Bioremediation of cadmium-contaminated paddy soil using an autotrophic and heterotrophic mixture

Abstract

Cadmium (Cd) pollution poses a serious risk to human health and ecological security. Bioremediation can be a promising and effective remediation technology for treating Cd contaminated soils. In this study, seven heterotrophic strains were isolated from Cd contaminated soil and 7 autotrophic strains were isolated from acid mine drainage. Cd removal efficiencies were compared after leaching with autotrophic bacteria (Att-sys), heterotrophic isolates (Htt-sys) and cooperative leaching systems (Co-sys) in laboratory agitating reactors. The results indicated that Cd removal efficiency of Co-sys (32.09%) was significantly higher than that of Att-sys (23.24%) and Htt-sys (0.74%). By analyzing the soil microbial community in different bioleaching systems, we found that the addition of heterotrophic isolates significantly promoted the growth of some heavy metal resistant inhabitants (Massilia, Alicyclobacillus, Micromonospora, etc.), and Co-sys had a minor effect on the growth of soil indigenous microbes. In Co-sys, the content of the four Cd fractions all decreased compared with other leaching systems. The analysis of soil physicochemical parameters during the leaching process showed that pH and ORP (oxidation reduction potential) were not the only determinants for Cd removal efficiency in Co-sys, synergistic metabolic activities of autotrophic and heterotrophic strains may be other determinants. This study demonstrated that cooperative bioremediation may prove to be a safe and efficient technique for field application in heavy metal soil pollution.

Fig. 1. Phylogenetic tree based on ITS gene sequence of F1, F2, F3, F4, zp1, zp2 and zp3, derived from Neighbour Joining method using MEGA X software.

Fig. 2. (a) Removal efficiency of Cd in three leaching systems (means ± SD). The different small letters represented the significantly different according to the Fisher's LSD test. (b) Relative Cd fractions in residual soils of five leaching systems, figures represent the contents of each Cd fraction in soils. (c) Variations of pH and ORP in three leaching systems (means ± SD).

Fig. 3. Changes of OTU number (a), Shannon diversity (b), Simpson diversity (c) and Evenness (d) of five leaching systems during the whole leaching process.

Fig. 4. (a) Relative abundance of microbial community structure using different treatments during the leaching process (Genus level). (b) Changes in microbial community structure after various treatments based on NMDS, the same symbol represents the same leaching group, with light colors for 0 days, darker for 2 days, and darkest for 4 days.

Fig. 5. (a) A heatmap showing the relationship between relative abundances (>1.0% of total OTU abundance) and different leaching groups. Colour codes: Red rectangles = high relative abundance, purple rectangles = low relative abundance and white rectangles = intermediate abundance. Groups are clustered in columns and OTUs in rows. (b) LEfSe analysis of microbial community in Cd-contaminated soils of five leaching systems (Yellow circles represent non-significant (p > 0.05) differences in abundance among five leaching system, other colors represent biomarkers with significant (p < 0.05) differences in different leaching groups).