Abstract

While intertidal macroalgae are exposed to drastic changes in solar photosynthetically active radiation (PAR) and ultraviolet radiation (UVR) during a diel cycle, and to ocean acidification (OA) associated with increasing CO2 levels, little is known about their photosynthetic performance under the combined influences of these drivers. In this work, we examined the photoprotective strategies controlling electron flow through photosystems II (PSII) and photosystem I (PSI) in response to solar radiation with or without UVR and an elevated CO2 concentration in the intertidal, commercially important, red macroalgae Pyropia (previously Porphyra) yezoensis. By using chlorophyll fluorescence techniques, we found that high levels of PAR alone induced photoinhibition of the inter-photosystem electron transport carriers, as evidenced by the increase of chlorophyll fluorescence in both the J- and I-steps of Kautsky curves. In the presence of UVR, photoinduced inhibition was mainly identified in the O2-evolving complex (OEC) and PSII, as evidenced by a significant increase in the variable fluorescence at the K-step (Fk) of Kautsky curves relative to the amplitude of FJ−Fo (Wk) and a decrease of the maximum quantum yield of PSII (Fv/Fm). Such inhibition appeared to ameliorate the function of downstream electron acceptors, protecting PSI from over-reduction. In turn, the stable PSI activity increased the efficiency of cyclic electron transport (CET) around PSI, dissipating excess energy and supplying ATP for CO2 assimilation. When the algal thalli were grown under increased CO2 and OA conditions, the CET activity became further enhanced, which maintained the OEC stability and thus markedly alleviating the UVR-induced photoinhibition. In conclusion, the well-established coordination between PSII and PSI endows P. yezoensis with a highly efficient photochemical performance in response to UVR, especially under the scenario of future increased CO2 levels and OA.

Highlights

  • Living in the intertidal zone, macroalgae are often exposed to periodic harsh light fluctuations and air exposure associated with changes in tide levels

  • The photosystems II (PSII) acceptor side activity, quantum yield of electron transport, photosystem I (PSI) donor side activity, and quantum yield for reduction of PSI acceptor side were significantly increased by ultraviolet radiation (UVR), as shown here by the negative inhibition values (t-test, p < 0.05 for these four parameters)

  • An increase in the re-reduction rate of P700+ showed that UVR significantly stimulated the activity of cyclic electron transport (CET) around PSI, especially in the presence of UVB, FIGURE 1 | Values (a.u., left pair of bars) and UVR-induced inhibition (%, right two pairs of bars) on the O2-evolving complex (OEC) activity of Pyropia yezoensis growing for 9 days at low (~400 μatm, open bars) and high (~1000 μatm, closed bars) CO2 conditions

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Summary

Introduction

Living in the intertidal zone, macroalgae are often exposed to periodic harsh light fluctuations and air exposure associated with changes in tide levels. Increased ocean temperatures result in stratification and shoaling of the upper mixed layer and expose organisms to increased levels of solar photosynthetically active radiation (PAR) and UVR (Häder and Barnes, 2019 and reference therein), and the global warming-induced melting of ice and snow would aggravate the transmission of UVR and increase UVR exposure in polar regions (Williamson et al, 2019; Neale et al, 2021 and references therein) These interactive effects control the levels of exposure of macroalgae to UVR, and may modulate their photosynthetic performance, production of photoprotective compounds and/or repair mechanisms in response to UVR (see the review by Ji and Gao, 2020 and references therein)

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