AbstractChanges in atmospheric CO₂ concentration have played a central role in algal and plant adaptation and evolution. The commercially important red algal genus, Pyropia (Bangiales) appears to have responded to inorganic carbon (C_i) availability by evolving alternating heteromorphic generations that occupy distinct habitats. The leafy gametophyte inhabits the intertidal zone that undergoes frequent emersion, whereas the sporophyte conchocelis bores into mollusk shells. Here, we analyze a high-quality genome assembly of Pyropia yezoensis to elucidate the interplay between C_i availability and life cycle evolution. We find horizontal gene transfers from bacteria and expansion of gene families (e.g. carbonic anhydrase, anti-oxidative related genes), many of which show gametophyte-specific expression or significant up-regulation in gametophyte in response to dehydration. In conchocelis, the release of HCO₃^- from shell promoted by carbonic anhydrase provides a source of C_i. This hypothesis is supported by the incorporation of ¹³C isotope by conchocelis when co-cultured with ¹³C-labeled CaCO₃.