Changes in the microbial community structure of bacteria, archaea and fungi in response to elevated CO 2 and warming in an A ustralian native grassland soil

The microbial community structure of bacteria, archaea and fungi is described in an Australian native grassland soil after more than 5 years exposure to different atmospheric CO2 concentrations (CO2) (ambient, +550 ppm) and temperatures (ambient, + 2°C) under different plant functional types (C3 and C4 grasses) and at two soil depths (0-5 cm and 5-10 cm). Archaeal community diversity was influenced by elevated CO2, while under warming archaeal 16S rRNA gene copy numbers increased for C4 plant Themeda triandra and decreased for the C3 plant community (P < 0.05). Fungal community diversity resulted in three groups based upon elevated CO2, elevated CO2 plus warming and ambient CO2. Overall bacterial community diversity was influenced primarily by depth. Specific bacterial taxa changed in richness and relative abundance in response to climate change factors when assessed by a high-resolution 16S rRNA microarray (PhyloChip). Operational taxonomic unit signal intensities increased under elevated CO2 for both Firmicutes and Bacteroidetes, and increased under warming for Actinobacteria and Alphaproteobacteria. For the interaction of elevated CO2 and warming there were 103 significant operational taxonomic units (P < 0.01) representing 15 phyla and 30 classes. The majority of these operational taxonomic units increased in abundance for elevated CO2 plus warming plots, while abundance declined in warmed or elevated CO2 plots. Bacterial abundance (16S rRNA gene copy number) was significantly different for the interaction of elevated CO2 and depth (P < 0.05) with decreased abundance under elevated CO2 at 5-10 cm, and for Firmicutes under elevated CO2 (P < 0.05). Bacteria, archaea and fungi in soil responded differently to elevated CO2, warming and their interaction. Taxa identified as significantly climate-responsive could show differing trends in the direction of response ('+' or '-') under elevated CO2 or warming, which could then not be used to predict their interactive effects supporting the need to investigate interactive effects for climate change. The approach of focusing on specific taxonomic groups provides greater potential for understanding complex microbial community changes in ecosystems under climate change.