Stimulatory effects of aluminum on in vitro growth of Frankia

To investigate the effects of aluminum (Al) on in vitro growth (total prote in content) of Frankia, seven isolates were incubated for 25 days at five n ominal Al concentrations (0-500 mu M) at pH 4.8. The concentrations of mono meric Al ranged between 25.6 and 106.7 mu M. The experimental medium was de void of inorganic phosphate and Fe-EDTA to avoid Al precipitation. Then was an Al-free control at pH 6.4 to assess the effects of low pH alone on Fran kia growth. Growth estimated as total protein of an strains was notably inc reased at pH 6.4, but only Frankia CcI3 was able to grow in the Al-free med ium at pH 4.8. Nominal Al concentrations from 125 to 500 mu M enhanced the growth of all Frankia isolates. The Al response was strain dependent, but a ll strains were stimulated at the 500 mu M (107 mu M monomeric) Al concentr ation. The final pH of media decreased as Al concentrations increased. At t he end of the experiment, the presence of citric, oxalic, malic, and lactic acids as possible Al binding agents in media at 500 mu M Al was assayed by enzymatic techniques. However, none of them could be detected. These resul ts suggest that the survival and growth of Frankia strains in acid soils th at are characteristically high in Al are more likely than previously estima ted from in vitro growth assays.