Phytoplankton primary productivity: A dual-incubation approach for direct comparison of photosystem II photosynthetic flux (JVPII) and 14C-fixation experiments

Singe-turnover active chlorophyll a fluorometry (STAF) can be used to assess phytoplankton photosynthetic rates in terms of the photosystem II photochemical flux (JV_PII, μmol e⁻ m⁻³ s⁻¹) instantaneously, autonomously, and at high resolution. While JV_PII provides an upper limit to rates of phytoplankton primary productivity in units of carbon fixation, the conversion between these two rates is variable, limiting our ability to utilize high-resolution JV_PII data to monitor phytoplankton primary productivity. Simultaneous measurements of JV_PII and ¹⁴C-fixation help in understanding the factors controlling the variable ratio between the two rates. However, to date, methodological inconsistencies, including differences in incubation lengths and light quality, have greatly inhibited practical assessment of such electron to carbon ratios (Φ_e,C, mol e⁻ mol C⁻¹). We here present data from a range of dual-incubation experiments in northeast Atlantic waters during which JV_PII and ¹⁴C-fixation were measured simultaneously on the same sample. Time-course experiments show how Φ_e,C increases with incubation length, likely reflecting the transition from gross to net ¹⁴C-fixation. Dual-incubation experiments conducted under different light levels show a tendency for increased Φ_e,C under (super-)saturating light. Finally, data from a diurnal experiment demonstrate how Φ_e,C increases over the course of a day, due to downregulation of ¹⁴C-fixation. We provide a detailed description of our methodological approach, including a critical discussion of improvements to the calculation of JV_PII implemented in the LabSTAF instrument used for active fluorescence measurements and the limitations of the well-established ¹⁴C-fixation approach.