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The present paper compares light-response patterns of electron transport rate (ETR) measured by imaging and conventional pulse-amplitude-modulated fluorescence (PAM) with that of gross O2 evolution in naturally thin-leaved (Ulva lactuca) and thick-leaved (Fucus serratus) algae, as well as in 1 to 8 cell layer thick 'artificial thalli' built from layers of Enteromorpha intestinalis. At sub-saturating and saturating irradiances imaging ETR/ETRmax provided a very accurate reflection of P/Pmax, which was different in thin and thick algae. There was no effect of tissue thickness on con- ventional ETR/ETRmax, which was unrelated to P/Pmax at low irradiances. At super-saturating irradi- ances, imaging ETR/ETRmax was reduced significantly compared to P/Pmax, which instead corre- sponded to conventional ETR/ETRmax. Imaging Fm was only related to thallus thicknesses from 1 to 4 cell layers but not beyond. Photoinhibition expressed by imaging PAM in thick thallus may have been compensated for by photosynthesis in deeper cells. Conventional Fm was linearly related to thallus thickness from 1 to 8 cell layers. No photoinhibition was indicated at super-saturating irradi- ances by conventional PAM and O2 evolution measurements that integrate the photosynthetic perfor- mance of the entire thallus of thick algae. At irradiances 0.25 and increased further at super-saturating irradiances, in partic- ular in thin algae. In conclusion, the type of PAM instrument may influence the light response of ETR. Likewise, the thallus thickness may affect the relationship between O2 evolution and ETR. ETR recordings should be treated with caution and regarded as a compliment to O2 measurements, particularly when thick-leaved macroalgae are studied.

Evaluation of imaging and conventional PAM as a measure of photosynthesis in thin- and thick-leaved marine macroalgae