The Science Behind Chelsea Technologies: Excerpts & Updates

We probably don't talk about the science behind Chelsea Technologies as much as we might, but academia isn't short of research papers that reference our science, scientists and apparatus, and this small overview highlights some of the recent publications relevant to our technology

We probably don’t talk about the science behind Chelsea Technologies as much as we might, but academia isn’t short of research papers that reference our science, scientists and apparatus, and this small overview highlights some of the recent publications relevant to our technology.

Paper: Single-Turnover Variable Chlorophyll Fluorescence as a Tool for Assessing Phytoplankton Photosynthesis and Primary Productivity: Opportunities, Caveats and Recommendations

In this paper, the authors assist the existing and upcoming user community by providing an overview of current approaches and consensus recommendations for the use of ST-ChlF measurements to examine in-situ phytoplankton productivity and photo-physiology. The authors argue that a consistency of practice and adherence to basic operational and quality control standards is critical to ensuring data inter-comparability.

Large datasets of inter-comparable and globally coherent ST-ChlF observations hold the potential to reveal large-scale patterns and trends in phytoplankton photo-physiology, photosynthetic rates and bottom-up controls on primary productivity. As such, they hold great potential to provide invaluable physiological observations on the scales relevant for the development and validation of ecosystem models and remote sensing algorithms.

Single-turnover variable chlorophyll fluorescence methods provide a powerful tool for high resolution photo-physiological measurements, with significant potential to examine aquatic productivity and its environmental controls over a range of spatial and temporal scales. Recent advances in instrumentation and data analysis are now beginning to significantly expand the application of ST-ChlF methods to a range of research questions. As the field continues to expand, it is essential to promote global coordination in the development of best practice, using flexible, open-source tools to disseminate information, software, and data products. Through the application of consensus recommendations and a robust system of documenting user-specific protocols, inter-comparison among emerging datasets will be greatly facilitated. This, in turn, will enable the synthesis of synoptic ST-ChlF observations at global scales, providing new insights into the response of marine productivity to a range of perturbations.

The UN Decade of Ocean Science for Sustainable Development (2021–2030) alongside the UN Sustainable Development Goals 6 and 14 (dealing with clean and productive inland and marine waters, respectively) will provide the opportunity to revolutionize the collection, storage, and analysis of ocean data, leading to better understanding of global-scale patterns in key ocean properties and their response to various environmental factors. ST-ChlF-derived observations are important supplements to existing observations that represent phytoplankton standing stocks.

Current advances in automation of measurements will enable data compilations at unprecedented resolution in both time and space. Such data will provide information on physical-biological coupling, including the impacts of localized hydrographic fronts, river plumes, and glacial discharge. At larger scales, regional patterns of phytoplankton physiology can be examined in relation to climate forcing, providing empirical correlations and mechanistic understanding for the improvement of ecosystem models, and remote sensing algorithms. Through concerted international cooperation, we are confident that the expansion of ST-ChlF measurements will significantly advance our understanding of global aquatic ecosystems.

Link to paper

Paper: Improving the accuracy of single turnover active fluorometry (STAF) for the estimation of phytoplankton primary productivity (PhytoPP)

Much of the science behind Chelsea Technologies is involved with Phytoplankton. Phytoplankton photosynthesis is responsible for approximately half of the carbon fixed on the planet. As a process, photosynthesis is responsive to variability in multiple environmental drivers including light, temperature and nutrients across spatial scales from meters to ocean basins, and time scales from minutes to tens of years.

This poses significant challenges for measurement and monitoring. While direct measurement of the carbon fixed by photosynthesis can only be applied on very limited spatial and temporal scales, active chlorophyll fluorescence has enormous potential for the accurate measurement of phytoplankton photochemistry, which provides the reducing power for carbon fixation, on much wider spatiotemporal scales and with much lower operational costs.

This study identifies practical measures that can be taken to improve the accuracy of such measurements. We are confident that these measures will have minimal impact on the frequency at which phytoplankton photochemistry is assessed and that they will be suitable for application on autonomous measurement platforms.

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, the authors assess potential errors in the calculation of JVPII arising from sources other than photochemically active PSII complexes (baseline fluorescence) and the package effect. Although the data show that such errors can be significant, the authors 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. the authors 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.

Link to paper

Paper: Roadmaps and Detours: Active Chlorophyll-a Assessments of Primary Productivity Across Marine and Freshwater Systems

Assessing phytoplankton productivity over space and time remains a core goal for oceanographers and limnologists. Fast Repetition Rate fluorometry (FRRf) provides a potential means to realize this goal with unprecedented resolution and scale yet has not become the “go-to” method despite high expectations.

A major obstacle is difficulty converting electron transfer rates to equivalent rates of C-fixation most relevant for studies of biogeochemical C-fluxes. Such difficulty stems from methodological inconsistencies and our limited understanding of how the electron requirement for C-fixation (Φ_e,C) is influenced by the environment and by differences in the composition and physiology of phytoplankton assemblages.

This paper outlines a “roadmap” for limiting methodological bias and to develop a more mechanistic understanding of the ecophysiology underlying Φ_e,C. In this paper, the authors 1) re-evaluate core physiological processes governing how microalgae invest photosynthetic electron transport-derived energy and reductant into stored carbon versus alternative sinks. Then, 2) outline steps to facilitate broader uptake and exploitation of FRRf, which could transform our knowledge of aquatic primary productivity. they argue it is time to 3) revise our historic methodological focus on carbon as the currency of choice, to 4) better appreciate that electron transport fundamentally drives ecosystem biogeochemistry, modulates cell-to-cell interactions, and ultimately modifies community biomass and structure.

FRRf delivers a series of closely spaced excitation flashlets to cumulatively close all PSII reaction centers typically within 50–200 μs, thereby ensuring that the first acceptor molecule within the photosynthetic electron transport chain (QA) is reduced only once for each PSII during a given flashlet series. As PSII reaction centers cumulatively receive a photon they become photochemically “closed” for a period of ∼1000 μs until diffusion of the plastoquinone electron carrier (QB) removes the photochemically generated electrons. The temporary closure of the photochemical yield stimulates a transient increase in the complementary fluorescence yield, measured as a fluorescence rise.

Such FRRf protocols are termed “single-turnover” which has important implications for the mechanistic interpretation of the resulting fluorescence rise. One of the main advantages of the single turnover protocol is that it does not increase the redox state of the plastoquinone pool, making FRRf measurements less intrusive and simpler to interpret than instrumentation which induces multiple-turnover of PSII (e.g., PAM). FIRe fluorescence is conceptually similar to FRRf, except that a single turnover pulse is provided during which the fluorescence yield is rapidly subsampled to characterize the fluorescence rise. FIRe potentially simplifies FRRf-type technology and can lower power requirements, provided the excitation source is stable.

By fitting the FRRf (or FIRe) rise with a biophysical model describing photochemistry, we can extract minimal fluorescence, F0, maximal fluorescence, FM, the effective absorption cross section for PSII, σPSII, and a connectivity coefficient, ρ, which describes the probability of an exciton from a closed center being transferred to an open PSII center. It is important to note that the mechanistic meaning (and correct terminology) for these variables depends upon the state of the sample at the instant of the measurement. For example, if the FRRf rise is imposed upon a sample that is already under a level of actinic illumination, the F0 extracted from the FRRf fit is actually steady-state fluorescence, Ft.

Similarly, the Fm extracted from an illuminated sample will be some version of Fm′, the “prime” notation indicating a measurement performed under actinic illumination. From the core FRRf variables a number of derived photosynthetic parameters describing PSII activity can be constructed. These parameters can, in turn, be used to estimate the rate at which electrons are generated by PSII (ETRPSII), the initial step in both linear photosynthetic electron flow and in multiple forms of pseudocyclic electron fluxes from water back to oxygen to regenerate water.

Subsequent to the rapid succession of excitation flashlets that drive PSII closure, FRR fluorometers also allow programming of probe flashes spaced far enough apart to allow progressive reopening of the PSII centers. In this relaxation phase, each probe flash tracks the instantaneous fluorescence yield (intensity) of the PSII pool at that instant and, by complementary inference, tracks the photochemical-reopening of the PSII pool by downstream electron transport processes.

These processes can be resolved into a number of kinetic lifetimes, τ (μs), which are the reciprocals of exponential decay rates (μs–1) of the fluorescence signal with time. These lifetimes therefore track the rates and relative amplitudes of processes consuming photochemical electrons. Even so, while routinely measured by chlorophyll-a fluorescence induction techniques, τ has been generally underutilized in primary productivity studies despite immense potential to resolve electron turnover processes and productivity.

Link to paper