TESTING FOR PRESENCE OF ALTERNATIVE PHOTOSYNTHETIC PATHWAYS IN PLANTS DURING THE LOWER CARBONIFEROUS

Plants are an integral part of terrestrial ecosystems and are important agents in the interactions between the biosphere, hydrosphere, lithosphere, and atmosphere. Modern plants use one of three biochemical pathways to conduct photosynthesis C₃, C₄, and the crassulacean acid metabolism (CAM), with the C₃ pathway being both the most predominate pathway today and the most ancient. The evolutionary emergence of CAM/CAM-like photosynthetic pathway is unknown, but some studies have speculated it to have emerged during the Paleozoic. The Devonian-Carboniferous, a time of vascular plant invasion and proliferation on land, may have been an ideal environment for the emergence of CAM as a response to low atmospheric pCO₂ and arid climatic conditions. Additionally, some Devonian-Carboniferous arborescent lycopsids are noted to have morphological features (i.e., aerenchyma) similar to modern CAM plants. This study aimed to test for the presence of CAM/CAM-like photosynthetic pathway in Devonian-Carboniferous plants through their carbon isotope (δ¹³C) composition; the CAM pathway can produce δ¹³C signatures that can be distinct from the C₃ and C­₄ pathways. Analyses were performed on four Devonian-Carboniferous taxa from assemblages that contained both arborescent lycopsids (e.g., Lepidodendropsis) and known C₃ plants (e.g., Genselia and Rhodeopteridium). Additionally, different anatomical parts of the plants were tested to evaluate potential isotopic differences among plant tissue (i.e., stem and leaf). The inter-deposit comparison was used to help control for environmental factors that may have influenced the δ¹³C of the plants (pCO₂, local climate, etc.). The result of this study shows that there was no distinction between the δ¹³C values of known C₃ plants and those of Lepidodendropsis suggesting that they did not use CAM/CAM-like photosynthetic pathway. However, the lack of isotopic distinction could have also derived from lack of a need for a water-use efficiency mechanism, the ability to switch photosynthetic pathways due to environmental changes, and/or differences in ancient plant anatomy from the modern CAM plant. Overall, our finding contributes to the isotopic archive of early land plants, the effects of the Devonian-Carboniferous environmental condition, and adds insight into the emergence of the different photosynthetic pathways in land plants.