Adaptability assessment of Aspergillus niger and Aspergillus terreus isolated from long-term municipal/industrial effluent-irrigated soils to cadmium stress

Abstract

Heavy metals (HMs) contamination is a major issue produced by industrial and mining processes, among other human activities. The capacity of fungi to eliminate HMs from the environment has drawn attention. However, the main process by which fungi protect the environment against the damaging effects of these HMs, such as cadmium (Cd), is still unknown. In this study, some fungi were isolated from HMs-polluted soil. The minimum inhibitory concentrations (MICs) and the tolerance indices of the tested isolates against Cd were evaluated. Moreover, molecular identification of the most tolerant fungal isolates (Aspergillus niger and A. terreus) was done and deposited in the GenBank NCBI database. The results showed that the colony diameter of A. niger and A. terreus was decreased gradually by the increase of Cd concentration. Also, all the tested parameters were influenced by Cd concentration. Lipid peroxidation (MDA content) was progressively increased by 12.95-105.95% (A. niger) and 17.27-85.38% (A. terreus), respectively, from 50 to 200 mg/L. PPO, APX, and POD enzymes were elevated in the presence of Cd, thus illustrating the appearance of an oxidative stress action. Compared to the non-stressed A. niger, the POD and PPO activities were enhanced by 92.00 and 104.24% at 200 mg/L Cd. Also, APX activity was increased by 58.12% at 200 mg/L. Removal efficiency and microbial accumulation capacities of A. niger and A. terreus have also been assessed. Production of succinic and malic acids by A. niger and A. terreus was increased in response to 200 mg/L Cd, in contrast to their controls (Cd-free), as revealed by HPLC analysis. These findings helped us to suggest A. niger and A. terreus as the potential mycoremediation microbes that alleviate Cd contamination. We can learn more about these fungal isolates' resistance mechanisms against different HMs through further studies.

Keywords: A. terreus; Aspergillus niger; Antioxidant enzymes; Bioaccumulation capacity; Heavy metals; Tolerance index.

Fig. 1

The preliminary qualitative screening of fungal isolates on Cd (100 mg/L)- supplemented agar plates. Notice that isolates number 3 and 8 were the most tolerant to 100 mg/L CdCl₂ as compared to the others

Fig. 2

(A) and (D) show the growth of A. niger and A. terreus, respectively on PDA media supplemented with CdCl₂ (100 mg/L), (B) and (E) show the morphological normal untreated hypha (Control) of A. niger and A. terreus, (C) and (F) show morphological abnormalities in the fungal mycelia as intensively swelling deformed mycelium of A. niger and segmented deformed mycelium of A. terreus under Cd stress. Picture was taken after 7 days of incubation at 28 °C

Fig. 3

The phylogenetic tree of the 18 S rRNA genes for A. niger (A) and A. terreus (B) and the others presented on GenBank based on the DNA sequence

Fig. 4

Effect of different Cd conc. on the bioaccumulation capacity (Q) and removal efficiency (Re%) of A. niger and A. terreus on PDB medium after 7 days

Fig. 5

Effect of different Cd conc. on MDA and enzymatic antioxidants (POD, APX and PPO) enzymes activities of A. niger and A. terreus after 7 days

Fig. 6

Effect of different Cd conc. on non-enzymatic antioxidants (proline, total soluble carbohydrates [TSC], total soluble protein [TSP]), total thiol, non-protein thiol and protein thiol of A. niger and A. terreus

Fig. 7

HPLC chromatograms of Cd-free culture filtrate (control) of A. niger and A. terreus (A and C; respectively)) and Cd-supplemented culture filtrate (200 mg/L) of A. niger and A. terreus (B and D; respectively)