Photosynthetic efficiency of Microcystis ssp. under salt stress

Microcystis is a genus of unicellular blue-green algae that is toxic and can be troublesome in freshwater, especially where organic material is abundant. The aim of this work was to analyse photosystem I (PSI) and photosystem II (PSII) photochemistry of Microcystis spp. colonies under salt stress by the use of comprehensive chlorophyll a fluorescence signals measurements (prompt, delayed and modulated 820 nm reflection-MR 820). The cyanobacteria samples were taken from a natural aquatic environment. The following salt solutions were applied with 10 days intervals: 0.00 (control), 0.05, 0.10, 0.20, 0.40, 0.80, and 1.00 M. Data was collected three times (10, 20 and 30 days after treatment), prior to increasing the salt concentration in the solution. We found that the salt stress inhibited electron transfer from the PQ pool to PSI (P700(+)) and the inhibition of reoxidation of the plastoquinone molecule tightly bound to PSII (QA). An inhibition of quantum yields of photoinduced electron transport in PSII reaction center to QA and a reduction of the fast phase of photoinduced kinetics of the MR was also noted under this stress. We recommend the combined analyses of the prompt and delayed chlorophyll fluorescence along with MR820 to be used as a novel qualitative method for studying photosynthetic machinery reactions not only in higher plants, but also in cyanobacteria.