Photobiont type influences the relative humidity threshold at which photosystem II activates in green algal lichens. Water vapor uptake alone can activate photosynthesis in lichens with green algal photobionts. However, the minimum relative humidity needed for activation is insufficiently known. The objective of this study was to quantify the humidity threshold for photosystem II (PSII) activation in a range of chlorolichen species associated with photobionts from Trebouxiaceae, Coccomyxaceae and Trentepohliaceae. These lichens exhibit distribution, habitat and substrate patterns that are likely coupled to their efficiency in utilizing water vapor at lower levels of relative humidity (RH) for photosynthesis. Using chlorophyll fluorescence imaging during water uptake from humid air of 25 species of chlorolichens representing the above photobiont groups, we monitored PSII activation within controlled chambers with constant RH at five levels ranging from 75.6 to 95.4%. The results demonstrate clear photobiont-specific activation patterns: the trentepohlioid lichens activated PSII at significantly lower RH (75.6%) than trebouxioid (81.7%) and coccomyxoid (92.0%) lichens. These responses are consistent with a preference for warm and sheltered habitats for trentepohlioid lichens, with cool and moist habitats for the coccomyxoid lichens, and with a more widespread occurrence of the trebouxioid lichens. Within each photobiont group, lichen species exposed to marine aerosols in their source habitats seemed to be activated at lower RH than lichens sampled from inland sites. High osmolyte concentration may therefore play a role in lowering a photobiont’s activation threshold. We conclude that photobiont type influences water vapor-driven photosynthetic activation of lichens, thereby shaping the ecological niches in which they occur.

中文翻译：

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绿地衣光系统 II 激活的光生物依赖湿度阈值

光生物类型影响绿藻地衣中光系统 II 激活的相对湿度阈值。仅吸收水蒸气就可以激活具有绿藻光生物的地衣的光合作用。然而，激活所需的最小相对湿度尚不清楚。本研究的目的是量化一系列绿地衣物种中光系统 II (PSII) 激活的湿度阈值，这些绿地衣物种与来自 Trebouxiaceae、Coccomyxaceae 和 Trentepohliaceae 的光生物相关。这些地衣表现出的分布、栖息地和底物模式可能与其在较低相对湿度 (RH) 水平下利用水蒸气进行光合作用的效率有关。在代表上述光生物组的 25 种绿地衣从潮湿空气中吸收水分期间使用叶绿素荧光成像，我们在 75.6% 至 95.4% 的五个水平的恒定 RH 的受控室内监测 PSII 活化。结果显示了清晰的光生物特异性激活模式：trentepohlioid 地衣在显着低于 trebouxioid (81.7%) 和 coccomyxoid (92.0%) 地衣的相对湿度 (75.6%) 下激活 PSII。这些反应与trentepohlioid 地衣喜欢温暖和有遮蔽的栖息地、coccomyxoid 地衣喜欢凉爽和潮湿的栖息地以及更广泛地出现treboxioid 地衣是一致的。在每个光生物组内，与从内陆地区取样的地衣相比，在其源栖息地暴露于海洋气溶胶的地衣物种似乎在较低的相对湿度下被激活。因此，高渗透质浓度可能在降低光生物的活化阈值方面发挥作用。我们得出结论，光生物类型影响水蒸气驱动的地衣光合激活，从而塑造了它们发生的生态位。