Improving the Accuracy of Single Turnover Active Fluorometry (STAF) for the Estimation of Phytoplankton Primary Productivity (PhytoPP)

Tobias G Boatman,Richard J Geider,Kevin Oxborough

doi:10.3389/fmars.2019.00319

Tobias G Boatman, Richard J Geider + Show 1 more

Open Access

https://doi.org/10.3389/fmars.2019.00319

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Abstract

Photosystem II (PSII) photochemistry is the ultimate source of reducing power for phytoplankton primary productivity (PhytoPP). Single turnover active chlorophyll fluorometry (STAF) provides a non-intrusive method that has the potential to measure PhytoPP on much wider spatiotemporal scales than is possible with more direct methods such as 14C fixation and O2 evolved through water oxidation. Application of a STAF-derived absorption coefficient for PSII light-harvesting (aLHII) provides a method for estimating PSII photochemical flux on a unit volume basis (JVPII). Within this study, we assess potential errors in the calculation of JVPII arising from sources other than photochemically active PSII complexes (baseline fluorescence) and the package effect. Although our data show that such errors can be significant, we identify fluorescence-based correction procedures that can be used to minimize their impact. For baseline fluorescence, the correction incorporates an assumed consensus PSII photochemical efficiency for dark-adapted material. The error generated by the package effect can be minimized through the ratio of variable fluorescence measured within narrow wavebands centered at 730 nm, where the re-absorption of PSII fluorescence emission is minimal, and at 680 nm, where re-absorption of PSII fluorescence emission is maximal. We conclude that, with incorporation of these corrective steps, STAF can provide a reliable estimate of JVPII and, if used in conjunction with simultaneous satellite measurements of ocean color, could take us significantly closer to achieving the objective of obtaining reliable autonomous estimates of PhytoPP.

Highlights

Phytoplankton contribute approximately half the photosynthesis on the planet (Field et al, 1998), forming the base of marine food webs
The absorption method is insensitive to Stern–Volmer quenching while baseline fluorescence can introduce a significant error in the calculation of JVPII
In the context of these tests, the lower values of both sample-specific Ka and Fv/Fm values observed within the N-limited cultures, when compared to the nutrient-replete values, are entirely consistent with a baseline fluorescence-induced error being introduced by, for example, the accumulation of photoinactivated Photosystem II (PSII) complexes and/or other energetically uncoupled complexes

Summary

Introduction

Phytoplankton contribute approximately half the photosynthesis on the planet (Field et al, 1998), forming the base of marine food webs. Reliable assessment of Phytoplankton Primary Productivity (PhytoPP) is crucial to an understanding of the global carbon and oxygen cycles and oceanic ecosystem function. Minimizing STAF Errors spatial scales from meters to ocean basins, and time scales from minutes to tens of years. This poses significant challenges for measuring and monitoring PhytoPP. Despite the widespread use of the 14C method, which has led to measurements of PhytoPP by the 14C method providing the database against which remote sensing estimates of primary production are calibrated (Bouman et al, 2018), there is considerable uncertainty in what exactly the 14C method measures and the accuracy of bottle-incubation based methods for obtaining PhytoPP in oligotrophic ocean waters (Quay et al, 2010)

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