Testing for presence of alternative photosynthetic pathways in plants during the Mississippian

Abstract

The evolutionary emergence of the CAM (Crassulacean Acid Metabolism) photosynthetic pathway is unknown, but some studies have speculated it may have emerged during the Devonian-Carboniferous when vascular plants first proliferated in terrestrial environments. Specifically, some Carboniferous arborescent lycopsids (e.g., Lepidodendropsis) have morphological features similar to modern CAM plants. In this study, we systematically examine the carbon isotope compositions of an assemblage of Tournaisian (Mississippian) Lepidodendropsis for evidence of alternative photosynthetic pathways. Previous work suggests that the CAM pathway can produce δ¹³C signatures that are distinct from the C₃ pathway due to differences in isotopic discrimination during carbon fixation. Analyses were performed on Lepidodendropsis sp. and coeval presumed C₃ plants (i.e. Genselia and Rhodeopteridium sp.) from the same fossil plant assemblage within the Price Formation of southwestern Virginia, U.S.A. The investigated plant fossils were limited to single assemblages from four localities within the Price Formation in an attempt to control differences in environmental conditions (differences in CO₂, local climate, etc.) and diagenetic history that could affect the carbon isotope compositions of the fossil plants. The full dataset yielded a mean δ¹³C of −22.5 ± 0.7 ‰ for the CAM/CAM-like plants (n = 35) and a mean δ¹³C of −22.7 ± 0.7 ‰ for the suggested C₃ plants (n = 36). The lack of isotopic difference between the coeval C₃ plants and that of Lepidodendropsis does not preclude the possibility that these plants were using a CAM pathway and leaves open three possibilities for the photosynthetic pathway that Lepidodendropsis employed: 1) the C₃ metabolism 2) aquatic CAM or 3) facultative CAM. More broadly, our study provides a more systematic framework for future studies to test for the presence of a CAM or CAM-like pathway in ancient plants.