Summary Environmental microorganisms have been widely applied in heavy metal remediation. This study explored the mechanisms of lead tolerance of two typical filamentous fungi, Aspergillus niger and Penicillium oxalicum. It is shown that the mechanisms of reducing Pb toxicity by these two fungi have three major pathways. The secreted oxalic acid can react with Pb (II) to form insoluble Pb minerals, primarily lead oxalate. Then, the enhanced biosorption via forming new border of cell wall prevents the transportation of Pb (II) into hypha. In addition, the fungal activity could be maintained even at high Pb concentration due to the intracellular accumulation. It was confirmed that A. niger has the higher Pb tolerance (up to 1500 mg l−1 Pb level) compared with P. oxalicum (up to 1000 mg l−1). Meanwhile, Pb levels below 1000 mg l−1 partially stimulate the bioactivity of A. niger, which was confirmed by its elevated respiration (from 53 to 63 mg C l−1 medium h−1). This subsequently enhanced microbial functions of A. niger to resist Pb toxicity. A better understanding of Pb tolerance of these two fungi sheds a bright future of applying them to remediate lead‐contaminated environments.

A 5x2x3 factorial experiment was used to investigate the effects of 5 ME levels (12.55, 12.30, 12.05, 11.80, and 11.55 MJ/kg) supplemented with or without exogenous enzymes in diets of broilers on the nutrient digestibility and energy improving efficiency over the starter, grower, and finisher phases of growth. The results indicated that the apparent digestibility of DM decreased linearly with a reduction in the ME level in diets for the starter (R2=0.234, P<0.001) and grower (R2=0.362, P<0.001) phases, and increased with enzyme supplementation for all diets. The greatest improvement occurred in the diet with the lowest ME level. The AME value also decreased linearly with the reduction of ME level in diets (R2=0.418, P<0.001 for starter; R2=0.398, P<0.001 for grower; R2=0.097, P=0.027 for finisher). Enzyme supplementation enhanced the AME value of diets in the starter, grower, and finisher phases by 0.07-0.62, 0.15-0.56, and 0.12-0.43 MJ/kg, respectively, and the optimal improvement of AME value occurred when the ME of diet was 11.55 MJ/kg in the starter phase. The effects of enzyme addition on AME for the starter phase were significantly greater than for the other phases. A significant interaction between ME level and enzyme supplementation in growth stage (P<0.05) was observed. The retention of CP decreased linearly with the reduction of ME level in diets (R2=0.245, P<0.001 for starter; R2=0.367, P<0.001 for grower). The retention of CP was increased by enzyme supplemented into the diets with ME levels of 11.55 and 11.80 MJ/ kg. Together, our results suggested that the ME level of diet affected the digestibility of DM, energy, and CP, and enzyme supplementation improved energy digestibility in diets with lower levels of ME.

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