Extracellular redox cycling and hydroxyl radical production occurs widely in lichenized Ascomycetes

Large differences in carbohydrate degradation and transport potential among lichen fungal symbionts

Lichen symbioses are thought to be stabilized by the transfer of fixed carbon from a photosynthesizing symbiont to a fungus. In other fungal symbioses, carbohydrate subsidies correlate with reductions in plant cell wall-degrading enzymes, but whether this is true of lichen fungal symbionts (LFSs) is unknown. Here, we predict genes encoding carbohydrate-active enzymes (CAZymes) and sugar transporters in 46 genomes from the Lecanoromycetes, the largest extant clade of LFSs. All LFSs possess a robust CAZyme arsenal including enzymes acting on cellulose and hemicellulose, confirmed by experimental assays. However, the number of genes and predicted functions of CAZymes vary widely, with some fungal symbionts possessing arsenals on par with well-known saprotrophic fungi. These results suggest that stable fungal association with a phototroph does not in itself result in fungal CAZyme loss, and lends support to long-standing hypotheses that some lichens may augment fixed CO₂ with carbon from external sources.

Lichen symbioses are thought to be stabilized by the transfer of fixed carbon from a photosynthesizing symbiont to a fungus. Here, Resl et al. show that, contrary to other fungal symbioses, fungal association with a phototroph in lichens does not result in loss of fungal enzymes for plant cell-wall degradation.

Fungal-Mineral Interactions Modulating Intrinsic Peroxidase-like Activity of Iron Nanoparticles: Implications for the Biogeochemical Cycles of Nutrient Elements and Attenuation of Contaminants

Fungal-mediated extracellular reactive oxygen species (ROS) are essential for biogeochemical cycles of carbon, nitrogen, and contaminants in terrestrial environments. These ROS levels may be modulated by iron nanoparticles that possess intrinsic peroxidase (POD)-like activity (nanozymes). However, it remains largely undescribed how fungi modulate the POD-like activity of the iron nanoparticles with various crystallinities and crystal facets. Using well-controlled fungal-mineral cultivation experiments, here, we showed that fungi possessed a robust defect engineering strategy to modulate the POD-like activity of the attached iron minerals by decreasing the catalytic activity of poorly ordered ferrihydrite but enhancing that of well-crystallized hematite. The dynamics of POD-like activity were found to reside in molecular trade-offs between lattice oxygen and oxygen vacancies in the iron nanoparticles, which may be located in a cytoprotective fungal exoskeleton. Together, our findings unveil coupled POD-like activity and oxygen redox dynamics during fungal-mineral interactions, which increase the understanding of the catalytic mechanisms of POD-like nanozymes and microbial-mediated biogeochemical cycles of nutrient elements as well as the attenuation of contaminants in terrestrial environments.

Oxidative stress in moss Bryum caespiticium (Bryaceae) under the influence of high temperature and light intensity in a technogenically transformed environment

Mosses are pioneer plants in post-technogenic areas. Therefore, the question of adaptive reactions of mosses from these habitats represents a scientific interest. The research is devoted to the study of adaptive changes in the metabolism of the dominant moss species Bryum caespiticium Hedw., collected in the devastated territories of the Novoyavorivsk State Mining and Chemical Enterprise (SMCE) “Sirka (Sulfur)” exposed to hyperthermia and insolation, which cause oxidative stress in plants. The influence of these stressors on the activity and thermal stability of antioxidant enzymes, hydrogen peroxide content, anion radical generation and accumulation of prooxidant components in moss shoots was studied. The activity and thermal stability of peroxidase and superoxide dismutase (SOD) were analysed forB. caespiticium moss from different locations of northern exposure at the sulfur mining dump No 1 in summer and autumn. We established the dependence of the activity of antioxidant enzymes of moss on the intensity of light and temperature on the experimental plots of the dump No 1. In summer, the highest activity and thermal stability rates of peroxidase and SOD were observed. Under the conditions of the experiment in shoots of В. caespiticium from the northern peak of the dump under the influence of 2 hours temperature action (+ 42 ºС) the most significant increase in peroxidase activity was found by 1.78 times and SOD by 1.89 times, as well as increase in its thermal stability by 1.35–1.42 times, respectively. The increase in peroxidase and SOD activity, as well as the increase in their thermal stability caused by hyperthermia were negated by pre-processing with a protein biosynthesis inhibitor cyclohexamide, which may indicate the participation of the protein-synthesizing system in this process. The effect of increasing the thermal stability of enzymes can be considered as a mechanism of adaptation of the protein-synthesizing system to the action of high temperatures. Increase in the activity and thermal stability of antioxidant enzymes is caused primarily by changes in the expression of stress protein genes, which control the synthesis of specific adaptogens and protectors. The obtained results indicate that the extreme conditions of the anthropogenically transformed environment contribute to the development of forms with the highest potential abilities. The mechanism of action of high temperatures is associated with the development of oxidative stress, which is manifested in the intensification of lipid peroxidation and the generation of superoxide anion radical. It was found that temperature stress and high insolation caused an increased generation of superoxide anion radical as the main inducers of protective reactions in the samples of B. caespiticium from the experimental transect of the sulfur mining heap. It is known that the synthesis of Н2О2 occurs under stress and is a signal to start a number of molecular, biochemical and physiological processes of cells, including adaptation of plants to extreme temperatures. It is shown that high temperatures initiate the generation of hydrogen peroxide. Increased reactive oxygen species (ROS) formation, including Н2О2, under the action of extreme temperatures, can cause the activation of signaling systems. Therefore, the increase in the content of Н2О2 as a signaling mediator is a component of the antioxidant protection system. It is determined that adaptive restructuring of the metabolism of the moss В. caespiticium is associated with the accumulation of signaling prooxidant components (diene and triene conjugates and dienketones). The increase in primary lipid peroxidation products, detected by us, under the action of hyperthermia may indicate the intensification of free radical oxidation under adverse climatic conditions in the area of the sulfur production dump, which leads to the intensification of lipid peroxidation processes. The accumulation of radical and molecular lipid peroxidation products are signals for the activation of protective systems, activators of gene expression and processes that lead to increased resistance of plants.

Role of quinone reductases in extracellular redox cycling in lichenized ascomycetes

Our previous work showed that many lichenized Ascomycetes can generate hydroxyl radicals using quinone-based extracellular redox cycling. During cycling, hydroquinones must be formed and subsequently regenerated from quinones using a quinone reductase (QR). However, we also showed that no simple correlation exists between QR activity and rates of hydroxyl radical formation. To further investigate the role of QR in hydroxyl radical formation, three model lichen species, Leptogium furfuraceum, Lasallia pustulata and Peltigera membranacea were selected for further investigation. All possessed QR activity and could metabolize quinones, and both Leptogium furfuraceum and Lasallia pustulata actively produced hydroxyl radicals. By contrast, P. membranacea produced almost no hydroxyl radicals, and although the lichen readily metabolized quinones, no hydroquinone production was detected. Peltigera had laccase (LAC) activity that was c. 50 times higher than in the other two species, suggesting that LAC rapidly oxidizes the hydroquinones, preventing radical formation deriving from auto-oxidation. It appears that in some lichens hydroxyl radical formation is blocked by the presence of high redox enzyme activity. QR from P. didactyla was studied further and found to display similar properties to the enzyme from free-living fungi, although it possessed an unusually high molecular mass (c. 62 kDa).

Sulfide drives hydroxyl radicals production in oxic ferric oxyhydroxides environments

Perturbation of Fe(III)-bearing oxic environments by reduced species such as sulfide occurs widely in natural and engineered systems. However, whether hydroxyl radicals (OH) can be produced in these environments remains unexplored. Here we show that sulfide drives OH production in Fe(III) oxyhydroxides suspensions under neutral and oxic conditions. For lepidocrocite, ferrihydrite and goethite suspensions at 11.2 mM Fe, the addition of 0.5 mM sulfide produced 14.2, 14.3 and 22.4 μM OH within 120 min, respectively. With addition of sulfide to lepidocrocite suspensions at 11.2 mM Fe, the cumulative OH concentration within 120 min increased from 0 to 14.2, 25.2, 52.6 and 63.1 μM when sulfide dosage increased from 0 to 0.5, 2.5, 5 and 7.5 mM, respectively. At a fixed sulfide dosage of 5 mM, the cumulative OH concentration increased with increasing the number of sulfide additions. The mechanisms of OH production were attributed to the generation of surface-bound Fe(II), most likely in the form of >Fe^IIOH₂⁺, and Fe(II) in the solid phase or FeS from the reactions between sulfide and Fe(III), followed by O₂ activation. OH production could take place until depletion of sulfide. Finally, we found that the generated OH could oxidize the coexisting redox-active substances like phenol under neutral and oxic conditions. Our findings reveal that sulfide perturbation of Fe(III)-bearing oxic environments is a new source of OH, and contaminants oxidation by OH necessitates consideration in these environments.