A comparison of photosynthetic electron transport rates in macroalgae measured by pulse amplitude modulated chlorophyll fluorometry and mass spectrometry

The relationship between whole chain photosynthetic electron transport and PSII activity was investigated in Parphyra columbina (Montagne) (Rhodophyta ), Uva australis (Areschoug) (Chlorophyta), and Zonaria crenata (J. Agardh) (Phaeophyta). Mass spectrometric measurements of gross O-2 evolution and g ross O-2 uptake were combined with simultaneous measurement of pulse-modula ted chl fluorescence under a range of irradiances and inorganic carbon (C-i ) concentrations. At light-limiting irradiance, a good correlation between gross O-2 evolution and the electron transport rate (ETR) calculated from c hl fluorescence ((F-m' - F-s)/ F-m') was found in the optically thin specie s (Ulva and Porphyra). The calculated ETR was equivalent to the theoretical electron requirement in these species but overestimated gross O-2 evolutio n in the thicker species Zonaria. In saturating light, especially when C-i availability was low, ETR overestimated gross O-2 evolution in all species. Excess electron flow could not be accounted for by an increase in gross O- 2 uptake; thus neither Mehler-ascorbate-peroxidase reaction nor the photosy nthetic carbon oxidation cycle were enhanced at high irradiance or low C-i. Alternative explanations for the loss of correlation include cyclic electr on flow around PSII that may be engaged under these conditions or nonphotoc hemical energy quenching within PSII centers. The loss of correlation betwe en ETR and linear photosynthetic electron flow as irradiance increased from limiting to saturating or at low C-i availability and in the case of optic ally thick thalli limits the application of this technique for measuring ph otosynthesis in macroalgae.