Fast repetition rate and pulse amplitude modulation chlorophyll<i>a</i>fluorescence measurements for assessment of photosynthetic electron transport in marine phytoplankton

Abstract

Pulse amplitude modulation (PAM) and fast repetition rate (FRR) fluorescence are currently used to estimate photosynthetic quantum yields and photosynthetic rates in aquatic systems. Here we compare simultaneous measurements of the photochemical efficiency of photosystem II obtained from the two techniques and independent estimates of the rate of light absorption by photosystem II. We measured the light-dependencies of the photochemical efficiency of photosystem II (Fq′/Fm′) in five phytoplankters using FRR and Xe-PAM approaches. The FRR and PAM estimates were related in a non-linear fashion. At low irradiances, Fq′/Fm′ measured using PAM fluorescence exceeded Fq′/Fm′ measured using FRR fluorescence by about 20%. At high irradiances, measurements of Fq′/Fm′ from the two approaches converged. The differences in Fq′/Fm′ reflect the distinct techniques by which FRR and PAM protocols excite PSII and are amplified when estimating electron transfer rates as a result of the irradiance term. We also found that measurements of the effective light absorption cross-section for photosystem II obtained by FRR fluorescence compared well with estimates obtained from measured light absorption and photosynthetic unit size. Finally, we compared the photon efficiency of gross oxygen evolution from measurements of gross oxygen evolution and light absorption (ΦPO2 18) with FRR measurements of Fq′/Fm′. We found that measurements of Fq′/Fm′ were highly linearly correlated, but were lower by a factor of ∼1.5, than ΦPO2 18.