In vivo assessment of mitochondrial respiratory alternative oxidase activity and cyclic electron flow around photosystem I on small coral fragments

Abstract

The mutualistic relationship existing between scleractinian corals and their photosynthetic endosymbionts involves a complex integration of the metabolic pathways within the holobiont. Respiration and photosynthesis are the most important of these processes and although they have been extensively studied, our understanding of their interactions and regulatory mechanisms is still limited. In this work we performed chlorophyll-a fluorescence, oxygen exchange and time-resolved absorption spectroscopy measurements on small and thin fragments (0.3 cm2) of the coral Stylophora pistillata. We showed that the capacity of mitochondrial alternative oxidase accounted for ca. 25% of total coral respiration, and that the high-light dependent oxygen uptake, commonly present in isolated Symbiodiniaceae, was negligible. The ratio between photosystem I (PSI) and photosystem II (PSII) active centers as well as their respective electron transport rates, indicated that PSI cyclic electron flow occurred in high light in S. pistillata and in some branching and lamellar coral species freshly collected in the field. Altogether, these results show the potential of applying advanced biophysical and spectroscopic methods on small coral fragments to understand the complex mechanisms of coral photosynthesis and respiration and their responses to environmental changes.

Toward bio‐optical phenotyping of reef‐forming corals using Light‐Induced FluorescenceTransient‐FastRepetition Rate fluorometry

“…Intriguingly in our study, of the various kinetic turnover parameters, τ 2 /PQ OX was often the dominant vector driving cluster separation, suggesting separation phenotypes of plating Acropora species—or indeed E. lamellosa vs. P. verrucosa —by divergent activity downstream of PSII (Szabó et al 2017; Hughes et al 2018). This finding supports previous observations of differential reliance on PSI for moderating excess energy flows among endosymbionts in culture (Roberty et al 2014) or in hospite of corals (Szabó et al 2017; Vega de Luna et al 2020; but see Hoogenboom et al 2012), and clearly warrants more targeted investigation in future. While we cannot resolve the underlying basis for the alternate phenotypes of Acropora retrieved via LIFT‐FRRf here, which were dispersed across different Acropora species (or morpho‐species), it is possibly an outcome of known plasticity of association between these various hosts and alternate Symbiodiniaceae taxa (e.g., A. cytherea , Rouzé et al 2019).…”

“…However, “instantaneous” measurements under transient conditions, such as the continuously dynamic light environments for benthic corals (Anthony et al 2004), are needed to capture the realized responses underpinning functional performance (Hughes et al 2018; Gorbunov and Falkowski 2021). Numerous factors operate to “downregulate” (in efforts to maintain optimum) PSII photochemistry in coral endosymbionts (Vega de Luna et al 2020; reviewed by Warner and Suggett 2016); as such, parameterizing light harvesting and utilization in the light (i.e., σ PSII ′, τ 1 ′, etc.) provide additional descriptors of the photochemical operation dynamics (Gorbunov et al 2001; Szabó et al 2017).…”

“…However, accurately resolving these kinetic parameters becomes increasingly more challenging (and potentially more inaccurate) as dynamic light exposure increases and variable fluorescence correspondingly decreases (Hughes et al 2018; see Gorbunov and Falkowski 2021). To overcome this constraint, active fluorescence induction signatures are typically parameterized to capture dynamic trade‐offs among photochemistry and “downregulation” via “quenching” terms calculated from minimum and maximum fluorescence; for example, we have shown that under the same conditions, coral endosymbionts as free living cultures (Suggett et al 2015) or in hospite (Nitschke et al 2018) employ different photosynthetic strategies based on trade‐offs in photochemical vs. nonphotochemical quenching (derived as [1 − C ] and [1 − Q ], respectively; see “Materials and procedures” section) reflecting different cellular machinery available for dissipating excitation energy (Hughes et al 2018; Vega de Luna et al 2020). We acknowledge that additional photobiological parameters could ultimately be applied for phenotyping (Kuhlgert et al 2016; Yao et al 2018; Keller et al 2019; McAusland et al 2019; Pleban et al 2020), but how dynamic quenching factors compliment a LHCII and the turnover time constants (Fig.…”

A phenomic modeling approach for using chlorophyll-a fluorescence-based measurements on coral photosymbionts: a step towards bio-optical bleaching prediction

“…The remaining three photo-physiological parameters (σ PSII , τ 1 and τ 2 ) can only be derived from single turnover instrumentation (Suggett et al, 2022). Overall, measurements of τ 1 and τ 2 indicate slower electron transport rates through the photochemical apparatus in Durusdinium phenotypes while the more dynamic responses to changing light observed for these two parameters may also be indicative of differences in how downstream photochemical activity is regulated across Symbiodiniaceae genera or species (Roberty et al, 2014; Vega de Luna et al, 2020). The high level of algal biometric parameterization demonstrated in our approach is valuable for identifying phenotypic differences.…”

Heat-Induced Photosynthetic Responses of Symbiodiniaceae Revealed by Flash-Induced Fluorescence Relaxation Kinetics

“…Chlorophyll fluorescence has been widely used to determine the photosynthetic efficiency and electron transfer rates in Symbiodiniaceae under various conditions (Gorbunov et al, 2001;Hill et al, 2004;Hill and Ralph, 2008a;Warner et al, 2010;Roth, 2014;Suggett et al, 2015;Warner and Suggett, 2016;Nitschke et al, 2018), and, in combination with other biophysical methods, the alternative electron transport pathways such as CEF (Reynolds et al, 2008;Roberty et al, 2014;Aihara et al, 2016;Dang et al, 2019;Vega De Luna et al, 2020). Active chlorophyll fluorescence kinetics therefore potentially enable bio-optical phenotyping of the coral endosymbiont algae based on an extensive set of parameters of Chl fluorescence induction and relaxation (Hoadley and Warner, 2017;Gorbunov and Falkowski, 2021;Suggett et al, 2022).…”