Influence of iron depletion on growth and production of catechol siderophores by different Frankia strains

Nine strains of Frankia isolated from six Casuarinaceae (including four Casuarina sp., one Allocasuarina and one Gymnostoma) and one Elaeagnaceae (Hippophae rhamnoides) were screened for growth and production of siderophores in an iron-deficient liquid medium. Siderophore production was detected only in four strains (Cj, G2, CH and G82) using the CAS and Arnow assays. Salicylates formed more than 90% and dihydroxybenzoates formed less than 10% of all catechol-type siderophores produced. Growth of the former strains was less affected by iron deficiency than that of strains Rif, Thr, URU, BR and RT which do not produce siderophores. Optimal siderophore production by strain Cj was noted when iron concentration reached 0.5 μM and was completely inhibited at an iron concentration of 10 μM. The kinetics of siderophore production by strain Cj showed that siderophore synthesis was detectable during the growth stationary phase. Growth of Cj (a siderophore-producing strain) and of RT (a non-siderophore-producing strain) differed when 2,2-dipyridyl or ethylene di(o-hydroxyphenyl) acetic acid (EDDHA) was added to the iron-deficient growth medium. Frankia strain RT was the most sensitive to the detrimental effect of both iron chelators.