Direct impact of silicate on the photosynthetic performance of the diatom Thalassiosira weissflogii assessed by on- and off-line PAM fluorescence measurements

Pulse-amplitude-modulation (PAM) fluorometry was used to investigate the ef fects of varying the silicate concentration on different fluorescence chara cteristics of batch and chemostat cultures of the diatom Thalassiosira weis sflogii. The fluorescence signals, measured both on- and offline, were used to calculate the actual (phi(p)) and potential (phi(P0)) photochemical eff iciencies of the reaction centres of photosystem II (PSII). Also the relati ve electron transport rate of the reaction centres of PSII (J(e)) was calcu lated. Fluorescence decreased in silicate-limited cells and increased rapid ly after silicate addition. The decrease was caused by strong nan-photochem ical quenching (q(N)) in silicate-limited cells. Continuous recording (on-l ine) of the minimum and maximum fluorescence provided data with high tempor al resolution revealing that first recovery reactions of silicate-limited c ells occurred 20 min after the addition of silicate. Based on these observa tions, we assume a strong influence of silicate metabolism on the photosynt hetic efficiency of the reaction centres of PSII assessed by PAM fluorescen ce. This implies that silicate, as well as other nutrients, has to be consi dered as a possible cause for variable photosynthetic efficiency of diatoms .