Effects of salinity stress on photosystem II function in cyanobacterial Spirulina platensis cells

The changes in PSII photochemistry in Spirulina platensis cells exposed to salinity stress (0-0.8 M NaCl) for 12 h were studied. Salinity stress induced a decrease in oxygen evolution activity, which correlated with the decrease in the quantum yield of PSII electron transport (Φ_PSII). Phycocyanin content decreased significantly while chlorophyll content remained unchanged in salt-stressed cells. Salinity stress induced an increase in non-photochemical quenching (q_N) and a decrease in photochemical quenching (q_P). Analyses of the polyphasic fluorescence transients (OJIP) showed that with the increase in salt concentration, the fluorescence yield at the phases J, I and P declined sharply and the transient almost levelled off at salt concentration of 0.8 M NaCl. The effects of DCMU on the polyphasic rise of fluorescence transients decreased significantly. Salinity stress resulted in a decrease in the efficiency of electron transfer from Q_A^- to Q_B. The slope at the origin of the relative variable fluorescence curves (dV/dt_O) and the relative variable fluorescence at phase J (V_J) increased in the absence of DCMU, but decreased in the presence of DCMU. The shape of the relative variable fluorescence transients in salt-stressed cells was comparable to that of the control cells incubated with DCMU. The results in this study suggest that salt stress inhibited the electron transport at both donor and acceptor sides of PSII, resulted in damage to phycobilisome and shifted the distribution of excitation energy in favour of PSI.