Root-nodule bacteria isolated from native Amphithalea ericifolia and four indigenous Aspalathus species from the acidic soils of the South African fynbos are tolerant to very low pH

Abstract

Indigenous root-nodule bacteria isolated from the acid sands of the Cape using Aspalathus linearis, Aspalathus hispida, Aspalathus carnosa, Aspalathus capensis and Amphithalea ericifolia as trap hosts showed considerable tolerance to low pH. Isolates from A. ericifolia and A. carnosa could even grow in YMB medium at pH 3. Although, all strains grew well at pH 4, 5 and 6, the isolates from A. carnosa exhibited the highest growth rate at each of the three pH regimes. The isolates from A. linearis subsp. linearis, A. capensis and A. carnosa grew significantly better when re-cultured from pH 3 in pH 5 or same pH 3 medium as compared to first-time culture in pH 3. With isolates from A. capensis and A. linearis, growth of cells from pH 3 re-cultured in pH 5 or same pH 3 was not significantly different. Except for isolates from A. carnosa, which showed a marked increase and A. capensis with a major decrease, no differences were observed in bacterial growth when cells from pH 5 were re-cultured in pH 3. Providing 0.5% of root metabolites from A. linearis subsp. linearis to its microsymbiont at pH 3 significantly reduced cell growth from 0.8 to less than 0.1 OD units. At pH 5, however, bacterial growth was neither inhibited nor promoted by the addition of root extract.

Key words: Bacterial isolates, acid soils, optical density, Western Cape.