Influence of Calcium and Magnesium on the Growth of Rhizobium

SUMMARY: Growth of Rhizobium trifolii in a defined medium reflected the supply of Ca2+ and Mg2+ (subsequently Ca and Mg, respectively) in distinctive fashion. Deficiency of Ca, in the presence of sufficient Mg, caused reduction in growth rate, the level of maximum growth and the proportion of viable cells. Such Ca-deprived cells were markedly swollen and vacuolated. On the other hand, although shortage of Mg (Ca sufficient) was without effect on growth rate down to the lowest concentration at which growth occurred, maximum growth and the proportion of viable organisms were markedly decreased. Mg-deficient organisms were appreciably elongated. Signs of Ca deficiency became apparent at less than 0.025 mM, and Mg deficiency at less than 0.1 mM, most markedly in the range below 0.5 mM. Additionally there was a need for total divalent cations of the order of 0.4-0.6 mM. This could be met by either Ca or Mg provided both were sufficient for their maximum specific effect.