Iodine toxicity in a plant-solution system with and without humic acid

Aqueous iodine (I_2(aq)) is a potent disinfectant that is being evaluated as a soil sanitizer for agricultural fields and a water purification treatment for the International Space Station. Rice (Oryza sativa L.) plants were grown in solution culture containing different I compounds at approximately 0, 18, or 30 μM total I [I_2(aq) + iodide (I⁻)] consisting of 0, 6, and 20 μM I as I_2(aq), respectively. In addition, humic acid (HA) was added to half the treatments. Most I_2(aq) was electrochemically reduced to the endpoint metabolite I⁻ within 24 h with HA promoting the response. Plants receiving the highest dose of I_2(aq), particularly those in treatments without HA, had the least growth and the greatest biomass I concentrations. Roots from both I_2(aq) treatments without HA were periodically sampled for bacteria. Viable and direct caints of bacterial cell density declined with increasing I_2(aq) concentrations within the first hour after treatment application. However, cell densities recovered within 96 hours and eventually surpassed the control treatment cell density. Additionally, the resulting high viable: direct count density ratio suggests that opportunistic species likely dominated the post I_2(aq) environment.