Impacts of Long-Term Elevated Atmospheric CO2 Concentrations on Communities of Arbuscular Mycorrhizal Fungi

The ecological implications of long-term elevated atmospheric CO2 (eCO2) levels on soil microbiota remains poorly understood. This is particular true for the arbuscular mycorrhizal (AM) fungi, which form mutualistic associations with over 1/3 of all terrestrial plant species. AM fungi are entirely dependent on their plant hosts for carbon. Thus any changes in photosynthates due to eCO2 could alter both AM fungal communities and their influence on plant community dynamics. Yet the long-term ecological impacts of eCO2 on natural AM fungal communities are unknown. Here we use high-resolution amplicon sequencing (Illumina, HiSeq) to quantify the AM fungal communities from plant roots from the longest running Free Air Carbon dioxide Enrichment (FACE) experiment in the northern hemisphere in Giessen (GiFACE), providing samples from an old-growth (>100 years) grassland subjected to >15 years of eCO2 (+20% of actual ambient CO2). eCO2 significantly increased AM fungal diversity, yet had a minimal impact on overall AM fungal community composition. However, while broad changes in the composition of AM fungal communities were not observed, the abundance of individual AM fungal taxa was significantly affected by eCO2, with one third of taxa responding negatively and the rest positively. Furthermore, three families (Archaeosporaceae, Claroideoglomeraceae, and Glomeraceae) were significantly more abundant under eCO2. This demonstrates the need to consider individual populations, alongside broad shifts in community composition when assessing how soil microbes respond to environmental change. AM fungal communities are likely to increase in diversity in the eCO2 world, but other factors involved in global change (e.g. water availability and changes in plant host fitness) may impact their communities significantly more than eCO2. eCO2-mediated changes in AM fungi have the potential to significantly influence terrestrial plant communities. Thus prediction of the future of terrestrial ecosystems must consider changes both aboveground and belowground.