| dc.description.abstracteng | Wood-decaying saprotrophic fungi of the phylum Basidiomycota occur almost ubiquitously. Due to their ability to degrade lignin, hemicellulose and cellulose, especially white-rot fungi are among the most important destroyer of lignocellulosic biomass such as deadwood in nature. For this purpose, they secrete enzymes at their hyphal tips during growth to catalyze the decomposition of the lignocellulosic substrates. During the decomposition processes, fungi from the class Agaricomycetes form a dense and stable mycelial matrix directly on the substrate surfaces and in the interstices of the substrates. The mycelial matrix forms by an initially polar negatively autotrophic rapid growth of leading hyphae and a subapical formation of primary, secondary and further lateral branches. Formation of anastomoses between lateral branches and also the leading hyphae in older, network-forming parts of the growing fungal colony create a stable 3-dimensional matrix that may bind and cover individual substrate particles. Fungal growth and properties of the mycelial matrix are influenced by many factors, for example genetic properties of the fungal species or the specific strain, the substrate for growth and further environmental factors such as temperature, air for gas-exchange and humidity, pH of the substrate and substrate moisture, light and darkness, and others.
Many innovative research studies in science and in industry have focused in recent years on testing fungal mycelium as biogenic binder for the production of bio-based composite materials from lignocellulosic substrates as wastes from agriculture and forestry. The results of the studies showed potential in the production of substitutes, e.g. for common building materials like bricks, insulation materials or wood-based materials glued by resins of petrochemical origin, for leather and synthetic leather or for plastics used as packaging materials. However, these studies rarely went beyond the status of a feasibility test.
In this work, the formation of 2-dimensional mycelium and 3-dimensional mycelial matrices of different Agaricomycetes was analyzed macro- and microscopically and the influence of various factors on the growth rate and properties of the mycelium was examined. The aim of the work was to achieve particularly rapid fungal growth with simultaneous formation of a dense and stable mycelial matrix for the use as a binder in the production of lightweight mycelial based thermal insulation materials from mixtures of chopped wheat straw and spelt husks as biogenic residues. From the initial selection of 27 different strains from 22 different fungal species, the five Agaricomycetes Pleurotus ostreatus, Ganoderma resinaceum, Schizophyllum commune, Trametes hirsuta and Trametes versicolor developed a particularly fast-growing and dense 2- and 3-dimensional mycelium and were selected for the further study.
All five fungi showed the fastest growth at 26 °C on malt extract agar medium (MEA), although S. commune may grow faster at higher temperatures depending on the nutrient medium. Acidic conditions around pH 5.5 were optimal for all fungi except for P. ostreatus, which grew faster on slightly basic medium. The growth rates of the colonies were negatively affected by acidic pH for P. ostreatus and by basic pH for T. versicolor, while the other fungi were not affected in their growth. All fungi adjusted the pH of the medium during incubation in species-specific manners. The light conditions during incubation also had an influence on mycelial density. In constant light, all fungi formed a thinner mycelium than in darkness interrupted by occasional light pulses. Also the composition of the substrate influenced mycelial densities as well as growth rates. Pure lignocellulosic substrate did not result in rapid growth and dense mycelium. A mixed substrate of chopped and sieved wheat straw, sieved spelt husks, and wheat flour as an additional nutrient source resulted in particularly dense mycelial growth and in compact product stability due to the structure-giving properties of the overgrown coarsely chopped straws and an optimal size of the voids in the substrate mediated by the spelt husks. Wheat flour adhered initially to the surface of the lignocellulosic substrate particles and was degraded by all fungi during incubation for the use as an easy C-source.
In the production of mycelium-bound insulation material in growth containers, gas exchange played a key role. On a small scale with a board-thickness of about 3 cm, composites could be produced in a closed growth container with marginal aeration. A higher thickness of about 7 cm resulted in inhomogeneous growth in the inner substrate and then inhomogeneous stability of the bio-based insulation material. Ventilation holes for better gas exchange in the bottom of the growth containers offered a good microclimate around holes but were overgrown and clogged by the fungi within a few days. With a continuous light air-flow underneath the growth container, the substrate in the area of the aeration holes dried out slightly, which locally slowed down the fungal growth. This allowed the evaporation of excessive water from the lower substrate layers, gas exchange during the whole incubation period and uniform growth throughout the whole composites material. T. hirsuta showed rapid growth under these conditions, but formed a weak, inhomogeneous mycelial matrix in the inner of the composites by sealing the substrate surface with fast-growing mycelium probably hindering gas-exchange. With this setup, composites of good quality were primarily produced with G. resinaceum.
A reduction of fungal growth time can reduce costs in the industrial production of insulation boards. All five fungi showed enhanced growth on substrate-mixtures containing starch. Starch degradation in fungi occurs by hydrolytic cleavage of the starch components amylose and amylopectin into the disaccharide maltose and the monosaccharide glucose by different types of amylases. Genome analysis for the five fungi (of Ganoderma lucidum as alternative performed for G. resinaceum lacking so far an established genome) revealed sets of genes for 7 – 13 α-amylases and 2 – 3 γ-amylases but no β amylases to be present in the species. All fungi were shown to produce a variety of secreted α- and γ-amylases to degrade starch. From former studies of α-amylases from other groups of organisms, it was known that α-amylases are metalloenzymes requiring Ca2+ as a cofactor for function and Cl- ions as allosteric activator. Addition of CaCl2 to liquid fungal cultures led to an increase in α-amylase activity by all five fungi and accelerated degradation of starch in the culture medium and with it mycelial growth. In the mixed lignocellulosic substrate, the addition of CaCl2 resulted in faster formation of the mycelial matrix and a higher 3-dimensional density of the aerial mycelium on the surface of the substrate. Observations on microslide cultures under the microscope revealed that supplementation of CaCl2 resulted in more frequent and also earlier branching of the leading hyphae. Addition of CaCl2 resulted in a smoother surface of the mycelial pellets and, in the case of P. ostreatus and G. resinaceum, to a significant decrease in pellet size. Smaller pellets have a potentially better internal gas exchange and seemed to be better physiologically active and provide therefore a better source for liquid inoculation of substrate mixtures in molds for bio-composite production. | de |