Combined toxicity of Cd and aniline to soil bacteria varying with exposure sequence.

Joint toxicity of organic-metal co-contamination can vary depending on organisms, toxicants, and even the sequence of exposure. This study examines how the combined toxicity of aniline (An) and cadmium (Cd) to soil bacteria in microcosms changes when the order of contaminant introduction is altered. Through analyzing biodiversity, molecular ecological network, functional redundancy, functional genes and pathways, we find the treatment of Cd followed by An brings about the strongest adverse impact to the bacterial consortium, followed by the reverse-ordered exposure and the simple mixture of the two chemicals. On the level of individual organisms, exposure sequence also affects the bacteria that are otherwise resistant to the standalone toxicity of both An and Cd. The dynamic behavior of aniline-cadmium composite is interpreted by considering the tolerance of organisms to individual chemicals, the interactions of the two toxicants, the recovery time, as well as the priority effect. The overall effect of the composite contamination is conceptualized by treating the chemicals as environmental filters screening the growth of the community.