Direct estimation of functional PSII reaction center concentration and PSII electron flux on a volume basis: a new approach to the analysis of Fast Repetition Rate fluorometry (FRRf) data

<jats:p>Phytoplankton primary productivity is most commonly measured by <jats:sup>14</jats:sup>C assimilation although less direct methods, such as O<jats:sub>2</jats:sub> exchange, have also been employed. These methods are invasive, requiring bottle incubation for up to 24 h. As an alternative, Fast Repetition Rate fluorometry (FRRf) has been used, on wide temporal and spatial scales within aquatic systems, to estimate photosystem II (PSII) electron flux per unit volume (<jats:italic>JV</jats:italic><jats:sub>PSII</jats:sub>), which generally correlates well with photosynthetic O<jats:sub>2</jats:sub> evolution. A major limitation of using FRRf arises from the need to employ an independent method to determine the concentration of functional photosystem II reaction centers ([RCII]); a requirement that has prevented FRR fluorometers being used, as stand‐alone instruments, for the estimation of electron transport. Within this study, we have taken a new approach to the analysis of FRRf data, based on a simple hypothesis; that under a given set of environmental conditions, the ratio of rate constants for RCII fluorescence emission and photochemistry falls within a narrow range, for all groups of phytoplankton. We present a simple equation, derived from the established FRRf algorithm, for determining [RCII] from dark FRRf data alone. We also describe an entirely new algorithm for estimating <jats:italic>JV</jats:italic><jats:sub>PSII</jats:sub>, which does not require determination of [RCII] and is valid for a heterogeneous model of connectivity among RCIIs. Empirical supporting evidence is presented. These data are derived from FRR measurements across a diverse range of microalgae, in parallel with independent measurements of [RCII]. Possible sources of error, particularly under nutrient stress conditions, are discussed.</jats:p>