Seawater carbonate chemistry and UVR-induced inhibition of photosynthetic light reactions and growth in an intertidal red macroalga

The commercially important red macroalga Pyropia (formerly Porphyra) yezoensis is, in its natural intertidal environment, subjected to high levels of both photosynthetically active and ultraviolet radiation (PAR and UVR, respectively). In the present work, we investigated the effects of a plausibly increased global CO2 concentration on quantum yields of photosystems II (PSII) and I (PSI), as well as photosynthetic and growth rates of P. yezoensis grown under natural solar irradiance regimes with or without the presence of UV-A and/or UV-B. Our results showed that the high-CO2 treatment (1000 μbar, which also caused a drop of 0.3 pH units in the seawater) significantly increased both CO2 assimilation rates (by 35%) and growth (by 18%), as compared with ambient air of 400 μbar CO2. The inhibition of growth by UV-A (by 26%) was reduced to 15% by high-CO2 concentration, while the inhibition by UV-B remained at ~6% under both CO2 concentrations. Homologous results were also found for the maximal relative photosynthetic electron transport rates (rETRmax), the maximum quantum yield of PSII (Fv/Fm), as well as the midday decrease in effective quantum yield of PSII (YII) and concomitant increased non-photochemical quenching (NPQ). A two-way ANOVA analysis showed an interaction between CO2 concentration and irradiance quality, reflecting that UVR-induced inhibition of both growth and YII were alleviated under the high-CO2 treatment. Contrary to PSII, the effective quantum yield of PSI (YI) showed higher values under high-CO2 condition, and was not significantly affected by the presence of UVR, indicating that it was well protected from this radiation. Both the elevated CO2 concentration and presence of UVR significantly induced UV-absorbing compounds. These results suggest that future increasing CO2 conditions will be beneficial for photosynthesis and growth of P. yezoensis even if UVR should remain at high levels.

具有重要经济价值的红藻条斑紫菜（Pyropia yezoensis，原称为Porphyra yezoensis）在其自然潮间带生境中，会暴露于高水平的光合有效辐射与紫外辐射（分别简称为PAR和UVR）。本研究探讨了合理预估升高的全球CO₂浓度，对光系统II（PSII）与光系统I（PSI）量子产额，以及在模拟自然太阳光辐照条件下（分别设置有无UV-A和/或UV-B处理组）条斑紫菜的光合速率与生长速率的影响。结果显示，相较于CO₂分压为400 μbar的环境空气对照组，高CO₂处理组（CO₂分压为1000 μbar，该处理同时导致海水pH下降0.3个单位）的CO₂同化速率与生长速率分别显著提升35%与18%。UV-A诱导的生长抑制率（26%）可通过高CO₂浓度处理降至15%，而两种CO₂浓度下UV-B所致的生长抑制率均维持在约6%。最大相对光合电子传递速率（rETRmax）、PSII的最大量子产额（Fv/Fm），以及PSII有效量子产额（YII）的午间下降现象与伴随的非光化学淬灭（NPQ）升高，均呈现出一致的变化趋势。双因素方差分析（two-way ANOVA）结果表明，CO₂浓度与辐照品质之间存在显著交互作用，提示高CO₂处理可缓解UVR诱导的生长与YII抑制效应。与PSII不同，PSI的有效量子产额（YI）在高CO₂条件下数值更高，且不受UVR存在的显著影响，表明PSI可免受该辐射的胁迫损伤。升高的CO₂浓度与UVR的存在均可显著诱导紫外吸收化合物的合成。上述结果表明，即便UVR仍维持在高水平，未来升高的CO₂环境仍将有益于条斑紫菜的光合作用与生长。