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Soiling of photovoltaic (PV) systems compromises their performance causing a significant power loss and demanding periodical cleaning actions. This phenomenon raises great concerns in the solar energy field, thus leading to notable research efforts over the last decades. Soiling is caused by a dual action of dust deposition and biofouling. However, surprisingly, the microbiological contribution to PV soiling is often overlooked or underestimated. In this study, a variety of qPCR-based methods have been developed to quantify the microbial load of fungi, bacteria and phototrophs on PV panels. These protocols were evaluated by comparison with culturedependent methods, and were implemented with real solar plants for two years. The results show that the developed molecular methods are highly sensitive and reliable to monitor the microbial component of the soiling. Fungal biomass was clearly dominant in all analysed PV modules, while bacteria and phototrophs showed much lower abundance. Light microscopy and qPCR results revealed that melanised microcolonial fungi and phototrophs are the main biofilm-forming microorganisms on the studied solar panels. In particular, the fungal qPCR protocol is proposed as a useful tool for monitoring of PV soiling, and investigating the microbial contribution to specific soiling cases.
Graphene and its derivatives have recently attracted much attention for sensing and deactivating pathogens. However, the mechanism of multivalent interactions at the graphene–pathogen interface is not fully understood. Since different physicochemical parameters of graphene play a role at this interface, control over graphene’s structure is necessary to study the mechanism of these interactions. In this work, different graphene derivatives and also zwitterionic graphene nanomaterials (ZGNMs) were synthesized with defined exposure, in terms of polymer coverage and functionality, and isoelectric points. Then, the switchable interactions of these nanomaterials with E. coli and Bacillus cereus were investigated to study the validity of the generally proposed “trapping” and “nano-knives” mechanisms for inactivating bacteria by graphene derivatives. It was found that the antibacterial activity of graphene derivatives strongly depends on the accessible area, i.e. edges and basal plane of sheets and tightness of their agglomerations. Our data clearly confirm the authenticity of “trapping” and “nano-knives” mechanisms for the antibacterial activity of graphene sheets.
Microbial activity and functioning in soils are strongly limited by carbon (C) availability, of which a great proportion is released by living roots. Rhizodeposition and especially root exudates stimulate microbial activity and growth, and may shift the stoichiometric balance between C, N, and P. Thereby, exudates heighten microbial nutrient demand and acquisition of N and P from organic matter, leading to an increase in enzyme production. Aim of this study was to determine environmental controls of extracellular enzyme production, and hence on potential enzyme activities (Vmax) and substrate affinities (Km). To determine the controlling factors, we worked on four spatial scales from the microscale (i.e. rhizosphere) through the mesoscale (i.e. soil depth) and landscape scale (relief positions), and finally to the continental scale (1200 km transect within the Coastal Cordillera of Chile). Kinetics of seven hydrolyzing enzymes of the C, N, and P cycles (cellobiohydrolase, β‑glucosidase, β‑xylosidase, β‑N‑acetylglucosaminidase, leucine‑aminopeptidase, tyrosine‑aminopeptidase, and acid phosphatase) were related to soil texture, C and N contents, pH, and soil moisture via redundancy analysis (RDA). Potential activities of C, N, and P acquiring enzymes increased up to 7-times on the continental scale with rising humidity of sites and C and N contents, while substrate affinities simultaneously declined. On the landscape scale, neither Vmax nor Km of any enzyme differed between north and south slopes. From top- to subsoil (down to 120 cm depth) potential activities decreased (strongest of aminopeptidases under humid temperate conditions with up to 90%). Substrate affinities, however, increased with soil depth only for N and P acquiring enzymes. Affinities of cellobiohydrolase and β‑xylosidase, on the contrary, were 1.5- to 3-times higher in top- than in subsoil. Potential activities of N and P acquiring enzymes and β‑glucosidase increased form bulk to roots. Simultaneously, substrate affinities of N and P acquiring enzymes declined, whereas affinities of β‑glucosidase increased. These trends of activities and affinities in the rhizosphere were significant only for acid phosphatase. The RDA displayed a strong relation of potential activities of C and P acquiring enzymes and β‑N‑acetylglucosaminidase to C and N contents in soil as well as to the silt and clay contents. Aminopeptidase activity was mainly dependent on soil moisture and pH. We conclude that substrate availability for microorganisms mainly determined enzyme activity patterns on the continental scale by the humidity gradient. Patterns on the meso- and microscale are primarily controlled by nutrient limitation, which is induced by a shift of the stoichiometric balance due to input of easily available C by roots in the rhizosphere.
In a controlled growth experiment we found that the cyanobacterium Nostoc punctiforme has a bulk cell 26Mg/24Mg ratio (expressed as δ26Mg) that is −0.27‰ lower than the growth solution at a pH of ca. 5.9. This contrasts with a recently published δ26Mg value that was 0.65‰ higher than growth solution for the black fungus Knufia petricola at similar laboratory conditions, interpreted to reflect loss of 24Mg during cell growth. By a mass balance model constrained by δ26Mg in chlorophyll extract we inferred the δ26 Mg value of the main Mg compartments in a cyanobacteria cell: free cytosolic Mg (−2.64‰), chlorophyll (1.85‰), and the nonchlorophyll-bonded Mg compartments like ATP and ribosomes (−0.64‰). The lower δ26Mg found in Nostoc punctiforme would thus result from the absence of significant Mg efflux during cell growth in combination with either (a) discrimination against 26Mg during uptake by desolvation of Mg or transport across protein channels or (b) discrimination against 24Mg in the membrane transporter during efflux. The model predicts the preferential incorporation of 26Mg in cells and plant organs low in Mg and the absence of isotope fractionation in those high in Mg, corroborated by a compilation of Mg isotope ratios from fungi, bacteria, and higher plants.
The development of a test to evaluate the degradation of semi-volatile fuels as diesel by microorganisms is presented. This method is based on the principles described in the CEC-L-103 Standard procedure that is exclusively meant for testing the biodegradability of non-volatile lubricants. Therefore, significant modifications involve aseptic conditions for testing specific microorganisms and conducting the test in closed vessels avoiding evaporation losses, while fuel quantification using gas chromatography-flame ionization detection (GC-FID) is retained. It is suggested that the modified procedure should enable routine application for semi-volatile hydrocarbon-based fuels. GC-FID provides additionally valuable information on the alteration of fuel component patterns during biodegradation. The procedure was successfully tested using two bacteria (Pseudomonas aeruginosa and Sphingomonas sp.) and two yeasts (Moesziomyces sp. and Candida sp.) isolated from real diesel contamination cases. All tested microorganisms caused a significant degradation of diesel fuel achieving hydrocarbon degradation percentages ranging from 23% to 35%. Specific aspects on the test modification and prospects for further modification regarding targeted investigations in the field of fuel contamination by microorganisms are briefly discussed.
How the rock-inhabiting fungus K. petricola A95 enhances olivine dissolution through attachment
(2020)
Free-living and mycorrhizal fungi are able to enhance the weathering of rock and other solid substrates. Deciphering the exact mechanisms of these natural processes requires their experimental simulation. Moreover, by performing these simulations with genetically amenable rock-weathering fungi, one can knock-out certain fungal traits and consequently identify their weathering-relevant function. Here, the effect of the rock-inhabiting fungus, Knufia petricola A95, on the dissolution kinetics of an Fe-bearing olivine (Mg1.86Fe0.19SiO4) is investigated at 25 °C and pH 6 using reproducible batch and mixed flow experiments. The availability of a melanin-deficient mutant (ΔKppks) of K. petricola A95, which produces more extracellular polymeric substances (EPS) than the wild type (WT), enables the comparative study of the role of melanin and EPS in olivine dissolution. In abiotic dissolution experiments, the olivine dissolution rate decreased considerably over time at pH 6 but not at pH 3.5. This inhibition of abiotic olivine dissolution at pH 6 was most likely caused by the in-situ oxidation of ferrous Fe and/or the precipitation of ferric hydroxides at the olivine surface. In corresponding biotic experiments at pH 6, both the wild type K. petricola and its melanin-deficient mutant ΔKppks solubilised and bound significant amounts of Fe released by olivine dissolution. Fe oxidation and precipitation were thus prevented and olivine dissolution proceeded faster than in the abiotic experiments. By sequestering Fe directly at the olivine surface, the attached wild type K. petricola cells were particularly efficient at preventing the oxidation of Fe at the mineral surface: the slowdown of olivine dissolution almost completely disappeared. The attachment capacity of these wild type cells is most likely mediated by wild type-specific EPS. Our presented experimental systems allow the oxidation of mineral-released Fe and include a rock-inhabiting fungus, thus simulating chemical, physical and biological conditions that set dissolution rates in a way that is relevant to natural ecosystems.
Stress is a normal part of life for fungi, which can survive in environments considered inhospitable or hostile for other organisms. Due to the ability of fungi to respond to, survive in, and transform the environment, even under severe stresses, many researchers are exploring the mechanisms that enable fungi to adapt to stress. The International Symposium on Fungal Stress (ISFUS) brings together leading scientists from around the world who research fungal stress. This article discusses presentations given at the third ISFUS, held in Sao Jose dos Campos, Sao Paulo, Brazil in 2019, thereby summarizing the state-of-the-art knowledge on fungal stress, a field that includes microbiology, agriculture, ecology, biotechnology, medicine, and astrobiology.
Fungi that share light-flooded habitats with phototrophs may profit from their excess photosynthetic products. But to cope with sunlight-associated stresses [e.g. high temperatures, UV radiation with associated DNA damage, accumulation of reactive oxygen species (ROS), desiccation and osmotic stresses] it is important for fungi to accurately sense and respond to changes in light. To test the hypothesis that light is an environmental cue that Ascomycota use to coordinate growth, stress responses as well as to establish pathogenic or symbiotic relationships, the photoreceptor (PR) distribution in species from different ecological niches was analysed. The genomes of black [dihydroxynaphthalene (DHN) melanin-containing] fungi from phyllosphere and exposed solid surfaces contain multiple photoreceptors (PRs). The plant pathogen Botrytis cinerea (Leotiomycetes) has a highly sophisticated photosensory and signalling system that helps to avoid light and to locate susceptible hosts. Rock-inhabiting Dothideomycetes and Eurotiomycetes including Knufia petricola possess equal numbers ofPRs along with the same set of protective pigments. This similarity between black fungi from plant and rock surfaces suggests that photoperception and -regulation are important for fungi that receive nutrients through cooperation with phototrophs. Genetic tools for manipulating K. petricola exist and will be used to test this idea.
Microcolonial black fungi are a group of ascomycetes that exhibit high stress tolerance, yeast-like growth and constitutive melanin formation. They dominate a range of hostile natural and man-made environments, from desert rocks and salterns to dishwashers, roofs and solar panels. Due to their slow growth and a lack of genetic tools, the underlying mechanisms of black fungi’s phenotypic traits have remained largely unexplored. We chose to address this gap by genetically engineering the rock-inhabiting fungus Knufia petricola (Eurotiomycetes, Chaetothyriales), a species that exhibits all characteristics of black fungi. A cell biological approach was taken by generating K. petricola strains expressing green or red fluorescent protein variants. By applying: (1) traditional gene replacement; (2) gene editing and replacement via plasmid-based or ribonucleoprotein (RNP)-based CRISPR/Cas9, and (3) silencing by RNA interference (RNAi), we constructed mutants in the pathways leading to melanin, carotenoids, uracil and adenine. Stable single and double mutants were generated with homologous recombination (HR) rates up to 100%. Efficient, partially cloning-free strategies to mutate multiple
genes with or without resistance cassettes were developed. This state-of-the-art genetic toolkit, together with the annotated genome sequence of strain A95, firmly established K. petricola as a model for exploring microcolonial black fungi.
Low-molecular-weight organic acids (LMWOAs) are crucial for the mobilization and acquisition of mineral phosphorus by plants. However, the role of LMWOAs in mobilizing organic phosphorus, which is the predominant phosphorus form in at least half of the world’s ecosystems, especially in humid climates, is unclear. The mechanisms of phosphorus mobilization by LMWOAs depend on climate, mainly precipitation, and shape the phosphorus nutrition strategies of plants. We disentangled the impact of roots and associated microorganisms on mechanisms of phosphorus cycling mediated by LMWOAs by studying soils along an ecosystem-sequence (ecosequence) from arid shrubland (~70 mm yr-1), and Mediterranean woodland (~370 mm yr-1) to humid-temperate forest (~1470 mm yr-1). Phosphorus speciation in soil was examined by X-ray absorption near edge structure analysis (XANES). LMWOAs were quantified as biological rock-weathering and organic phosphorus mobilization agents and compared to kinetics of acid phosphatase as a proxy for organic phosphorus mineralization. Calcium-bound phosphorus in topsoils decreased from 126 mg kg-1 in the arid shrubland, to 19 mg kg-1 in the Mediterranean woodland and was undetectable in the humid-temperate forest. In contrast, organic phosphorus in topsoils in close root proximity (0–2 mm distance to roots) was absent in the arid shrubland but raised to 220 mg kg-1 in the Mediterranean woodland and to 291 mg kg-1 in the humid-temperate forest. The organic phosphorus content in topsoils was 1.6 to 2.4 times higher in close root proximity (0–2 mm distance to roots) compared to bulk soil (4–6 mm distance to roots) in the Mediterranean woodland and humid-temperate forest, showing intensive phosphorus bioaccumulation in the rhizosphere. Redundancy analysis (RDA) revealed that LMWOAs were explained by the content of hydroxyapatite and variscite phosphorus-species in the arid shrubland, indicating that LMWOAs contribute to mineral weathering in this soil. LMWOA contents, phosphatase activity, and microbial biomass carbon correlated strongly with organic phosphorus in the humid-temperate forest soil, which implies a high relevance of LMWOAs for organic phosphorus recycling. In the Mediterranean woodland soil, however, oxalic acid correlated with organic phosphorus in the topsoil (suggesting phosphorus recycling), whereas in the subsoil malic and citric acid were correlated with primary and secondary phosphorus minerals (implying mineral weathering). We conclude that phosphorus acquisition and cycling depend strongly on climate and that the functions of LMWOAs in the rhizosphere change fundamentally along the precipitation gradient. In the arid shrubland LMWOAs facilitate biochemical weathering (rock eating), while in the humid-temperate forest their functions change towards supporting organic phosphorus recycling (vegetarian).