Effect of chromium and organic acids on microbial growth and exopolymeric substance (EPS) production by Pseudomonas bacteria

Abstract

Natural organic acids are capable of stimulating microbial chromium(VI) reduction, but little information is available about their behavior on microbial growth, exopolymeric substance (EPS) production, and subsequent microbial Cr(VI) reduction. Here, laboratory batch experiments were conducted to determine the effects of different natural organic acids (galacturonic, glucuronic, citric, and alginic acid) on microbial EPS production and the growth rates of four different naturally occurring soil bacteria (Pseudomonas putida P18, P. aeruginosa P16, P. fluorescens ATCC 55241, and P. stutzeri P40) as a function of pH and time in solutions containing toxic metal ions such as Cr(III) and Cr(VI). While the addition of Cr(VI) led to a negative impact on microbial growth in all strains studied, Cr(VI) significantly enhanced EPS release from cells due to extreme cell lysis. Organic acids diminished the toxic effects of Cr(VI) on cells, and thus significantly increased microbial cell growth and the EPS yield. The addition of Cr(III) with Cr(VI), on the other hand, led to a significant decrease in microbial cell growth rates relative to the systems containing only Cr(VI). This toxic effect decreased significantly in the presence of organic acids, and thus the EPS yield increased due to the formation of less toxic Cr(III)-EPS species. The overall results indicate that while the accumulation of free Cr(III) ion in aqueous phase during microbial Cr(VI) reduction may have an adverse influence on microbial cell growth, the EPS released by bacteria may bind with free Cr(III) ion in solution, and thus increase the cell growth rate due to the removal of toxic products of microbial reduction.