Photosystem II electron transport rates and oxygen production in natural waterblooms of freshwater cyanobacteria during a diel cycle

The relationship between electron transport rate through PSII and photosynthetic oxygen evolution in cyanobacterial surface waterblooms was followed over a diel cycle. Chlorophyll fluorescence and photosynthetic oxygen evolution (PSOE) measurements were performed in a small-volume incubation chamber an samples taken from a fish pond, Measurement of light-response curves showed a close to linear relationship between electron transport rates (ETR) and PSOE up to irradiancies of 800 pmol quanta m(-2) s(-1), except during mid-morning conditions. At higher irradiances, the relationship was non-linear. The regression coefficient kappa (= PSOE/ETR) exhibited wide variation during the day (3.8-9.2), indicating that the use of ETR as a measure of PSOE in cyanobacterial waterblooms should be approached with caution. The involvement of alternate oxygen-consuming electron transfer pathways is discussed as a possible explanation for this discrepancy.