Hierarchical structure and chemical composition of complementary segments of the fruiting bodies of Fomes fomentarius fungi fine-tune the compressive properties

Abstract

Humanity is often fascinated by structures and materials developed by Nature. While structural materials such as wood have been widely studied, the structural and mechanical properties of fungi are still largely unknown. One of the structurally interesting fungi is the polypore Fomes fomentarius. The present study deals with the investigation of the light but robust fruiting body of F. fomentarius. The four segments of the fruiting body (crust, trama, hymenium, and mycelial core) were examined. The comprehensive analysis included structural, chemical, and mechanical characterization with particular attention to cell wall composition, such as chitin/chitosan and glucan content, degree of deacetylation, and distribution of trace elements. The hymenium exhibited the best mechanical properties even though having the highest porosity. Our results suggest that this outstanding strength is due to the high proportion of skeletal hyphae and the highest chitin/chitosan content in the cell wall, next to its honeycomb structure. In addition, an increased calcium content was found in the hymenium and crust, and the presence of calcium oxalate crystals was confirmed by SEM-EDX. Interestingly, layers with different densities as well as layers of varying calcium and potassium depletion were found in the crust. Our results show the importance of considering the different structural and compositional characteristics of the segments when developing fungal-inspired materials and products. Moreover, the porous yet robust structure of hymenium is a promising blueprint for the development of advanced smart materials.

Fig 1. Macroscopic overview of the specimen of F. fomentarius under investigation.

a Tinder fungus on a fallen birch with a typical hoof shape and grayish coloration. b, c, and d show different orientations of the specimen in a. While b shows the front side and c the back side connected to the tree, d shows the underside facing the ground in the natural environment. e shows slices of the specimen in a. The slices contain different parts of the fruiting body. The circle marks the mycelial core (dashed arrow). Around the mycelial core is the trama (full arrow). Below the trama and mycelial core are tubes in the mm range called hymenium (dash-dotted arrow). The entire fruiting body is surrounded by a rigid outer layer, the crust (dotted arrow).

Fig 2. Microstructure by μCT of the four segments of F. fomentarius.

a The volume reconstruction of μCT data shows the anisotropic structure of the hymenium. Elongated tubes traverse the mycelium. The holes are arranged in parallel, resulting in a transversely isotropic structure. At the micrometer scale, the structure consists of hyphae arranged predominantly parallel to the holes. Loose hyphae can be seen within the holes. b The volume reconstruction of the trama shows a uniform distribution of hyphae over the millimeter length scale. The hyphae are oriented parallel to each other. In c the crust shows fused, dense hyphae at the bottom, a less dense region in the middle, and a thin dense layer (arrows) before the outermost rough surface layer. d shows the mycelial core, a patchwork of regions of lower and higher density and different kinds of cells namely fungal but also wood cells.

Fig 3. SEM images show the microstructure of different segments of F. fomentarius.

a, b, and d show the hymenium in parallel and in transverse direction. The mycelium has a distinct transverse isotropic structure and forms elongated tubes. b and c show that the hymenium near the mycelial core has holes filled with generative hyphae, whereas the holes in new layers, near the bottom of the fruiting body, are empty. In c, dense and less dense areas are seen, with mainly generative hyphae and probably wood cells forming the mycelial core. In e, the trama is seen, which is composed largely of skeletal hyphae forming a uniform mass with some denser areas. In f, the thin dense part of the crust can be seen. The hyphae appear to be compressed and form a dense protective layer.

Fig 4. Chitin/chitosan and glucan content.

The amount of glucosamine (wt.%) (the chitin/chitosan monomer) and glucan (wt.%) was measured by chitin and glucan assays, respectively. Mean values and standard deviations are given.

Fig 5. Micro X-ray fluorescence (μXRF) of a slice of F. fomentarius.

All four segments are shown in a. On the left side, the crust joins the trama surrounding the mycelial core. On the right side, the hymenium follows. The area marked by the red square was examined by μXRF. b shows potassium (K) and calcium (Ca) distributions, and c shows zinc (Zn) and manganese (Mn) distributions. The color scales give the measured intensities in counts per second of the fluorescence signal.

Fig 6. Magnification of μXRF of hymenium and crust.

Zooming in on a the hymenium (tubes facing the observer) and b the crust (cross-section) for the Ca K and K K distributions. The color scales give the measured intensities in counts per second of the fluorescence signal.

Fig 7. SEM images and corresponding SEM-EDX calcium maps of the crust and hymenium.

a Inside of the crust, viewed parallel to its outer surface, revealing circular patches rich in calcium. b Closer view of the hypha indicated by the arrow in a, highlighting the brighter regions surrounding the hypha, where Ca concentrations are notably higher. c Hymenium (view parallel to the tubes) showing elevated Ca levels, mainly in d crystals situated between the hyphae.

Fig 8. FTIR–ATR spectra X-ray diffraction patterns of the crust, hymenium, mycelial core, and trama.

a Full FTIR spectrum; the colored regions show: -OH and -NH stretching bands (grey), lipid region (blue), amides and proteins (red), and polysaccharides like glucans (green); b fingerprint region. c XRD patterns of the four segments.

Fig 9. Stress-strain curves from compression tests on cube-shaped specimens from the four segments of F. fomentarius.

a complete curves up to densification; b magnified view of the initial part.

Fig 10. Representative stress-strain curves and optical micrographs.

Cubes derived from the mycelial core a, hymenium b, and trama c at various time intervals during the experiment (marked by asterisks on the stress-strain curves). All specimens were loaded until densification.

Fig 11. Laser-cut specimen and schematic of hymenium with compression directions.

In a, a section with laser-cut square specimens is shown. The squares are numbered by carving with the laser, e.g. here in the segment of the mycelial core. b is a schematic representation of a cube specimen showing the possible compression directions (parallel and transverse) of the hymenium.