Limitations in the use of PAM fluorometry for measuring photosynthetic rates of macroalgae at high irradiances

Pulse amplitude modulated (PAM) fluorometry can be used for measuring photosynthetic electron transport rates (ETR) of marine angiosperms and macroalgae both in the laboratory and in situ. Regarding macroalgae, quantitative values and linear correlations between ETR and rates of photosynthetic O 2 evolution have so far been shown only for a few species under low irradiances. As a logical continuation of such work, the aim of the present study was to (a) assess to what degree high irradiances would limit such measurements and (b) evaluate whether PAM fluorometry could be used quantitatively also for other marine macroalgae from different phyla. This was done by comparing ETR with rates of gross O 2 evolution (net O 2 exhcange corrected for dark respiration) at various irradiances for the green alga Ulva lactuca grown at two irradiances, the brown algae Fucus serratus and Laminaria saccharina and the red algae Palmaria palmata and Porphyra umbilicalis. At low irradiances, there was a clear positive correlation between O 2 evolution and fluorescence-based ETR. At high irradiances, however, all algae featured an apparent decrease in ETR while O 2 evolution remained relatively constant, and this resulted in markedly increasing O 2 /ETR ratios. This anomaly could be nicely illustrated in plots of O 2 /ETR as a function of the effective quantum yield of photosystem II (Y). Such plots showed that the O 2 /ETR ratio generally started to increase when Y reached a critical low value of c. 0.1. It was further found that the irradiance at which this value was reached varied with species and previous light histories. Thus, it is the Y value, rather than the irradiance per se