Differential response of Frankia strains to heavy metals

Abstract

Shortage of water in Israel necessitates utilization of increasing volumes of marginal water for irrigation. Marginal water is characterized by higher concentrations of heavy metals then the potable water from which they were derived. Over time, irrigation with treated water carrying appreciable amounts of heavy metals can contribute to their build-up in soils. The actinomycete Frankia is a nitrogen-fixing root-nodule endosymbiont that is present free in the soil or in association with dicotyledonous plants roots. Frankia has the ability to bind and sequester several toxic heavy metals and is a potentially bioremediation agent. The sensitivity to heavy metals of seven Frankia strains (1F, 5F, 6F, 7F, Fb, Fc, and d) recently isolated in our lab and one reference strain (DSM-44251) was determined. A differential response of these strains to Cd, Al, and B was observed. Toxic levels of the different strains for Cd and B were determined as well as the deficiency levels of B. For all Frankia strains except 7F, increasing Al concentrations enhanced the growth at low pH. Strain d had the highest tolerance to Cd and toxic levels of B with no inhibitory effect of Al, albeit with low growth enhancement by Al compared to the other strains. Irrigation with treated wastewater may reduce growth of some Frankia strains and reduce their nodulation efficiency. Given the potential of Frankia in bioremediation and phytoremediation applications it is important to elucidate Frankia isolates' sensitivity and tolerance to water pollutants such as heavy metals.