Cadmium tolerance and removal from Cunninghamella elegans related to the polyphosphate metabolism

Insights into the beneficial roles of dark septate endophytes in plants under challenging environment: resilience to biotic and abiotic stresses

Dark septate endophytes (DSE) exert a plethora of effects in regulating plant growth, signalling and stress tolerance. The advent of metagenomics has led to the identification of various species of DSE to be associated with plant organs. They are known to modulate growth, nutrient uptake, phytohormone biosynthesis and production of active bioconstituents in several plants. The interactions between the DSE and host plants are mostly mutualistic but they can also be neutral or exhibit negative interactions. The DSE has beneficial role in removal/sequestration of toxic heavy metals from various environmental sites. Here, we discuss the beneficial role of DSE in enhancing plant tolerance to heavy metal stress, drought conditions, high salinity and protection from various plant pathogens. Furthermore, the underlying mechanism of stress resilience facilitated by DSE-plant interaction has also been discussed. The article also provides insights to some important future perspectives associated with DSE-mediated phytoremediation and reclamation of polluted land worldwide thus facilitating sustainable agriculture.

Iron stress response and bioaccumulation potential of three fungal strains isolated from sewage-irrigated soil

AIMS

Contamination with heavy metal (HM) is a severe environmental issue. Therefore, there is a pressing need to create environmentally safe and cost-effective HM bioremediation approaches.

METHODS AND RESULTS

Three iron-tolerant fungal strains were isolated from sewage-irrigated soils, molecularly identified and deposited in the GenBank as Aspergillus flavus MT639638, A. terreus MT605370 and Fusarium oxysporum MT605399. The fungal growth, minimum inhibitory concentration (MIC), tolerance index (TI), removal efficiency, bioaccumulation, and enzymatic and non-enzymatic antioxidants were determined. Based on MIC values, A. flavus MT639638 was the most resistant strain. F. oxysporum displayed the highest percent removal efficiency (93.65% at 4000 mg L^-1 ) followed by A. flavus (92.92%, at 11,000 mg L^-1 ), and A. terreus (91.18% at 3000 mg L^-1 ). F. oxysporum was selected based on its highly sensitivity for further characterization of its response to Fe(II) stress using TEM, SEM and EDX, in addition to HPLC analysis of organic acids. These analyses demonstrated the localization of bioaccumulated Fe(II) and ultrastructural changes induced by iron and indicated induction release of organic acids.

CONCLUSIONS

Our fungal strains showed an effective capacity for removal of Fe(II) via bioaccumulation and biosorption mechanisms which were supported by instrumental analyses. The iron tolerance potentiality was mediated by induction of selected antioxidative enzymes and biomolecules.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study depicts a potential utilization of the three fungal strains for the bioremediation of iron-contaminated soils.

Long-term acclimation to cadmium exposure reveals extensive phenotypic plasticity in Chlamydomonas

Increasing industrial and anthropogenic activities are producing and releasing more and more pollutants in the environment. Among them, toxic metals are one of the major threats for human health and natural ecosystems. Because photosynthetic organisms play a critical role in primary productivity and pollution management, investigating their response to metal toxicity is of major interest. Here, the green microalga Chlamydomonas (Chlamydomonas reinhardtii) was subjected to short (3 d) or chronic (6 months) exposure to 50 µM cadmium (Cd), and the recovery from chronic exposure was also examined. An extensive phenotypic characterization and transcriptomic analysis showed that the impact of Cd on biomass production of short-term (ST) exposed cells was almost entirely abolished by long-term (LT) acclimation. The underlying mechanisms were initiated at ST and further amplified after LT exposure resulting in a reversible equilibrium allowing biomass production similar to control condition. This included modification of cell wall-related gene expression and biofilm-like structure formation, dynamics of metal ion uptake and homeostasis, photosynthesis efficiency recovery and Cd acclimation through metal homeostasis adjustment. The contribution of the identified coordination of phosphorus and iron homeostasis (partly) mediated by the main phosphorus homeostasis regulator, Phosphate Starvation Response 1, and a basic Helix-Loop-Helix transcription factor (Cre05.g241636) was further investigated. The study reveals the highly dynamic physiological plasticity enabling algal cell growth in an extreme environment.

Bioremediation and tolerance of zinc ions using Fusarium solani

Evaluating the mechanism of tolerance and biotransformation Zn(II) ions by Fusarium solani based on the different physiological was the objective of this work. The physical properties of synthesized ZnONPs was determined by UV-spectroscopy, transmission electron microscope, and X-ray powder diffraction. The structural and anatomical changes of F. solani in response to Zn(II) was examined by TEM and SEM. From the HPLC profile, oxalic acid by F. solani was strongly increased by about 10.5 folds in response to 200 mg/l Zn(II) comparing to control cultures. The highest biosorption potential were reported at pH 4.0 (alkali-treated biomass) and 5.0 (native biomass), at 600 mg/l Zn(II) concentration, incubation temperature 30 °C, and contact time 40 min (alkali-treated biomass) and 6 h (native biomass). From the FT-IR spectroscopy, the main functional groups implemented on this remediation were C-S stretching, C=O C=N, C-H bending, C-N stretching and N-H bending. From the EDX spectra, fungal cellular sulfur and phosphorus compounds were the mainly compartments involved on ZN(II) binding.

Mechanistic insight to mycoremediation potential of a metal resistant fungal strain for removal of hazardous metals from multimetal pesticide matrix

Fungi have an exceptional capability to flourish in presence of heavy metals and pesticide. However, the mechanism of bioremediation of pesticide (lindane) and multimetal [mixture of cadmium (Cd), chromium (Cr), copper (Cu), nickel (Ni), lead (Pb), zinc (Zn)] by a fungus is little understood. In the present study, Aspergillus fumigatus, a filamentous fungus was found to accumulate heavy metals in the order [Zn(98%)>Pb(95%)>Cd(63%)>Cr(62%)>Ni(46%)>Cu(37%)] from a cocktail of 30 mg L^-1 multimetal and lindane (30 mg L^-1) in a composite media amended with 1% glucose. Particularly, Pb and Zn uptake was enhanced in presence of lindane. Remarkably, lindane was degraded to 1.92 ± 0.01 mg L^-1 in 72 h which is below the permissible limit value (2.0 mg L^-1) for the discharge of lindane into the aquatic bodies as prescribed by European Community legislation. The utilization of lindane as a cometabolite from the complex environment was evident by the phenomenal growth of the fungal pellet biomass (5.89 ± 0.03 g L^-1) at 72 h with cube root growth constant of fungus (0.0211 g^1/3 L^-1/3 h^-1) compared to the biomasses obtained in case of the biotic control as well as in presence of multimetal complex without lindane. The different analytical techniques revealed the various stress coping strategies adopted by A. fumigatus for multimetal uptake in the simultaneous presence of multimetal and pesticide. From the Transmission electron microscope coupled energy dispersive X-ray analysis (TEM-EDAX) results, uptake of the metals Cd, Cu and Pb in the cytoplasmic membrane and the accumulation of the metals Cr, Ni and Zn in the cytoplasm of the fungus were deduced. Fourier-transform infrared spectroscopy (FTIR) revealed involvement of carboxyl/amide group of fungal cell wall in metal chelation. Thus A. fumigatus exhibited biosorption and bioaccumulation as the mechanisms involved in detoxification of multimetals.

Tolerance and mycoremediation of silver ions by Fusarium solani

Silver ions discharged from various industries, are potentially toxic to living organisms at low concentrations, thus, there is an increasing need for development of an eco-friendly and cost-effective approach for its bioremediation. Filamentous fungi especially, Fusarium solani displayed a strong resistance to copper and cadmium ions as revealed from our previous study (El-Sayed 2014), however, the mechanisms of silver resistance by this fungus has not been resolved yet. Thus, this study was an extension to our previous work, to elucidate the mechanism of silver ions resistance and biotransformation by F. solani. The growth, bioaccumulation, thiol, total antioxidant, malondialdehyde (MDA), hydrogen peroxide (H₂O₂) contents and polyphenol oxidase (PPO) and catalase (CAT) activities of F. solani in response to silver ions were determined. Production and bioaccumulation of silver nanoparticles was characterized by UV-visible spectroscopy, TEM, and X-ray powder diffraction (XRD). The ultrastructural changes of F. solani induced by Ag(I) was examined by TEM and SEM. Production of oxalic acid by F. solani was increased by about 343.8% in response to 400 mg/l Ag(I), compared to control cultures (without silver ions) as revealed from HPLC analysis. The maximum biosorption levels by the native and alkali-treated biomass were carried out at pH 5.0, initial metal concentration 200 mg/l, biomass 0.5 g/l, temperature 35 °C, and contact time 1 h (native biomass) and 3 h (alkali-treated biomass). Fourier transform infrared spectroscopy (FTIR) results revealed that the main functional groups involved on this mycoremediation were C–S stretching, C=O C=N, C – H bending, C–N stretching and N–H bending. EDX spectra indicated the involvement of fungal cellular sulfur and phosphorus compounds in Ag(I) binding.

Subcellular distribution, chemical forms, and physiological response to cadmium stress in Hydrilla verticillata

This study investigated the subcellular distribution and chemical forms of cadmium (Cd) in Hydrilla verticillata and the physiological mechanism underlying H. verticillata responses to Cd stress. Hydrilla verticillata was grown in a hydroponic system and was treated with various Cd concentrations (0, 10, 50, 100, 125, and 150 µM) for 7 days. Cadmium analysis of the leaves at the subcellular level showed that Cd was mainly stored in the soluble fraction (77.98-83.62%) and in smaller quantities in the cell wall fraction (11.99-17.30%) and the cell organelles (4.30-4.88%). The Cd taken up by H. verticillata was in different chemical forms. In the leaves and stems, the Cd was mostly extracted using 1 M NaCl and smaller amounts of Cd were extracted using 2% acetic acid. The malondialdehyde content significantly increased at all Cd concentrations, which indicated oxidative stress. The superoxide dismutase, guaiacol peroxidase, and catalase activities were enhanced. The proline, ascorbate, and glutathione contents increased at lower Cd concentrations, but decreased consistently as the Cd concentration rose. These results suggest that H. verticillata can be successfully used in the phytoremediation of Cd-contaminated water.

Molecular basis of Cd+2 stress response in Candida tropicalis

This study examines the bioremediation potential and cadmium-induced cellular response on a molecular level in Candida tropicalis 3Aer. Spectroscopic analysis clearly illustrated the involvement of yeast cell wall components in biosorption. Cadmium bioaccumulation was confirmed by TEM, SEM, and EDX examination. TEM images revealed extracellular as well as cytoplasmic and vacuolar cadmium nanoparticle formation, further validated by presence of ycf1 gene and increased biosynthesis of GSH under cadmium stress. Fourteen proteins exhibited differential expression and during cellular redox homeostasis are found to involve in nitrogen metabolism, nucleotide biosynthesis, and carbohydrate catabolism. Interestingly, C. tropicalis 3Aer is equipped with nitrile hydratase enzyme, rarely been reported in yeast. It has the potential to remove nitriles from the environment. The Cd⁺² toxicity not only caused growth stasis but also upregulated the cysteine biosynthesis, protein folding and cytoplasmic detoxification response elements. The present study suggests that C. tropicalis 3Aer is a potential candidate for bioremediating environmental pollution by Cd⁺².

Subcellular distribution and chemical forms of cadmium in a dark septate endophyte (DSE), Exophiala pisciphila

Our objective was to understand the cadmium (Cd) tolerance mechanisms by investigating the subcellular distribution, chemical forms of Cd and adsorptive groups in the mycelia of Exophiala pisciphila. We grew E. pisciphila in the liquid media with increasing Cd concentrations (0, 25, 50, 100, 200, and 400 mg L(-1)). Increased Cd in the media caused a proportional increase in the Cd uptake by E. pisciphila. Subcellular distribution indicated that 81 to 97% of Cd was associated with the cell walls. The largest amount and proportion (45-86%) of Cd was extracted with 2% acetic acid, and a concentration-dependent extraction was observed, both of which suggest that Cd-phosphate complexes were the major chemical form in E. pisciphila. A large distribution of phosphate and Cd on the mycelia surface was observed by scanning electron microscopy-energy dispersive spectrometer (SEM-EDS). The precipitates associated with the mycelia were observed to contain Cd by transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDX). Fourier transform infrared (FTIR) identified that hydroxyl, amine, carboxyl, and phosphate groups were responsible for binding Cd. We conclude that Cd associated with cell walls and integrated with phosphate might be responsible for the tolerance of E. pisciphila to Cd.

A review on progress of heavy metal removal using adsorbents of microbial and plant origin

Heavy metals released into the water bodies and on land surfaces by industries are highly toxic and carcinogenic in nature. These heavy metals create serious threats to all the flora and fauna due to their bioaccumulatory and biomagnifying nature at various levels of food chain. Existing conventional technologies for heavy metal removal are witnessing a downfall due to high operational cost and generation of huge quantity of chemical sludge. Adsorption by various adsorbents appears to be a potential alternative of conventional technologies. Its low cost, high efficiency, and possibility of adsorbent regeneration for reuse and recovery of metal ions for various purposes have allured the scientists to work on this technique. The present review compiles the exhaustive information available on the utilization of bacteria, algae, fungi, endophytes, aquatic plants, and agrowastes as source of adsorbent in adsorption process for removal of heavy metals from aquatic medium. During the last few years, a lot of work has been conducted on development of adsorbents after modification with various chemical and physical techniques. Adsorption of heavy metal ions is a complex process affected by operating conditions. As evident from the literature, Langmuir and Freundlich are the most widely used isotherm models, while pseudo first and second order are popularly studied kinetic models. Further, more researches are required in continuous column system and its practical application in wastewater treatment.

Enhancement of Cunninghamella elegans UCP/WFCC 0542 biomass and chitosan with amino acid supply

Studies were carried out with Cunninghamella elegans UCP/WFCC 0542 to evaluate the effects of an abundant supply of amino acids, asparagine and corn steep liquor associated with sucrose on the production of biomass and chitosan by submerged fermentation. The concentrations of the components of the culture medium which were determined by a 2³ full factorial design evaluated the interactions and effects of the independent variables of the sucrose, asparagine and corn steep liquor in relation to carbon and nitrogen sources, on the production of chitosan regarding biomass. The best results were observed at the central point [asparagine 0.025%, sucrose 0.15% and 0.45% of corn steep liquor, ratio C:N=2:6], and produced maximum yields of 16.95 g/L biomass and 2.14 g/L chitosan, after 96 h of submerged fermentation. However, the lowest level of sucrose, asparagine and corn steep liquor produced a low amount of biomass (10.83 g/L) and chitosan (0.60g/L). The infrared spectrum absorption of the chitosan produced by C. elegans showed bands regarding OH-axial stretching between 3406 and 3432 cm⁻¹, superimposed on the NH stretching band with axial deformation of the amide C=O group at about 1639 cm⁻¹, NH angular deformation at approximately 1560 cm⁻¹; axial deformation of amide-CN at around 1421 cm⁻¹, symmetrical angular deformation in CH₃ at 1379 cm⁻¹, -CN axial deformation of amino groups from 1125 to 1250 cm⁻¹ and polysaccharide structure bands in the range of between 890–1150 cm⁻¹. The crystallinity index of chitosan was 60.92%, and its degree of deacetylation was 75.25%. A low percentage of a supply of sucrose and asparagine with corn steep liquor offered higher yields of biomass and chitosan production at low cost.