Carbon-concentrating mechanisms are a key trait in lichen ecology and distribution.

Carbon-concentrating mechanisms (CCMs) are a widespread phenomenon in photosynthetic organisms. In vascular plants, the evolution of CCMs ([C44-carbon compound] and crassulacean acid metabolism [CAM]) is associated with significant shifts, most often to hot, dry and bright, or aquatic environments. If and how CCMs drive distributions of other terrestrial photosynthetic organisms, remains little studied. Lichens are ecologically important obligate symbioses between fungi and photosynthetic organisms. The primary photosynthetic partner in these symbioses can include CCM-presenting cyanobacteria (as carboxysomes), CCM-presenting green algae (as pyrenoids) or green algae lacking any CCM. We use an extensive dataset of lichen communities from eastern North America, spanning a wide climatic range, to test the importance of CCMs as predictors of lichen ecology and distribution. We show that the presence or absence of CCMs leads to opposite responses to temperature and precipitation in green algal lichens, and different responses in cyanobacterial lichens. These responses contrast with our understanding of lichen physiology, whereby CCMs mitigate carbon limitation by water saturation at the cost of efficient use of vapor hydration. This study demonstrates that CCM status is a key functional trait in obligate lichen symbioses, equivalent in importance to its role in vascular plants, and central for studying present and future climate responses.