under the stress conditions can be the origin of reactive oxygen species. The confirmation of algal cultures performed in different organic carbon supply and presence or absence of light energy led to estimate the participation of chloroplast and mitochondrion in the induction of oxidative stress defence mechanism(s), while usage of the photosystems deleted mutants allowed to estimate the involvement of PS I and PS II in ROS generation.

Three strains of Desmodesmus obliquus: wild type 276-6, PS II deleted mutant 56.80. and PS I deleted mutant 57.80. were cultivated under continuous light (24 Wm^-2) and dark conditions on mineral BBM, yeast extract enriched BBM and yeast extract medium. In algal cells grown under photoautotrophic, mixotrophic and heterotrophic conditions photosynthetic activity characterized by chlorophyll a fluorescence measurements (PAM, OJIP) as well as superoxide dismutase isoforms activities and profiles were analysed. The mixotrophy occurred to be the most favourable conditions for algal growth, but simultaneously the highest SOD activities were observed in algal cells. Under heterotrophic conditions cells growth was only slightly lower, with unchanged michondrion and chloroplast Mn- and Fe-SOD isoforms activities and profiles. The inability of photosystems deleted mutants to grow under photoautotrophic conditions, 3 to 6 times higher activities of Fe- and Cu/Zn-SOD as well as marked increase of qNP value indicates chloroplast as the main source of ROS in Desmodesmus cells, where any interruption in either PS I or PS II function results in oxidative stress mediated cell damage.

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