The slow-phase of chlorophyll fluorescence induction curve reflects the electron transport rates of Photosystem II in vivo in Chlorella vulgaris

Ideally methods and parameters to assess photosynthesis in the microalga Chlorella vulgaris must be rapid, non-invasive, accurate, and simple to implement. Methods such as the Quantum Yield of photosynthesis (ϕ_Po or F_v/F_m) yield rapid results but do not reflect the actual electron turnover of PSII (ETR_II). Alternatively, methods that calculate ETR_II take several minutes to execute, require expensive instrumentation, and manual input. In this work, we describe a method to estimate ETR_II in real-time via predictive statistic modelling. This method utilises the Kautsky induction curve and a low-cost chlorophyll a fluorometer. Our model optimises the fit of linear regressions between the normalized fluorescence intensity after F_m (named S_ETR) and ETR_II measurements using traditional methods. This allows for an estimation of ETR_II in real-time through Kautsky induction curves alone. S_ETR can then be used as a faster alternative to quenching analysis to determine bioenergetics performance during steady state and can be assessed with any chlorophyll fluorometer capable of measuring the Kautsky induction curve. The S_ETR parameter allows for a high-throughput and high-resolution assessment of photosynthesis while being non-invasive, having lower production costs, and with lower technical requirements. Furthermore, the biophysical bases of the method are discussed.