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 effects of varying the silicate concentration on different fluorescence characteristics of batch and chemostat cultures of the diatom Thalassiosira weissflogii. The fluorescence signals, measured both on-and offline, were used to calculate the actual ( P ) and potential ( P0 ) photochemical efficiencies of the reaction centres of photosystem II (PSII). Also the relative electron transport rate of the reaction centres of PSII (J e ) was calculated. Fluorescence decreased in silicate-limited cells and increased rapidly after silicate addition. The decrease was caused by strong non-photochemical quenching (q N ) in silicate-limited cells. Continuous recording (on-line) of the minimum and maximum fluorescence provided data with high temporal resolution, revealing that first recovery reactions of silicate-limited cells occurred 20 min after the addition of silicate. Based on these observations, we assume a strong influence of silicate metabolism on the photosynthetic efficiency of the reaction centres of PSII assessed by PAM fluorescence. This implies that silicate, as well as other nutrients, has to be considered as a possible cause for variable photosynthetic efficiency of diatoms.