Photosynthetic rates of Ulva (Chlorophyta) measured by pulse amplitude modulated (PAM) fluorometry

In this work, we attempt to quantify pulse amplitude modulated (PAM) chloro phyll fluorescence measurements in marine macroalgae in terms of photosynth etic rates. For this, the effective electron transfer quantum yield of phot osystem II measured for two Ulva species, at various irradiances and inorga nic carbon (Ci) concentrations, was multiplied by the estimated flux of pho tons absorbed by the photosynthetic pigments associated with this photosyst em. The rates of electron transport (ETR) calculated in this way were then compared with rates of photosynthetic O-2 evolution as measured in associat ion with the fluorescence measurements. It was found that the calculated ET Rs correlated linearly with rates of 'gross' O-2 evolution (net O-2 exchang e corrected for dark respiration as measured immediately after turning off each irradiance level) within the range of irradiances applied (up to 608 m u mol photons m(-2) s(-1)). The average molar O-2/ETR ratio was 0.238 for U lva lactuca and 0.261 for Ulva fasciata, which is close to the theoretical maximal value of 0.25. Rates of O-2 evolution at various concentrations of Ci also showed linear correlations with ETR, and the average molar O-2/ETR ratio was 0.249. These results show that PAM fluorometry can be used as a p ractical tool for quantifying photosynthetic rates at least under moderate irradiances in thin-bladed macroalgae such as Ulva possessing a CO2-concent rating system. A comparison between the PAM-101 (which was used in Sweden f or the light- and Ci-response measurements of Ulva lactuca) and the newly d eveloped portable Diving PAM (used for Ulva fasciata in Israel) showed that such fluorescence-based photosynthetic rate measurements can also be carri ed out in situ.