Effect of Drying-Rewetting cycles on the metal adsorption and tolerance of natural biofilms

Drying-rewetting (D-RW) cycles can induce changes in biofilms by forcing the microbial community to tolerate and adapt to environmental pressure. Existing studies have mostly focused on the impact of D-RW cycles on the microbial community structure, and little attention has been paid to how D-RW cycles may change the biofilm tolerance and adsorption of heavy metals. We experimentally evaluated the effect of repeated D-RW cycles on the Cd2+ and Pb2+ adsorption and tolerance of biofilms. The equilibrium adsorption capacity of the biofilm decreased as the number of D-RW cycles was increased, which was attributed to a change in affinity between the biofilm and metal ions. For a binary metal system, the D-RW cycles affected the competitive adsorption of Cd2+ and Pb2+ by the biofilm. A synergistic effect was observed with one and three D-RW cycles, while an antagonistic effect was observed for the control film and five D-RW cycles. The tolerance of the biofilm to Cd2+ and Pb2+ increased with the number of D-RW cycles. The stress from the D-RW cycles may have increased the relative abundance of drought-tolerant bacteria, which altered the biofilm functions and thus indirectly affected the heavy metal adsorption capacity.