Mercury and lead tolerance in hypersaline sulfate-reducing bacteria

Sulfate-reducing bacteria (SRB) HSR1, HSR4, and HSR14 isolated from the salt pans of Goa grew best at 90–100‰ salinity on substrates like formate, acetate, lactate, butyrate, ethanol and benzoate. They were gram negative, non-sporulating, non-motile rods lacking in desulfoviridin and cytochromes. Examination of these isolates for heavy metal tolerance and response studies in terms of growth and sulfate-reducing activity (SRA) were carried out using HgCl2 and Pb(NO3)2 at final concentration of 50, 100, and 200 and 100, 200 and 500 μg ml−1 respectively. With Hg, HSR1 showed ≈80% of the controlʹs growth at 100 and 200 μg ml−1 but SRA reached only 60% of the control values at the end of 14 days. HSR14 could reach >100% of the controlʹs growth at 200 μg ml−1 but the SRA reached only up to 60% of the control without metal at 100 μg ml−1. Though the concentration of Pb was double that of Hg, HSR4 could grow and respire better than the control, the growth being stimulated by 160% and respiration by 170% in the presence of 500 μg ml−1 of Pb(NO3)2. It is probable that some hypersaline SRB are more tolerant to heavy metals than the mesohaline counterparts and could be more effectively used for precipitating these metals in bioremediatory measures. Further examination of their responses to varied concentration of metals under different salinities would indicate their range of applicability.