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. Chlorophyll/metabolism, Humidity, Lichens/*metabolism, Optical Imaging, Photosystem II Protein Complex/*metabolism, Symbiosis, Chlorophyll fluorescence, Ecophysiology, Green algae, Osmolytes