Bio-optical modelling of oxygen evolution using in vivo fluorescence: Comparison of measured and calculated photosynthesis/irradiance (P-I) curves in four representative phytoplankton species

The measurement of variable fluorescence yields relative electron transport rates which can not directly be converted into absolute values of photosynthetic rates. We have developed a measuring device which provides identical optical geometry for simultaneous fluorescence and oxygen measurements in single cell or chloroplast suspensions and also allows the determination of the absorbed photosynthetic radiation (Qphar). In this set-up we have simultaneously measured photosynthesis-irradiance curves (P-I curves) of algal cell suspensions with both PAM fluorometry and a Clark-type electrode. From the electron flow through photosystem II per absorbed light quanta (Qphar) we have modeled oxygen based P-I curves. In order to evaluate the validity of the model we have compared modeled with directly measured oxygen based P-I curves. This comparison was extended to four different algal taxa which exhibit significantly different absorption spectra leading to changes in Qphar. The results show that in the initial slope of the P-I curves (α), calculated and measured oxygen production is quite similar on an absolute scale, whereas at the transition part (lk = Pmax/α) and at maximal photosynthetic rates (Pmax) both methods can deviate significantly from each other depending on the species used. Therefore, the conversion of fluorescence based electron transfer rates through photosystem II into oxygen production is a reliable procedure for light intensities below the lk. However, at light intensities close to light saturation, physiological regulation processes can cause problems in the interconversion of fluorescence- and oxygen-based photosynthetic rates.