The effect of cadmium on earthworms (Eisenia andrei) and their intestinal bacteria

Abstract

Cadmium contamination, predominantly from anthropogenic activities such as mining, have a significant impact on soil organisms. It alters the abundance, diversity, community structure, ecological functions and the species present in the soil. The reproduction of earthworms are adversely affected by very low cadmium concentrations. Consequently, earthworms are recognised bioindicators of cadmium contamination in soil. Soil bacterial communities also change dramatically with cadmium contamination. As bacteria provide essential molecules through their metabolic processes, a disturbance in bacterial community structure and function does have significant implications on other organisms. Both earthworms and bacteria have methods of resisting cadmium toxicity. Some bacteria are known for their ability to bind and detoxify cadmium, not only in soil but also within organisms. In this manner, intestinal bacteria may contribute to the cadmium resistance observed in earthworms after long-term exposure. This study aimed to determine if Eisenia andrei acquired cadmium resistance after long-term exposure in comparison to E. andrei that have not had long-term exposure. The study compared earthworm resistance in terms of; mortality, reproduction, cadmium body burden and, aerobically culturable bacteria present in their casts. Bacterial results were compared in relation to: bacterial levels, diversity of the morphologically distinct culturable bacteria colonies, the species present in the casts and the overall Gram-positive to Gram-negative ratios. The OECD guidelines for testing the effects of chemicals on earthworms was utilised to compare the earthworm resistance over a range of 10 to 400 μg Cd2+ g-1 after four weeks. Thereafter, the bacteria present in the casts were cultured on soil and nutrient agar augmented with a range of 10 to 400 μg Cd2+ L-1. The cocoons produced during the four weeks and the juveniles that emerged after an additional four weeks were counted to determine difference in reproductive output. Bacterial levels and morphologically distinct colony diversity was compared from the cultured bacteria. The 16S rDNA from the cultured bacteria was amplified, sequenced and compared to GenBanks’ identified sequences for species identification. Earthworms under long-term exposure acquired significantly greater resistance to cadmium according to their reproductive output. Furthermore, they had significantly less cadmium body burden at the highest soil-cadmium concentration. The bacteria from the long-term exposure group required a greater concentration of cadmium to significantly reduce bacterial levels. There were no conclusive results about the difference in diversity of culturable bacteria from the earthworm casts. The highly cadmium resistant species isolated, are all known to be metal resistant. Different species were isolated at the highest cadmium concentration from the two groups. Cellulomonas persica and Bacillus subtilis were the only Gram-positive bacteria isolated at the highest cadmium concentration and both of these were isolated from the more resistant earthworm group. The implications are that earthworms that have acquired greater resistance to cadmium have a reduced cadmium body burden and since the overall cadmium resistance of the bacteria are greater and the community structure of resistant bacteria are different, it is concluded that intestinal bacteria may contribute to earthworm resistance.