Carbon for nutrient exchange between Lycopodiella inundata and Mucoromycotina fine root endophytes is unresponsive to high atmospheric CO.

Non-vascular plants associating with arbuscular mycorrhizal (AMF) and Mucoromycotina 'fine root endophyte' (MFRE) fungi derive greater benefits from their fungal associates under higher atmospheric [CO] (a[CO]) than ambient; however, nothing is known about how changes in a[CO] affect MFRE function in vascular plants. We measured movement of phosphorus (P), nitrogen (N) and carbon (C) between the lycophyte Lycopodiella inundata and Mucoromycotina fine root endophyte fungi using P-orthophosphate,  N-ammonium chloride and CO isotope tracers under ambient and elevated a[CO] concentrations of 440 and 800 ppm, respectively. Transfers of P and  N from MFRE to plants were unaffected by changes in a[CO]. There was a slight increase in C transfer from plants to MFRE under elevated a[CO]. Our results demonstrate that the exchange of C-for-nutrients between a vascular plant and Mucoromycotina FRE is largely unaffected by changes in a[CO]. Unravelling the role of MFRE in host plant nutrition and potential C-for-N trade changes between symbionts under different abiotic conditions is imperative to further our understanding of the past, present and future roles of plant-fungal symbioses in ecosystems.