{'fraction_non_alphanumeric': 0.017289719626168223, 'fraction_numerical': 0.014018691588785047, 'mean_word_length': 5.567484662576687, 'pattern_counts': {'":': 0, '<': 0, '<?xml version=': 0, '>': 0, 'https://': 0, 'lorem ipsum': 0, 'www.': 0, 'xml': 0}, 'pii_count': 0, 'substrings_counts': 0, 'word_list_counts': {'cursed_substrings.json': 0, 'profanity_word_list.json': 0, 'sexual_word_list.json': 0, 'zh_pornsignals.json': 0}}
Low atmospheric CO2 induces nocturnal carbon accumulation in the lycophyte genus Isoëtes

Metabolic shifts play an essential role in the survival of plants in extreme habitats. Certain desert plant species, for example, avoid water loss by temporally segregating the light and dark reactions of photosynthesis. By only opening their stomata at night these plants inhibit significant water loss during the day. At night CO2 is incorporate into 4-carbon acids for subsequent daytime fixation1-5. This behavior is known as Crassulacean Acid Metabolism (CAM) and leads to noticeable diel cycles in pH of photosynthetic organs1-5. Oddly, similar acidity cycles are found in the submerged aquatic lycophyte genus Isoëtes2,6-12, which are not water limited in any sense. It has long been assumed that their nocturnal CO2 accumulation is an adaptation to low daytime carbon levels in aquatic ecosystems, but this has never been empirically tested2,6-13. Here, we offer direct evidence that CO2 starvation induces CAM-like nocturnal carbon accumulation in terrestrial Isoëtes. Populations of terrestrial Isoëtes engelmannii grown in climate-controlled chambers and starved of atmospheric CO2 during the day displayed diel acidity cycles similar to those in both xerophytic CAM plants and submerged Isoëtes species. These results substantiate the hypothesis that carbon starvation provides a selective pressure for nocturnal carbon accumulation in Isoëtes. Furthermore, while aquatic CO2 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 response to low atmospheric CO2 levels during the Carboniferous period14-17. Our findings therefore both provide empirical support for a long-standing assumption about nocturnal carbon accumulation in the lycophyte lineage2,6-12,18 and suggest an earlier evolution of this behavior, leading to the notion that CAM in xerophytes may only represent a subset of metabolisms that employ nocturnal carbon accumulation in response to variable environmental pressures.