Validation of parameters and protocols derived from chlorophyll

Abstract

This study documents the first validation of the suitability of the most common parameters and protocols used in marine ecophysiology to characterise photosynthesis by means of chlorophyll a fluorescence tools. We demonstrate that the effective yield of PSII (ΔF/Fm′) is significantly underestimated when using short inductions times (≤1 min) following the rapid light curve protocol (RLC). The consequent electron transport rates (ETR) underestimations are species-specific and highly variable with irradiance and the photoacclimatory condition of the sample. Our analysis also questions the use of relative descriptors (relETR), as they not only overestimate photosynthesis, but overlook one of the fundamental components of the photosynthetic response: light absorption regulation. Absorptance determinations were fundamental to characterise the ETR response of low-pigmented seagrass leaves, and also uncovered relevant differences between two coral species and the accclimatory response of a cultured dinoflagellate to temperature. ETR and oxygen evolution determinations showed close correlations for all organisms tested with the expected slope of 4 e– per O2 molecule evolved, when correct photosynthesis inductions and light absorption determinations were applied. However, ETR curves cannot be equated to conventional photosynthetic response to irradiance (P vs E) curves, and caution is needed when using ETR to characterise photosynthesis rates above photosynthesis saturation (Ek). This validation strongly supports the utility of fluorescence tools, underlining the need to correct two decades of propagation of erroneous concepts, protocols and parameters in marine eco-physiology. We aim also to emphasise the importance of optical descriptions for understanding photosynthesis, and for interpreting fluorescence measurements. In combination with conventional gross photosynthesis (GPS) approaches, optical characterisations open an extraordinary opportunity to determine two central parameters of photosynthesis performance: the quantum yield (φmax) of the process and its minimum quantum requirements (1/φmax). The combination of both approaches potentiates the possibilities of chlorophyll a fluorescence tools to characterise marine photosynthesis biodiversity.

Experimental Assessment of Vulnerability to Warming in Tropical Shallow-Water Marine Organisms

“…Direct comparison of ETR max across treatments (Table 1) should be interpreted with care as heat stress may have more pronounced effects on ETR than reported in our results. This points to limitation of the use of RLCs in repeated measurements in experiment which may induce variation in AF, in line with previous discussions by Enríquez and Borowitzka (2010) and González-Guerrero et al (2021).…”

Sand supplementation favors tropical seagrass Thalassia hemprichii in eutrophic bay: implications for seagrass restoration and management

“…Considering the large variation of seagrass leaf light absorption [70,71], the leaf light absorption needs to be measured. Field observations concerning seagrass response to sediment type by applying chlorophyll fluorescence and oxygen evolution [72][73][74], are needed at an ecosystem level to determine the operable habitat requirements of seagrasses [64]. It is also very important to change the sediment type to improve the growth conditions of seagrass and enhance the success rate of planting and transplanting seagrass shoots in eutrophic ecosystems.…”

“…This type of determination cannot be used to estimate the equivalent descriptors derived from the classic photosynthetic response curve to irradiance. However, the present study took standardized measurements on the RLCs, which might provide useful information for the description of relative changes in photosynthetic activity [73,74,77]. The process of measuring RLCs and the determination of the relative maximum electron transport rate (rETR max ), minimum saturating irradiance (Ek ETR ) and α ETR (the initial slope of the light-limited relationship) by curvefitting were according to Ralph and Gademann [76] and Jiang et al [78].…”

Combined effects of high temperature and light on the photosynthetic parameters and recovery of temperate microphytobenthos in Browns River, Tasmania

“…Studies have shown that low temperatures are likely to significantly impact cell growth with cells grown at colder temperatures absorbing less light than those grown at a warmer temperature (Mock & Hoch, 2005; Gleich et al ., 2020). However, using very short exposure steps of different irradiances (such as in RLCs) may lead to an overestimation of rETRmax (Serôdio et al ., 2005; González-Guerrero et al ., 2022), unlike using O 2 or CO 2 gas exchange methods. In particular, rETRmax measurements generated from RLCs do not achieve steady-state condition during each light step, and down-regulation of photosynthesis can occur.…”