In-line recording of PAM fluorescence of phytoplankton cultures as a new tool for studying effects of fluctuating nutrient supply on photosynthesis

Turbidostat cultures of Dunaliella salina (Chlorophyceae) and Thalassiosira weissflogii (Bacillariophyceae) were grown at fluctuating concentrations o f nitrate. phosphate and silicate. In-Line measurements of PAM fluorescence were used to monitor the effects of fluctuating nutrient supply on the pho tochemical efficiency of photosystem II reaction centres of light-adapted c ells (DeltaF/F-m'). Besides the maximal photochemical efficiency of photosy stem II reaction centres of dark-adapted cells (F-v/F-m), chlorophyll a, pa rticulate organic carbon, nitrogen and phosphorus. and the cell number were measured frequently during the experiments. Following nutrient-replete gro wth, the cells were supplied with medium from which either nitrate, phospha te or silicate was omitted. When significant effects of nutrient starvation were indicated by the fluorescence parameters, a pulse of the deficient nu trient was added to the cultures. Our experimental set-up for in-line fluor escence measurements provided sensitive and reproducible detection of the v arious fluorescence signals, revealing strong influences of nutrient supply on the photochemical efficiency of photosystem II. In general the fluoresc ence values changed substantially within 1-30 min after re-addition of the deficient nutrient. Addition of phosphate and silicate induced an immediate characteristic decrease in fluorescence, whereas nitrate addition was char acterized by a strong, delayed increase in fluorescence. Complete recovery to pre-starvation fluorescence Values took about 48 h in all experiments. T he physiological background of nutrient uptake is used to explain the obser ved tight couplings between fluorescence responses and nutrient re-addition Our study clearly demonstrates that in-line fluorescence measurements prov ide a new tool for the investigation of phytoplankton reactions to fluctuat ing nutrients and offer the possibility to detect nutrient starvation in th e field.