Changes in macromolecular allocation in nondividing algal symbionts allow for photosynthetic acclimation in the lichen Lobaria pulmonaria.

•   The lichen Lobaria pulmonaria survives large seasonal environmental changes through physiological acclimation to ambient conditions. •   We quantitated algal cell population, cell division and key macromolecular levels associated with photosynthesis and nitrogen metabolism in L. pulmonaria sampled from four seasons with contrasting environmental conditions in a deciduous forest. •   The algal symbiont population did not vary seasonally and cell division was restricted to the newest thallus margins. Nevertheless the symbiont concentrations of chlorophyll, PsbS, PsbA, and RbcL changed significantly through the seasons in the nondividing algal cells from older thallus regions. •   L. pulmonaria reversibly allocated resources toward photochemical electron generation and carbohydrate production through the spring, summer and fall, and towards photoprotective dissipation in the cold, high-light winter. Our study shows that large seasonal molecular acclimation in L. pulmonaria occurs within a nearly stable, nondividing algal cell population that maintains photosynthetic capacity through many years of changing environmental cues.