Effects of pH on the comparative culturability of fungi and bacteria from acidic and less acidic forest soils

Under aerobic conditions, the culturable microbial population of acidic forest soils was more tolerant to acidic cultivation conditions than was the culturable microbial population of less acidic soils. The number of culturable bacteria decreased sharply under acidic cultivation conditions, while the number of culturable fungi remained relatively constant over the pH range 2.2-6.5. The ratios of culturable bacteria to culturable fungi were greater than one at pH 6.5; in contrast, the bacteria-to-fungi ratios were less than one at pH 2.2-4. At pH's approximating those of the soils examined, culturable fungi predominated the culturable microbial community in acidic soils. However, relative to the populations resolved, acidic forest soils displayed a more acid tolerant bacterial population than did less acidic forest soils. The culturable fungal population contained both filamentous and yeast morphologies. An acid-tolerant fungal isolate that grew at pH 1 was identified as a subspecies of Penicillium frequentans, and an acid-tolerant yeast that grew at pH 2 was identified as the yeast stage of the basidiomycetes Ustilago maydis.