| Abstract: | Gross oxygen evolution was compared with the electron transport rate (ETR), estimated from chl a fluorescence parameters on the common tropical green macro alga Ulva fasciata Delile with confirmatory carbon saturation curves from U. reticulata Forskål. Theoretically, the relationship between estimated ETR and gross oxygen evolution should be 4:1, that is, four electrons are transported through PSII for each molecule of oxygen evolved. However, deviations of the 4:1 relationship have previously been reported. Measurements were conducted with two commercially available and portable pulse amplitude modulated (PAM) chl fluorometers. We sought experimental approaches that minimize discrepancies between the two different measuring techniques of photosynthetic rates, both for in situ and laboratory conditions. Using fresh algal tissue for each of the different irradiances gave the best fit of gross oxygen evolution and ETR even at irradiances above light saturation, where large discrepancies between oxygen evolution and ETR are common. With increasing dissolved inorganic carbon (DIC) concentrations, there was a curvilinear response of gross oxygen evolution in relation to ETR. We therefore suggest to establish DIC saturation curves in the laboratory, oxygen evolution is probably the most relevant choice. Photorespiration could not readily explain a curvilinear response of O2 evolution and proportionally higher ETR at high irradiances. ETRs measured with the rapid light curve function of the PAM were compared with steady‐state rates of gross and net oxygen evolution, and the ETR was found to decrease at higher irradiances whereas oxygen evolution was constant. |
