Non-photochemical quenching (NPQ) of chlorophyll a fluorescence (ChlF): an undervalued optical indicator of photosynthetic energy conversion efficiency

Light energy absorbed by the pigments of PSII must follow one of three complementary pathways: charge separation (photosynthesis), fluorescence (ChlF), or heat dissipation (including NPQ). Under conditions of excess absorbed light, the upregulation of NPQ reduces ChlF. This effect complicates the use of in situ ChlF as a biomass proxy, and the calculation of electron transport rates from induced ChlF approaches such as fast repetition rate fluorometry (FRRF). Here, we argue that estimates of NPQ in natural phytoplankton assemblages holds undervalued potential as an indicator for energy conversion efficiency, an important physiological input parameter for many models of primary productivity.

We present data from multiple oceanic regions showing that NPQ correlates to the conversion factor needed to estimate carbon-based productivity from FRRF-derived electron transport rates. Our results also show that NPQ correlates with the photosynthetic efficiency, Φ_C, which quantifies carbon fixation per quanta absorbed and is a crucial parameter in absorption-based productivity algorithms.

NPQ is an optical signal amiable to quantification in multiple approaches. While NPQ induction dynamics can be most precisely studied with active fluorometry (eg FRRF), the realized NPQ signal can also be estimated from the decoupling of continuous measurements of ChlF (quenched) and absorption line height at 680 nm (not quenched), or from the extent of ChlF quenching relative to incident PAR over diurnal cycles and with depth. Data can thus be collected from ship-based optical sensors connected to underway water supplies or moorings, floats and gliders, and interpreted in the context of physiological status of the resident phytoplankton community. It may also be possible to derive estimates of NPQ from remote sensing of ocean color, in an approach analogous to the photochemical reflectance index used in terrestrial primary productivity algorithms.