Degradation of Pinus sylvestris and Populus tremula by laccate Ganoderma species

1 Introduction

Forest industries generate significant volumes of wood side-streams worldwide. Sawmills and plywood industries process approximately 50 % of the harvested timber and the remaining mass fraction consist of bark, chips, and sawdust (Lindberg and Tana 2012), which in Europe are mostly used for energy and, to a minor extent, for pulp and panel production (Verkasalo et al. 2019). The European Union is promoting a transition towards more sustainable uses of material resources. The consequential increase of wood side-streams has sparked efforts from industry and research communities to discover innovative applications for these underutilized secondary resources.

The cultivation of mushrooms is one strategy that allows the transformation of wood side-streams into high-value products, such as carbohydrates. Among cultivated fungal species, Ganoderma lucidum has been broadly studied in regards of their nutritional values and biological activity. To supply the demand of the bioactive compounds derived from G. lucidum, the species has been cultivated in a wide variety of substrates originating from agricultural or forestry side-streams (Atila 2020; Cortina-Escribano et al. 2020a,b).

The commercial cultivation of G. lucidum in Northern Europe is still at its infancy. Wood-derived side-streams have been tested for the cultivation of G. lucidum with variable results (Cortina-Escribano et al. 2020a,b): Populus tremula increased the yield of fruiting bodies, while Pinus sylvestris prevented the mycelial growth and probability of fruiting.

The effect of wood structure and composition of Pinus sp. and Populus sp. on fungal metabolism have been previously studied (Witzell and Martín 2008; Kuhar et al. 2018). For instance, the phenolic compounds present in P. sylvestris and Populus tremuloides play a role in the defense mechanism against several fungal pathogens and inhibit their growth (Witzell and Martín 2008).

It is expected that some of the wood components of P. sylvestris are not degraded by G. lucidum and, therefore, the fungus is unable to produce fruiting bodies. In this study, the aim was to identify the changes in lignocellulosic composition of wood side-streams after they have been applied for the cultivation of G. lucidum.

2 Materials and methods

3 Results and discussion

The chemical composition of fresh (pine_fresh and poplar_fresh) and degraded (pine_SMS and poplar_SMS) substrate is shown in Table 1. The major components of pine _fresh and poplar _fresh are within the common average range for softwood and hardwood (Sixta 2006; Sjöström 1993). The lignocellulosic composition given in Table 1 does not include the proportion of acetyl groups (Sixta 2006) and methyl esterified pectin (Hafrén et al. 2000), which would explain that the composition does not add up to 100 %.

The component fractions of P. sylvestris wood-based substrate changed notably after the fungal degradation compared to P. tremula substrate. This indicates that the G. lucidum strain degraded evenly the main components of P. tremula and, on the contrary, some of the components present in P. sylvestris were not consumed by the fungal strain. A determined fungal species within the Ganoderma genus may cause different decay processes depending on the hosting wood species and its chemical and physiological characteristics (i.e. Loyd et al. 2018). These results agree with the findings of Baietto and Wilson (2010), where a strain of G. lucidum showed lower decay performance when grown on softwood species, including Pinus sp., compared to hardwood species such as Populus sp.

The proportion of water-soluble extractives increased in both species after the degradation by G. lucidum (Figure 1A). Ganoderma spp. exudate enzymes and secondary metabolites during the delignification process (Zhou et al. 2013), possibly increasing the extractive fraction in the degraded wood. However, the proportion of ethanol-soluble extractives decreased in pine_SMS and increased in poplar_SMS. Previous studies found that Scots pine sawdust inhibited the mycelium growth and fruiting body production of G. lucidum in comparison to other tree species, such as P. tremula and Betula spp. (Cortina-Escribano et al. 2020a,b). The reason behind the fungal growth inhibition might be the presence of toxic compounds in softwood species (Mata et al. 2019). Earlier biodegradation analyses for wood indicate that fungal inhibition is likely to arise from the resin acids, pinosylvins and polyphenolic extractives from P. sylvestris (Harju et al. 2003), such as stilbenes (Venäläinen et al. 2004).

Figure 1:

Chemical composition of Pinus sylvestris and Populus tremula fresh wood (pine_fresh and poplar_fresh) and degraded wood samples (pine_SMS and poplar_SMS). (A) Water and ethanol soluble (water soluble extractives subtracted) extractives. (B) Content of carbohydrates. (C) Content of uronic acids. (D) Content of Klason lignin and acid soluble lignin. (E) Content of acid insoluble ash and gravimetric ash.

The carbohydrates are the highest fraction per dry matter in both tree species prior and after fungal degradation (Table 1). Surprisingly, the P. sylvestris wood-based substrate increased the fraction of carbohydrates markedly after wood degradation from 59.1 to 73.6 w-%, from which most of the fraction corresponds to glucan (Figure 1B). The glucan fraction in P. tremula wood-based substrate remained around 40 % prior and after fungal degradation. A possible explanation of these results is the selective degradation ability of fungal species towards wood components. For example, Pleurotus pulmonarius consumed a lower proportion of glucan on Picea abies than in Alnus glutinosa (Chen et al. 2020).

The differences in glucan content in the substrates can also be explained by the formation and harvest of fruiting bodies. In a previous study, higher fruiting body yields were recorded from poplar_SMS than from pine_SMS (Cortina-Escribano et al. 2020b), therefore, it is possible that a fraction of the poplar glucans is transformed to G. lucidum fruiting bodies. G. lucidum produces branched polysaccharides, (1→3, 1→6)-β-glucans that could be seen in carbohydrate analyses (Chang and Lu 2004).

Pine_SMS presented lower content of uronic acids as compared to poplar_SMS (Figure 1C). Galacturonic and 4-0-methyl-D-glucuronic acids, which are part of pectin and xylan respectively, represent most of the uronic acid fraction prior and after wood degradation for both species. A small fraction of glucuronic acid has been also detected in all substrates. Mannuronic and guluronic acids were not present in any of the samples analyzed.

As reflected in the results, softwood contains higher content of lignin and galactan than hardwood (Figure 1D). This can be explained by the reaction of the tree to strain forces; compression wood is formed in softwood, whereas tension wood is formed in hardwood (Kandhola et al. 2017).

After fungal degradation, the substrate containing wood side-streams of P. tremula maintained the fraction of lignin at 19.6 %. On the contrary, the fungi degraded most of the lignin present in P. sylvestris wood-based substrate, in which case the lignin fraction was dropped to 6.6 % of the dry weight. Based on the results of this study, the wood degradation by G. lucidum significantly reduced the fraction of lignin in P. sylvestris. These results agree with Kuhar et al. (2018), who reported a reduction in the lignin fraction of Pinus radiata and Populus nigra after the cultivation of G. lucidum. The cultivation of G. lucidum could lead to novel biotechnological pre-treatment of wood side-streams of P. sylvestris, such as in the bleaching process or biomechanical pulping due to the successful delignification of the lignocellulose (Blanchette 1991; Kang et al. 2007). Further research analyzing the changes in lignin structure and other cell wall components (i.e. Loyd et al. 2018) in both P. tremula and P. sylvestris is needed.

The proportion of inorganic compounds in both wood species increased after the degradation by G. lucidum as an outcome of the reduced organic fractions (Figure 1E). Therefore, for the same volume of material, there is a greater weight of inorganic compounds in the degraded wood samples. The ash content in sound wood is often within 0.6–2.7 % (Sannigrahi et al. 2010), which aligns with the findings of this study.

4 Conclusions

Changes in the wood components were observed for P. tremula and P. sylvestris wood-based substrates after the cultivation of G. lucidum. Carbohydrates and lignin of P. tremula were consumed in even proportions during the cultivation of G. lucidum, allowing the production of fruiting bodies and fungal polysaccharides. The wood degradation by G. lucidum significantly reduced the fraction of lignin in P. sylvestris substrate. On the contrary, the carbohydrate proportion in P. sylvestris increased after degradation. Further research is needed to understand the changes in the lignin structures and in other cell wall components of both poplar and Scots pine wood.