Photoacclimation in Dunaliella tertiolecta reveals a unique NPQ pattern upon exposure to irradiance

Highly time-resolved photoacclimation patterns of the chlorophyte microalga Dunaliella tertiolecta during exposure to an off–on–off (block) light pattern of saturating photon flux, and to a regime of consecutive increasing light intensities are presented. Non-photochemical quenching (NPQ) mechanisms unexpectedly responded with an initial decrease during dark–light transitions. NPQ values started to rise after light exposure of approximately 4 min. State-transitions, measured as a change of PSII:PSI fluorescence emission at 77 K, did not contribute to early NPQ oscillations. Addition of the uncoupler CCCP, however, caused a rapid increase in fluorescence and showed the significance of qE for NPQ. Partitioning of the quantum efficiencies showed that constitutive NPQ was (a) higher than qE-driven NPQ and (b) responded to light treatment within seconds, suggesting an active role of constitutive NPQ in variable energy dissipation, although it is thought to contribute statically to NPQ. The PSII connectivity parameter p correlated well with F′, Fm′ and NPQ during the early phase of the dark–light transients in sub-saturating light, suggesting a plastic energy distribution pattern within energetically connected PSII centres. In consecutive increasing photon flux experiments, correlations were weaker during the second light increment. Changes in connectivity can present an early photoresponse that are reflected in fluorescence signals and NPQ and might be responsive to the short-term acclimation state, and/or to the actinic photon flux

Toward autonomous measurements of photosynthetic electron transport rates: An evaluation of active fluorescence-based measurements of photochemistry

This study presents a methods evaluation and intercalibration of active fluorescence-based measurements of the quantum yield (ϕ'ₚₛᵢᵢ) and absorption coefficient (aₚₛᵢᵢ) of photosystem II (PSII) photochemistry. Measurements of ϕ'ₚₛᵢᵢ, aₚₛᵢᵢ, and irradiance (E) can be scaled to derive photosynthetic electron transport rates (Pₑ), the process that fuels phytoplankton carbon fixation and growth. Bio-optical estimates of ϕ'ₚₛᵢᵢ and aₚₛᵢᵢ were evaluated using 10 phytoplankton cultures across different pigment groups with varying bio-optical absorption characteristics on six different fast-repetition rate fluorometers that span two different manufacturers and four different models. Culture measurements of ϕ'ₚₛᵢᵢ and the effective absorption cross section of PSII photochemistry (σₚₛᵢᵢ, a constituent of aₚₛᵢᵢ) showed a high degree of correspondence across instruments, although some instrument-specific biases are identified. A range of approaches have been used in the literature to estimate aₚₛᵢᵢ(λ) and are evaluated here.With the exception of ex situ aₚₛᵢᵢ(λ) estimates from paired σₚₛᵢᵢ and PSII reaction center concentration ([RCII]) measurements, the accuracy and precision of in situ aₚₛᵢᵢ(λ) methodologies are largely determined by the variance of method-specific coefficients. The accuracy and precision of these coefficients are evaluated, compared to literature data, and discussed within a framework of autonomous Pₑ measurements. This study supports the application of an instrument-specific calibration coefficient (KR) that scales minimum fluorescence in the dark (F₀) to aₚₛᵢᵢ as both the most accurate in situ measurement of aₚₛᵢᵢ, and the methodology best suited for highly resolved autonomous Pₑ measurements