Electron Transport Rate Efficiency Research Articles

Photoacclimation of cultured strains of the cyanobacteriumMicrocystis aeruginosato high-light and low-light conditions

The cyanobacterium Microcystis aeruginosa forms blooms that can consist of colonies. We have investigated how M.aeruginosa acclimatizes to changing light conditions such as can occur during blooms. Three different strains were exposed to two irradiance levels: lower (LL) and higher (HL) than the irradiance-onset saturation parameter. We measured the photosynthetic pigment concentrations, PSII photochemical efficiency, electron transport rate (ETR), irradiance-saturated ETR and ETR efficiency. The relationship between ETR and photosynthetic oxygen production and the excess in PSII capacity were also studied for one strain. Higher values of chlorophyll a and phycocyanin and lower values of total carotenoids were found under LL conditions in the three strains. The strains showed clear differences in the irradiance-saturated ETR and in ETR efficiency under both LL and HL treatments. No differences were found in the linear relationship between ETR and photosynthetic oxygen production under both irradiance treatments. LL-acclimated cells showed higher PSII excess capacity than HL ones, possibly because their higher pigment content could result in a higher light stress than HL cells when forming surface blooms. The fact that the genetically different strains show different photosynthetic physiologies suggests that the very dynamic light climate observed in lakes may allow their coexistence.

Photoinhibition and photosynthetic pigment reorganisation dynamics in light/darkness cycles as photoprotective mechanisms of <em>Porphyra umbilicalis</em> against damaging effects of UV radiation

Porphyra umbilicalis L. Kutzing collected from the upper intertidal zone at Helgoland, North Sea, was exposed to different spectral ranges of UV radiation under both 12/12 h light/dark cycles and continuous irradiation. In light/dark cycles, oscillations of the optimal quantum yield (Fv /Fm) were observed during the experiments, reaching maximal values at the end of the light phase followed by lower values during the dark phase. Decreased Fv /Fm was observed in thalli illuminated with photosynthetic active radiation (PAR) plus UV-A and PAR+UV-A+UV-B, compared with the PAR control, indicating a certain degree of UV-induced photoinhibition. In addition, a decrease in the percentage of change of the linear initial slope and maximum electron transport rate (ETR) estimated from ETR vs. irradiance curves was induced by UV radiation during the light phase. Recovery during the 12 h dark phase was almost completed in UV-A treated plants. PAR+UV-A seemed not to affect the photosynthesis, measured as O2 production. However, a decrease in O2 production was observed in the PAR+UV-A+UV-B treatment, but it recovered to initial values after 48 h of culture. No changes in total content of photosynthetic pigments were observed. However, thallus absorptance and the in vivo absorption cross-section in the PAR range (400-700 nm) normalised to Chl a (a* parameter) fluctuated during light/dark cycles and were positively correlated with changes in the optimum quantum yield, thus indicating that daily pigment reorganisation in the light-harvesting complex may play a key role in the photosynthetic performance of the algae. Both UV-A and UV-B treatments under continuous irradiation induced a significant reduction in the optimal quantum yield, ETR efficiency and photosynthetic oxygen production during the first 36 h to values around 30% of the initial ones. Thus, different protective mechanisms against UV stress can be observed in P. umbilicalis: dynamic photoinhibition when UVA is combined with PAR, followed by full recovery of photosynthesis during the dark phase, and a more pronounced photoinhibition under UV-B, with only partial recovery after longer time periods, in which photosynthetic pigment reorganisation plays an important role.