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Sub-aerial biofilms (SABs) are ubiquitous microbial communities that develop at the interface between hard surfaces and the atmosphere. Inherent SAB 'core-settlers' include phototrophic algae, cyanobacteria, heterotrophic bacteria and microcolonial fungi (MCF). SABs do not simply cover hard surfaces; they interact with them in myriads of ways and bind to the underlying substrate. Secretion of extracellular mucilage aids adhesion, while organic acids and acidic polysaccharides weather the surface. As protection against solar radiation, many members of the SAB consortia produce shielding pigments while the phototrophic inhabitants are laden with photosynthetic pigments. All absorb light of many wavelengths and in addition, the cells themselves scatter light. Both effects change the spectra of incoming radiation (including wavelengths that are converted to electricity by photovoltaic cells) and decrease its intensity. To quantify these effects on SABs as complex entities of organisms and pigments, we measured the spectral properties of model and natural biofilms transferred to glass. Here we show that SABs growing on solar panels and other substrates scatter incident radiation between 250 nm up to 1800 nm and block up to 70% of its transmission. Model biofilms have the advantage that their microbial components can be 'tuned' to resemble natural ones of different compositions thus providing a novel materials-testing tool.
Genetic transformation of Knufia petricola A95 - a model organism for biofilm-material interactions
(2014)
We established a protoplast-based system to transfer DNA to Knufia petricola strain A95, a melanised rock-inhabiting microcolonial fungus that is also a component of a model sub-aerial biofilm (SAB) system. To test whether the desiccation resistant, highly melanised cell walls would hinder protoplast formation, we treated a melanin-minus mutant of A95 as well as the type-strain with a variety of cell-degrading enzymes. Of the different enzymes tested, lysing enzymes from Trichoderma harzianum were most effective in producing protoplasts. This mixture was equally effective on the melanin-minus mutant and the type-strain. Protoplasts produced using lysing enzymes were mixed with polyethyleneglycol (PEG) and plasmid pCB1004 which contains the hygromycin B (HmB) phosphotransferase (hph) gene under the control of the Aspergillus nidulans trpC. Integration and expression of hph into the A95 genome conferred hygromycin resistance upon the transformants. Two weeks after plating out on selective agar containing HmB, the protoplasts developed cell-walls and formed colonies. Transformation frequencies were in the range 36 to 87 transformants per 10 µg of vector DNA and 106 protoplasts. Stability of transformation was confirmed by sub-culturing the putative transformants on selective agar containing HmB as well as by PCR-detection of the hph gene in the colonies. The hph gene was stably integrated as shown by five subsequent passages with and without selection pressure.
Introduction. Throughout a remarkably long period land colonisation on Earth has been proceeding by biofilm growth on bare rock surfaces. This very long history of sub-aerial biofilm development resulted in a high degree of their specialisation in challenging environments including desert rocks and high mountain altitudes. Presently the solid substrate/atmosphere interface habitat is frequently anthropogenic and includes e.g. building surfaces as well as industrial energy-producing facilities like solar plants. Those can be looked upon as pseudodeserts inhabited by complex microbial communities metabolising under limited water availability and high sun irradiation.
Aim. We intended to compare microbial settlers from sub-aerial biofilms that grow on photovoltaic panels and painted building facades surfaces to the known database of the rock-inhabiting fungi.
Materials and methods. Characteristic organisms' that dominate specific rock-inhabiting as well as buildings' facades- and photovoltaic panel-dwelling communities were isolated, identified, and characterized by microbiological and molecular biological methods.
Results. Melanised meristematic ascomycetes are the most enduring and numerous dwellers on sub-aerial rock, facade and solar panel surfaces. Obviously, environmental changes perturb sub-aerial biofilm development but over a number of seasons, these changes result in relatively stable microbial communities peculiar to this particular environmental niche.
Conclusions. A broad selection of melanised meristematic ascomycetes is indicative of sub-aerial biofilm on all atmosphere-exposed surfaces and can thus be referred to as "reference organisms" for these habitats. Our experimental evidence confirms that colonization of such substrates is facilitated by a symbiosis between a photosynthesizing organism and a fungus that are equipped to cope with the stress associated with sub-aerial existence. Melanised ascomycetes possess a very special stress-tolerant life style that arises under the influence of atmosphere and the solid support. It should be pointed out, that (1) these biofilm communities cannot be considered as "primitive" ones regarding the very long history of their development and high degree of their specialization; and (2) symbiotically competent but free-living bacterial/fungal biofilms cannot be compared to lichen communities, evolving much later than biofilm ecosystems. A genetically tractable laboratory system that includes the key participants of sub-aerial biofilm ecosystems is currently used for the development of standard test procedures in material sciences.
Keywords. Sub-aerial biofilms, rock-inhabiting melanised fungi, reference organisms, solar panels biofilms
A laboratory biofilm consisting of the phototrophic cyanobacterium Nostoc punctiforme ATCC 29133 and the rock-inhabiting ascomycete Knufia petricola CBS 726.95 was tested for its mineral weathering potential. Minerals with different grain sizes and mineralogy were incubated with and without biofilm in batch and in flow-through column experiments. After incubation, the mineral dissolution was quantified analysing (i) leachate chemistry via ICP-OES/MS (inductively coupled plasma optical emission spectrometry/mass spectrometry) and (ii) the residual grains as thin polished sections via SEM/TEM-EDX (scanning electron microscopy/transmission electron microscopy-energy dispersive X-ray spectrometry). Mineral dissolution was enhanced in biotic experiments as compared to abiotic ones, for both batch culture and flow-through approaches. Analyses of thin polished sections confirmed the leaching of these elements near the surface of the mineral grains. These results clearly indicate a biotic effect on the weathering of minerals produced by the laboratory biofilm.