Photosynthetic efficiency in Polish and Australian Raphidiopsis raciborskii strains

Raphidiopsis raciborskii, as a potentially toxic, invasive cyanobacteria has attracted great attention. Worldwide distribution could have contributed to its phenotypic plasticity and even the formation of ecotypes adapted to the local environment. To determine mechanisms responsible for the omnipresence of R. raciborskii, photosynthetic efficiency in several strains differing in origin and toxicity has been investigated. Three non-toxic Polish strains and two toxic Australian strains were grown under standardized light and temperature conditions. Concentration of photosynthetic pigments, qualitative and quantitative analysis of carotenoid composition, and photosynthetic performance (chlorophyll fluorescence in vivo) were analysed. No significant differences between strains were observed in the photosystem II quantum efficiency (Fv/Fm), the photochemical (qP) and non-photochemical quenching (NPQ) and dark and light levels of plastoquinone reduction. Nonetheless, a positive correlation between Fv/Fm and overall NPQ was found. The phycocyanin concentration was negatively correlated with the carotenoid concentration in all five examined strains. Furthermore, in four strains roughly one third of pigments consisted of myxoxanthophylls. The ability to accumulate pigments involved both in light harvesting and photoprotection may contribute to the optimization of photosynthetic efficiency of investigated R. raciborskii strains in different environmental conditions. Documented parameters could not be ascribed to the difference in growth conditions, but could result from variations in their genetics

Elevated Growth Temperature Can Enhance Photosystem I Trimer Formation and Affects Xanthophyll Biosynthesis in Cyanobacterium Synechocystis sp. PCC6803 Cells.

In the thylakoid membranes of the mesophilic cyanobacterium Synechocystis PCC6803, PSI reaction centers (RCs) are organized as monomers and trimers. PsaL, a 16 kDa hydrophobic protein, a subunit of the PSI RC, was previously identified as crucial for the formation of PSI trimers. In this work, the physiological effects accompanied by PSI oligomerization were studied using a PsaL-deficient mutant (DeltapsaL), not able to form PSI trimers, grown at various temperatures. We demonstrate that in wild-type Synechocystis, the monomer to trimer ratio depends on the growth temperature. The inactivation of the psaL gene in Synechocystis grown phototropically at 30 degrees C induces profound morphological changes, including the accumulation of glycogen granules localized in the cytoplasm, resulting in the separation of particular thylakoid layers. The carotenoid composition in DeltapsaL shows that PSI monomerization leads to an increased accumulation of myxoxantophyll, zeaxanthin and echinenone irrespective of the temperature conditions. These xanthophylls are formed at the expense of beta-carotene. The measured H2O-->CO2 oxygen evolution rates in the DeltapsaL mutant are higher than those observed in the wild type, irrespective of the growth temperature. Moreover, circular dichroism spectroscopy in the visible range reveals that a peak attributable to long-wavelength-absorbing carotenoids is apparently enhanced in the trimer-accumulating wild-type cells. These results suggest that specific carotenoids are accompanied by the accumulation of PSI oligomers and play a role in the formation of PSI oligomer structure