Edaphic controls on genome size and GC content of bacteria in soil microbial communities

Genomic traits, such as genome size, GC content, codon usage, and amino acid content, shed insight into the evolutionary processes of bacteria and selective forces behind microbial community composition. Nutrient limitation has been shown to reduce bacterial genome size and influence nucleotide composition, yet little research has been conducted in the soil environment, and the factors which shape soil bacterial genomic traits remain largely unknown. Here we determined average genome size, GC content, codon usage, and amino acid content from 398 soil metagenomes across the United States from the National Ecological Observation Network (NEON) and observed the distribution of these traits across numerous environmental gradients. We found that genomic trait averages were most strongly related to pH, which we suggest results in both physiological constraints on growth as well as affects availability of nutrients in soil. Low pH soils had higher carbon to nitrogen ratios (C:N) and tended to have communities with larger genomes and lower GC-content, potentially a result of increased physiological stress and increased metabolic diversity. Conversely, smaller genomes with high GC content were associated with high pH and low soil carbon to nitrogen ratios, indicating potential resource driven selection against carbon-rich AT base pairs. We found that this relationship of nutrient conservation also applied to amino acid stoichiometry, where bacteria in soils with C:N ratios tended to code for amino acids with lower C:N. Together, these relationships point towards fundamental mechanisms which underpin nucleotide and amino acid selection in soil bacterial communities. ### Competing Interest Statement The authors have declared no competing interest.