Recent advancements in cadmium-microbe interactive relations and their application for environmental remediation: a mechanistic overview

Reproductive strategy response of the fungi Sarocladium and the evaluation for remediation under stress of heavy metal Cd(II)

Cadmium (Cd) is documented as one of the most lethal metals and poses a major threat to all life forms in the environment due to its toxic effects. Bioremediation of hazardous metals has received considerable and growing interest over the years. The functional fungi with tolerance to the heavy metal Cd were screened from the mining soil samples. Two fungi isolates from coal mine soil were characterized as Sarocladium sp. M2 and Sarocladium sp. M6 based on morphological and partial ITS sequencing analysis. M2 and M6 exhibited high levels of resistance to cadmium, and they were investigated for their micro-morphology and application in heavy metal removal with different concentration Cd(II) (0, 50, 100, 150 and 200 mg/L). The colony morphology of M2 and M6 gradually become very similar to that of bacteria with the increase of cadmium concentration (150-200 mg/L). Micro-morphological studies showed that Cd(II) exposure caused the disappearance of conidial heads and the occurrence of hyphae breakage (100-200 mg/L Cd(II), which is consistent to the colony morphology results. The surface/volume ratio of the spores decreased with the presence of Cd(II). The removal potential of fungi for cadmium was quantified by atomic absorption spectrometry. M2 and M6 showed great potential as bioremediators for highly Cd(II)-contaminated environment. The highest Cd(II) biosorption capacity was 5.13 ± 0.21 mg/g for M2 and 6.04 ± 0.21 mg/g for M6. The highest heavy metal sorption by M2 removed 57.11% ± 4.45% Cd(II) while that of M6 removed 48.35% ± 1.44% Cd(II) in 200 mg/L initial concentration Cd(II). To the best of our knowledge, this is the first report that cadmium induced the change of reproduction mode of the Sarocladium, from conidia to arthrospores, which made the colony morphological modifications, from the fungi colony morphology to the bacteria colony morphology. The arthrospore-modified (hyphae breakage) seemed to accumulate greater amounts of heavy metals than filamentous hyphae formation.

The role of arbuscular mycorrhizal fungi in the alleviation of cadmium stress in cereals: A multilevel meta-analysis

The symbiotic relationships between crop species and arbuscular mycorrhizal fungi (AMF) are crucial for plant health, productivity, and environmental sustainability. The roles of AMF in reducing crop stress caused by cadmium (Cd) toxicity and in the remediation of Cd-contaminated soil are not fully understood. Here we report on a meta-analysis that sought to identify the functions of AMF in cereals under Cd stress. A total of 54 articles published between January 1992 and September 2022 were used to create the dataset, which provided 7216 data sets on mycorrhizal cereals under Cd stress examined. AMF effects on colonization rate, biomass, physiological level, nutritional level, and plant Cd level were measured using the logarithmic response ratio (Ln R). The results showed that AMF overall greatly reduced 5.14 - 33.6 % Cd stress on cereals in greenhouse experiments under controlled conditions. AMF colonization significantly stimulated crop biomass by 65.7 %, boosted the formation of photosynthetic pigments (23.2 %), and greatly increased plant nitrogen (24.8 %) and phosphorus (58.4 %) uptake. The dilution effect of mycorrhizal plants made the Cd concentration decline by 25.2 % in AMF plants compared to non-mycorrhizal ones. AMF also alleviated Cd stress by improving osmotic regulators (soluble protein, sugar, and total proline, from 14.8 to 36.0 %) and lowering the membrane lipid peroxidation product (MDA, 12.9 %). Importantly, the results from the random forest and model selection analysis demonstrated that crop type, soil characteristics, chemical form, and Cd levels were the main factors determining the function of AMF in alleviating Cd stress. Additionally, there was a significant interaction between AMF colonization rate and Cd addition, but their interactive effect was less than the colonization rate alone. This meta-analysis demonstrated that AMF inoculation could be considered as a promising strategy for mitigation of Cd stress in cereals.

Effects of different remediation methods on phosphorus transformation and availability

The effects of different heavy metal pollution remediation methods on soil nutrient transformation and soil health remain unclear. In this study, the effects of phytoextraction (PE) and passivation remediation (PR) on Cd-polluted soil phosphorus transformation and availability were compared by pot experiment. The results showed that PE significantly reduced the concentrations of total and available Cd (both H2O-Cd and DTPA-Cd) in soil, PR also decreased available Cd content but had no significant effect on total Cd content. PE slightly increased soil pH and NH4+-N content, while PR significantly increased soil pH, NO3--N and AK content. PE promoted the conversion of stable P (including HCl-Pi and residual-Pt), and increased the content of labile P (including H2O-Pi, NaHCO3-Pi and NaHCO3-Po) and the proportion of moderately labile P (including NaOH-Pi and NaOH-Po), while PR showed the opposite trend. PE showed a higher soil phoC gene abundance and acid phosphatase (ACP) activity, while PR showed a higher phoD gene copies and alkaline phosphatase (ALP) activity. Soil bacteria and phoD-harboring bacteria community was significantly affected by remediation methods and soil types. Compared with PR, PE reduced phoD-harboring bacterial diversity but significantly increased the abundance of genera associated with P dissolution (Streptomyces) and P conversion (Bradyrhizobium and Frankia), both of which were significantly positively correlated with labile P or moderately labile P. In general, compared with PR, PE can effectively remove soil Cd pollution, while maintaining a higher content of labile P and a higher proportion of moderately labile P, which can be considered as a green and sustainable remediation strategy conducive to soil quality.