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

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 O-2 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/P-max, which was different in thin and thick algae. There was no effect of tissue thickness on conventional ETR/ETRmax, which was unrelated to P/P-max, at low irradiances. At super-saturating irradiances, imaging ETR/ETRmax, was reduced significantly compared to P/P-max, which instead corresponded to conventional ETR/ETRmax. Imaging F-m 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 F-m was linearly related to thallus thickness from 1 to 8 cell layers. No photoinhibition was indicated at super-saturating irradiances by conventional PAM and O-2 evolution measurements that integrate the photosynthetic performance of the entire thallus of thick algae. At irradiances < 80 mu mol m(-2) S-1, the O-2/conventional ETR ratio was close to 0.25, whereas it decreased at higher irradiances in both thin and thick macroalgae. The O-2/imaging ETR ratio was > 0.25 and increased further at super-saturating irradiances, in particular 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 O-2 evolution and ETR. ETR recordings should be treated with caution and regarded as a compliment to O-2 measurements, particularly when thick-leaved macroalgae are studied.