Low atmospheric CO2 levels induce nocturnal carbon accumulation in the lycophyte genus Isoëtes

Background and Aims Crassulacean Acid Metabolism (CAM) is an adaptation to increase water use efficiency in dry environments. Oddly, similar biochemical patterns occur in the submerged aquatic lycophyte genus Isoëtes . It has long been assumed that CAM-like nocturnal carbon accumulation in aquatic plants is an adaptation to low daytime carbon levels in aquatic ecosystems, but this has never been directly tested. Methods To test this hypothesis, populations of terrestrial Isoëtes engelmannii and I. tuckermanii were grown in climate-controlled chambers and starved of atmospheric CO 2 during the day while pH was measured for 24-hours. Key results We demonstrate that terrestrial plants exposed to low atmospheric CO 2 display diel acidity cycles similar to those in both xerophytic CAM plants and submerged Isoëtes . Conclusions CO 2 starvation induces CAM-like nocturnal carbon accumulation in terrestrial Isoëtes , substantiating the hypothesis that carbon starvation is a selective pressure for nocturnal carbon accumulation in Isoëtes . Furthermore, while aquatic carbon levels undoubtedly promote nocturnal carbon accumulation in extant Isoëtes , the induction of this behavior in terrestrial plants suggests a possible earlier terrestrial evolution of this metabolism in Isoetalean ancestors in response to low atmospheric CO 2 levels. We both provide support for a long-standing assumption about nocturnal carbon accumulation Isoëtes as well as suggest an earlier evolution of this behavior.