(a) Hierarchical clustering of closed-reference OTUs based on mean pH; (b) balance of low-pH-associated organisms (3.8 < mean pH < 6.7) and high-pH-associated organisms (6.8 < mean pH < 8.2); (c) observed OTU counts sorted by pH; (d) predicted OTU proportions from ordinary least-squares linear regression on balances sorted by pH. (a) Hierarchical clustering of closed-reference OTUs based on mean pH; (b) balance of low-pH-associated organisms (3.8 < mean pH < 6.7) and high-pH-associated organisms (6.8 < mean pH < 8.2); (c) observed OTU counts sorted by pH; (d) predicted OTU proportions from ordinary least-squares linear regression on balances sorted by pH. The coefficient of determination was 35%, showing that 35% of the variation in the microbial community abundance data can be predicted by pH alone. James T. Morton et al. mSystems 2017; doi:10.1128/mSystems.00162-16