Abstract
Reef building corals (phylum Cnidaria) harbour endosymbiotic dinoflagellate algae (genus Symbiodinium) that generate photosynthetic products to fuel their host’s metabolism. Non-invasive techniques such as chlorophyll (Chl) fluorescence analyses of Photosystem II (PSII) have been widely used to estimate the photosynthetic performance of Symbiodinium in hospite. However, since the spatial origin of PSII chlorophyll fluorescence in coral tissues is uncertain, such signals give limited information on depth-integrated photosynthetic performance of the whole tissue. In contrast, detection of absorbance changes in the near infrared (NIR) region integrates signals from deeper tissue layers due to weak absorption and multiple scattering of NIR light. While extensively utilised in higher plants, NIR bio-optical techniques are seldom applied to corals. We have developed a non-intrusive measurement method to examine photochemistry of intact corals, based on redox kinetics of the primary electron donor in Photosystem I (P700) and chlorophyll fluorescence kinetics (Fast-Repetition Rate fluorometry, FRRf). Since the redox state of P700 depends on the operation of both PSI and PSII, important information can be obtained on the PSII-PSI intersystem electron transfer kinetics. Under moderate, sub-lethal heat stress treatments (33 ˚C for ~20 min), the coral Pavona decussata exhibited down-regulation of PSII electron transfer kinetics, indicated by slower rates of electron transport from QA to plastoquinone (PQ) pool, and smaller relative size of oxidised PQ with concomitant decrease of a specifically-defined P700 kinetics area, which represents the active pool of PSII. The maximum quantum efficiency of PSII (Fv/Fm) and functional absorption cross-section of PSII (σPSII) remained unchanged. Based on the coordinated response of P700 parameters and PSII-PSI electron transport properties, we propose that simple P700 kinetics parameters as employed here serve as indicators of the integrity of PSII-PSI electron transfer dynamics in corals.
Highlights
Coral reefs are one of the world’s most productive and diverse ecosystems, sustained through a symbiosis between reefbuilding corals and their endosymbiont eukaryotic microalgal partner (Symbiodinium sp.) and a complex bacterial consortia (Ainsworth et al, 2010)
As a result of the high intensity of the saturation pulse, electron flow from Photosystem II (PSII) led to the re-reduction of P700+, with a decline in P700+ below the initial steady-state P700+
As the absolute values of maximum P700+ signal in weak FR light (Pm), steady-state P700+ and P700 kinetics area varies from specimen to specimen due to potential differences in symbiont density and optical properties etc., heat induced changes in P700 parameters are represented as relative changes (Figure 2)
Summary
Coral reefs are one of the world’s most productive and diverse ecosystems, sustained through a symbiosis between reefbuilding corals and their endosymbiont eukaryotic microalgal partner (Symbiodinium sp.) and a complex bacterial consortia (Ainsworth et al, 2010). Nutrient exchange between partners is fuelled primarily by Symbiodinium’s photosynthetic productivity and light utilization is a key factor driving coral metabolism from the organismal (Roth, 2014) to the entire ecological (Muir et al, 2015) reef scale. The importance of understanding the photosynthetic processes is critical to revealing the light utilization efficiency and niche specialization of corals across a range of biogeographical habitats. Such knowledge characterizes a range of regulatory mechanisms that respond to environmental stress that define the resilience of the coral holobiont (Roth, 2014; Warner and Suggett, 2016). Photosynthetic productivity of Symbiodinium sp. is highly regulated via the coupled inter-system electron transfer processes between photosystem II (P680) and photosystem I (P700), but the regulation mechanisms of this process in Symbiodinium are still poorly understood
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