A wood-decay or xylophagous fungus is any species of

fungus A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

that digests moist

wood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin ...

, causing it to rot. Some

species In biology, a species is the basic unit of classification and a taxonomic rank of an organism, as well as a unit of biodiversity. A species is often defined as the largest group of organisms in which any two individuals of the appropriat ...

of wood-decay fungi attack dead wood, such as brown rot, and some, such as ''

Armillaria ''Armillaria'' is a genus of fungi that includes the ''Armillaria mellea, A. mellea'' species known as honey fungi that live on trees and woody shrubs. It includes about 10 species formerly categorized summarily as ''A. mellea''. ''Armill ...

'' (honey fungus), are

parasitic Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson ha ...

and colonize living trees. Excessive moisture above the fibre saturation point in wood is required for fungal colonization and proliferation. In nature, this process causes the breakdown of complex molecules and leads to the return of nutrients to the soil. Wood-decay fungi consume wood in various ways; for example, some attack the

carbohydrate In organic chemistry, a carbohydrate () is a biomolecule consisting of carbon (C), hydrogen (H) and oxygen (O) atoms, usually with a hydrogen–oxygen atom ratio of 2:1 (as in water) and thus with the empirical formula (where ''m'' may o ...

s in wood and some others decay

lignin Lignin is a class of complex organic polymers that form key structural materials in the support tissues of most plants. Lignins are particularly important in the formation of cell walls, especially in wood and bark, because they lend rigidity a ...

. The rate of decay of wooden materials in various climates can be estimated by empirical models.Viitanen, T. et al. (2010). Towards modelling of decay risk of wooden materials. European Journal of Wood and Wood Products 68:303-313. Wood-decay fungi can be classified according to the type of decay that they cause. The best-known types are brown rot, soft rot, and white rot. Each produce different enzymes, can degrade different plant materials, and can colonise different environmental niches.Vane, C. H., et al. (2005). "Decay of cultivated apricot wood (''Prunus armeniaca'') by the ascomycete ''Hypocrea sulphurea'', using solid state 13C NMR and off-line TMAH thermochemolysis with GC–MS." International Biodeterioration & Biodegradation 55(3): 175-185. Brown rot and soft rot both digest a tree's

cellulose Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...

and hemicellulose but not its lignin; white rot digests lignin as well. The residual products of decomposition from fungal action have variable pH, solubility and redox potentials. Over time this residue will become incorporated in the soil and sediment, so can have a noticeable effect on the environment of that area.

Brown rot

Cubical rot checked quercus bialowieza 1 beentree

Brown-rot fungi break down

hemicellulose A hemicellulose (also known as polyose) is one of a number of heteropolymers (matrix polysaccharides), such as arabinoxylans, present along with cellulose in almost all terrestrial plant cell walls.Scheller HV, Ulvskov Hemicelluloses.// Annu Rev ...

and

that form the wood structure. Cellulose is broken down by

hydrogen peroxide Hydrogen peroxide is a chemical compound with the formula . In its pure form, it is a very pale blue liquid that is slightly more viscous than water. It is used as an oxidizer, bleaching agent, and antiseptic, usually as a dilute solution (3 ...

(H₂O₂) that is produced during the breakdown of hemicellulose. Because hydrogen peroxide is a small

molecule A molecule is a group of two or more atoms held together by attractive forces known as chemical bonds; depending on context, the term may or may not include ions which satisfy this criterion. In quantum physics, organic chemistry, and b ...

, it can diffuse rapidly through the wood, leading to a decay that is not confined to the direct surroundings of the fungal

hypha A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one or ...

e. As a result of this type of decay, the wood shrinks, shows a brown discoloration, and cracks into roughly cubical pieces, a phenomenon termed cubical fracture. The

fungi A fungus ( : fungi or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, separately fr ...

of certain types remove

compounds from wood and hence the wood turns brown. Brown rot in a dry, crumbly condition is sometimes incorrectly referred to as ''dry rot'' in general. The term ''brown rot'' replaced the general use of the term ''dry rot'', as wood must be damp to decay, although it may become dry later.

Dry rot Dry rot is wood decay caused by one of several species of fungi that digest parts of the wood which give the wood strength and stiffness. It was previously used to describe any decay of cured wood in ships and buildings by a fungus which resul ...

is a generic name for certain species of brown-rot fungi. Brown-rot fungi of particular economic importance include ''

Serpula lacrymans ''Serpula lacrymans'' is one of the fungi that cause damage to timber referred to as dry rot. It is a basidiomycete in the order Boletales. The ''Serpula lacrymans'' has the ability to rapidly colonise sites through unique and highly specialise ...

'' (true dry rot), ''

Fibroporia vaillantii ''Fibroporia vaillantii'', also known as mine fungus, white pore fungus, ''Antrodia vaillantii'', ''Polyporus vaillantii'', and various other namesConiophora puteana'' (cellar fungus), which may attack

timber Lumber is wood that has been processed into dimensional lumber, including beams and planks or boards, a stage in the process of wood production. Lumber is mainly used for construction framing, as well as finishing (floors, wall panels, w ...

in buildings. Other brown-rot fungi include the sulfur shelf, '' Phaeolus schweinitzii'', and '' Fomitopsis pinicola''. Brown-rot fungal decay is characterised by extensive demethylation of lignins whereas white-rot tends to produce low yields of molecules with demethylated functional groups.Vane, C. H., et al. (2001). "The effect of fungal decay (''Agaricus bisporus'') on wheat straw lignin using pyrolysis–GC–MS in the presence of tetramethylammonium hydroxide (TMAH)." Journal of Analytical and Applied Pyrolysis 60(1): 69-78. There are very few brown rot fungi in

tropical climate Tropical climate is the first of the five major climate groups in the Köppen climate classification identified with the letter A. Tropical climates are defined by a monthly average temperature of 18 °C (64.4 °F) or higher in the cool ...

s or in southern

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout ...

zones. Most brown rot fungi have a geographical range north of the

Tropic of Cancer The Tropic of Cancer, which is also referred to as the Northern Tropic, is the most northerly circle of latitude on Earth at which the Sun can be directly overhead. This occurs on the June solstice, when the Northern Hemisphere is tilted towa ...

(23.5° latitude), and most of these are found north of the 35° latitude, corresponding to a roughly

boreal Boreal may refer to: Climatology and geography *Boreal (age), the first climatic phase of the Blytt-Sernander sequence of northern Europe, during the Holocene epoch *Boreal climate, a climate characterized by long winters and short, cool to mild ...

distribution. Those brown rot fungi between latitudes 23.5° and 35° are typically found at high elevations in pine forest regions, or in

coniferous forest Conifers are a group of cone-bearing seed plants, a subset of gymnosperms. Scientifically, they make up the division Pinophyta (), also known as Coniferophyta () or Coniferae. The division contains a single extant class, Pinopsida. All exta ...

regions such as the

Rocky Mountains The Rocky Mountains, also known as the Rockies, are a major mountain range and the largest mountain system in North America. The Rocky Mountains stretch in straight-line distance from the northernmost part of western Canada, to New Mexico ...

or the

Himalayas The Himalayas, or Himalaya (; ; ), is a mountain range in Asia, separating the plains of the Indian subcontinent from the Tibetan Plateau. The range has some of the planet's highest peaks, including the very highest, Mount Everest. Over 10 ...

Soft rot

Soft-rot fungi secrete

cellulase Cellulase (EC 3.2.1.4; systematic name 4-β-D-glucan 4-glucanohydrolase) is any of several enzymes produced chiefly by fungi, bacteria, and protozoans that catalyze cellulolysis, the decomposition of cellulose and of some related polysaccha ...

from their

e, an

enzyme Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products ...

that breaks down

in the wood. This leads to the formation of microscopic cavities inside the wood, and sometimes to a discoloration and cracking pattern similar to brown rot. Soft-rot fungi need fixed nitrogen in order to synthesize enzymes, which they obtain either from the wood or from the environment. Examples of soft-rot-causing fungi are '' Chaetomium'', '' Ceratocystis'', and '' Kretzschmaria deusta''. Soft-rot fungi are able to colonise conditions that are too hot, cold or wet for brown or white-rot to inhabit. They can also decompose woods with high levels of compounds that are resistant to biological attack. Bark in woody plants contains a high concentration of

tannin Tannins (or tannoids) are a class of astringent, polyphenolic biomolecules that bind to and precipitate proteins and various other organic compounds including amino acids and alkaloids. The term ''tannin'' (from Anglo-Norman ''tanner'' ...

, which is difficult for fungi to decompose, and

suberin Suberin, cutin and lignins are complex, higher plant epidermis and periderm cell-wall macromolecules, forming a protective barrier. Suberin, a complex polyester biopolymer, is lipophilic, and composed of long chain fatty acids called suberin aci ...

which may act as a microbial barrier.Vane, C. H., et al. (2006). "Bark decay by the white-rot fungus Lentinula edodes: Polysaccharide loss, lignin resistance and the unmasking of suberin." International Biodeterioration & Biodegradation 57(1): 14-23. The bark acts as form of protection for the more vulnerable interior of the plant. Soft-rot fungi do not tend to be able to decompose matter as effectively as white-rot fungi: they are less aggressive decomposers.

White rot

White-rot fungi are a type of fungi comprising agaricomycetes, basidiomycetes, and some ascomycetes that are capable of decomposing many tree species. It is now recognized that saprotrophic interactions have profound effects on forest biomes. White-rot fungi are characterized by their ability to break down the lignin, cellulose, and hemicellulose of wood. As a result of this ability, white-rot fungi are considered a vital component of the carbon cycle, due to their ability to access carbon pools that would otherwise remain inaccessible. The name “White-rot” derives from the white color and rotting texture of the remaining crystalline cellulose from wood degraded by these fungi.Martinez, D., Larrondo, L. F., Putnam, N., Gelpke, M. D. S., Huang, K., Chapman, J., Helfenbein, K. G., Ramaiya, P., Detter, J. C., Larimer, F., Coutinho, P. M., Henrissat, B., Berka, R., Cullen, D., & Rokhsar, D. (2004). Genome sequence of the lignocellulose degrading fungus Phanerochaete chrysosporium strain RP78. Nature Biotechnology, 22(6), 695–700. Most knowledge of white rot fungi comes from ''Coriolus versicolor'' and ''Phanerochaete chrysosporium''. White-rot fungi show strong participation in interspecific competition, culminating the in the evolution of lignin catabolism specificity. The current and future applications of white-rot fungi as a potential component of mycoremediation merits greater study of this saprotroph.

Biochemistry

Compared to other saprotrophs, white-rot fungi possess the specialized ability to cleave lignin into smaller, more processable molecules. Lignin is a biopolymer which combines with cellulose to form the lignocellulose complex, an important complex that confers strength and durability to plant cell walls. Lignin itself is a macromolecule formed from the combination of many phenolic aromatic groups via oxidative coupling. Due to its high stability, lignin is incapable of being broken down through simple decomposition. As a result, white-rot fungi employ a series of enzymes that break lignin down into smaller aromatic rings. The relative abundance of phenylpropane alkyl

side chain In organic chemistry and biochemistry, a side chain is a chemical group that is attached to a core part of the molecule called the "main chain" or backbone. The side chain is a hydrocarbon branching element of a molecule that is attached to a ...

s of lignin characteristically decreases when decayed by white-rot fungi. Since lignin is the specialized food source of white-rot fungi, understanding the two different catabolic pathways is important.

Lignin metabolism through peroxidases

The first way white-rot fungi can break down lignin involves a high-redox-potential catalyzed

peroxidase Peroxidases or peroxide reductases ( EC numberbr>1.11.1.x are a large group of enzymes which play a role in various biological processes. They are named after the fact that they commonly break up peroxides. Functionality Peroxidases typically ca ...

attack on the heme pocket, thus reducing the stability of lignin. The process starts with creation of extracellular hydrogen peroxide (H₂O₂), a process completed via glyoxal oxidase (GLX). Extracellular hydrogen peroxide may be responsible for creation of hydroxyl radical (·OH) via the Fenton reaction: Fe²⁺ + H₂O₂ → Fe³⁺ + ·OH + OH⁻ The peroxidases used to oxidize lignin are lignin peroxidase (LiP), manganese peroxidase (MnP), and versatile peroxidase (VP). These peroxidases are commonly referred to as fungal class II peroxidases (PODs). Research suggests there may be another group of POD enzymes: basal peroxidases, including novel peroxidase (NoP). The NoP of ''Postia placenta'' is characterized by its inability to bind Mn²⁺ and its low redox potential. PODs developed in the common ancestor of white-rot, brown-rot and mycorrhizal fungi but these enzyme families have undergone secondary loss or contraction in the latter two groups. LiPs are oxidioreductases specific to lignin degradation. VPs are a class of peroxidase that combines elements of both LiPs and MnPs. LiPs and VPs are specific to heme product architecture allowing direct oxidation of benzene groups regardless of linkages. Direct oxidation of benzene groups results in the creation of an unstable radical aromatic. However, the hydrogen peroxide, bound to the heme group on the heme pocket, is unable to access the bulky lignin due to steric hindrance. As a result, LiP and VP enzymes create a

tryptophan Tryptophan (symbol Trp or W) is an α-amino acid that is used in the biosynthesis of proteins. Tryptophan contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a polar molecule with a non-polar aromatic ...

radical on their protein surface which allows long-range electron transfer from the aromatic substrate to the activated cofactor.

Lignin metabolism through laccase

Illustration of the cellulose composition

The second mechanism for breaking down lignin involves

laccase Laccases () are multicopper oxidases found in plants, fungi, and bacteria. Laccases oxidize a variety of phenolic substrates, performing one-electron oxidations, leading to crosslinking. For example, laccases play a role in the formation of l ...

, a low-redox-potential oxidase incapable of direct attack. Laccase can be used both in breaking and forming lignin. It cleaves lignin by reducing oxygen, creating a free radical which allows a hydroxyl radical (·OH) to attack the ring and deposit an alcohol group (OH). Deprotonation follows, resulting in the breaking of C-C (aryl-alphaC) bond into two aromatic rings. These products enter the fungal hyphae to be further broken down via catabolic processes. After the lignin complex is broken down, other saprotrophs can enter and begin degrading the newly created products. The final products of these transformations are carbon dioxide and water. While it is known that brown-rot fungi can also target lignin, they are only capable of modifying and are not capable of completely recycling it with a few exceptions. The ability to degrade lignin, previously supposed to only occur in white-rot fungi which have PODs, was found in ''Botryobasidium botryosum'' and ''Jappia argillacea'', two brown-rot fungi, lacking PODs. While the general pathway is currently unknown, research supports the existence of a continuum of features that separate the two fungal types rather than distinct categories.

Cellulose metabolism

While white-rot fungi specialized in catabolizing lignin, they are also capable of metabolizing other common organic forms of carbon like cellulose. Cellulose is also a laborious molecule to cleave. First, cellobiohydrolases, found in all white-rot fungi, hydrolyze the 1,4-beta-D-glycosidic bonds partially degrading cellulose. GH61 enzymes initiate a copper-dependent oxidative (LMPO) attack on crystalline cellulose. LMPOs boost degradation by activating oxygen using a copper-containing histidine brace that increases glycoside hydrolase activity, effectively lowering the activation cost of the reaction, making cleavage much cheaper, and therefore, more profitable for the fungi. Products from the cleavage are glucose and cellobiose. Another method involves endoglucanases hydrolyzing cellulose at random points before cellobiohydrolases cleave the chains, resulting in cellobiose. At the end of both processes, Beta-glucosidases further catabolize cellobiose into glucose.

Hemicellulose metabolism

Another main food source of white-rot fungi is

, a heteropolymer like cellulose that is not exclusively catabolized by white-rot fungi. The prevalent hemicellulose found in soft wood trees is Galactoglucomannan, a molecule made up of b-1,4-linked D-mannopyranose and D-glucopyranose units. Endo-1,4-b-D-mannanase breaks the prior linkages along the main chain of galactoglucomannan. Recent studies have found that LPMOs, previously only thought to be used in cellulose cleavage, were also found to be important in the catabolism of hemicellulose in conjunction with glycoside hydrolase enzymes (GHs). The availability of non white-rot fungi to catabolize cellulose and hemicellulose results in the creation of interspecific competition for access to these resources. Understanding the methods white-rot fungi use to dominate a resource and prevent competition will prove an important facet to understanding white-rot fungi.

Ecology

White rot competitive ability

Since white-rot fungi aren't the only saprotrophs capable of accessing cellulose and hemicellulose, competition ensues. Researchers attempted to estimate the effect of competition on white rot fungi. They reported that in sterile environments with no microbiota competitors present, white-rot fungi had good growth, but in soil with natural microbiota present, white-rot growth was variable. Even though white-rot fungi have a very specialized process for acquiring carbon, they are still vulnerable to competitors. Researchers clarified that white-rot fungi survival is dependent on its ability to defend lignocellulose substrate against attack by soil microbiota and its ability to establish itself within the soil bulk. These findings suggest that white-rot fungi and soil microbiota remain largely antagonistic in interactions, with only the highly competitive ''Pleurotus'' species capable of establishing themselves with only negligible negative impact due to soil microbiota. Less competitive white-rot fungi either failed to establish or produced lower enzyme concentrations associated with respiration. Successful interactions are characterized by which microbe arrives first and establishes a foothold.Lang, E., Eller, G., & Zadrazil, F. (1997). Lignocellulose Decomposition and Production of Ligninolytic Enzymes during Interaction of White Rot Fungi with Soil Microorganisms. Microbial Ecology, 34(1), 1–10. Brown-rot fungi and white-rot fungi have similar interspecific mycelial interactions. When white-rot fungal species occupied the same host distinct districts formed known as ‘decay columns’. Interactions were classified as interspecific competition. There were two important results when competition occurs: ‘deadlock’, when neither species could dominate the other; and ‘replacement’ when once species achieved complete colonization and replaced the other. A different study noted a third option: ‘reciprocal replacement’ when fungi successfully captured some territory and simultaneously lost other territory. Mutualism between two white-rot fungi was noted to be very rare. Findings suggested the important distinction between primary competition, that is competition to colonize unoccupied territory and antagonistic capture and defense of territory. Many competitive interactions were ‘intransitive’, meaning interactions involved more than two fungal species each often deploying a different antagonistic mechanism that gave it an advantage over one species but a disadvantage over others. Research further highlighted the importance of environmental factors including temperature, water potential, and invertebrate interactions in influencing competition. Findings suggested that competition increased decay, due to competition being expensive and saprotrophs needing to access more resources to fund it. Similarly, decay rates increased in smaller environments where natural resources were limited and competition intense. Interestingly, even though brown-rot fungi lack the ability to decompose lignin, a relatively energetically expensive molecule, brown-rot fungi were slightly more competitive than white-rot fungi since they could still access the relatively cheaper cellulose and hemicellulose and devote more energy to competition and less to extracting nutrients. Further evidence for white-rot fungi possessing long-term advantages was found in a study that determined that a longer time was required for white-rot fungal invasion of wood chips than for foliage litters. The data they collected on white-rot mass loss was sigmoid shaped. This finding suggests that while white-rot fungi are not as competitive at decomposing carbon from common sources as other decomposers within the first year, but they proved to be more competitive after one year due to their specialized ability to access carbon from lignin. Competition is not just limited between fungi. The presence of white rot fungi, in this case ''Hypholoma fasciculare'' and ''Resinicium bicolor'', on sterilized beech wood blocks resulted in a lower number of wood-inhabiting bacteria, even though lignin is not a food source of these bacteria. This finding points to an antagonistic relationship between white-rot fungi and bacteria that both compete for cellulose and hemicellulose, as well as the existence of bactericidal and bacteriostatic weapons utilized by white-rot fungi against competitor bacteria. Though the mechanism is unknown, researchers suggested that white-rot fungi may utilize lignin decomposing enzymes, hydroxyl radicals, and aryl alcohols to create a toxic environment. Further environmental manipulation involved the release of PODs to lower the pH and create a more acidic habitat. The resulting conclusion is that peroxides not only make lignin accessible, but create a more accessible environment for white-rot fungi to compete in. Even with a specialized catabolic mechanism, competition remains a highly selective force on white-rot evolution.

Evolution

Insight on the evolutionary development of white-rot fungi comes from the evolution of lignin catabolism. Lignin is a precursor to the development of coal. During the Carboniferous (360-300 mya) and Permian (300-250 mya) there was a very high carbon accumulation. However, near the end of the Permian there was a sharp decline in carbon accumulation. White-rot fungi and their ability to cleave lignin evolved at the end of the Permian period. Researchers attempted to reconstruct the evolution of saprotrophic capabilities. Results suggested that white-rot saprotrophs were the common ancestors of brown-rot fungi and ectomycorrhiza (ECM), but that in the latter two groups genes coding for PODs were lost. To gain insight on the evolution of lignolytic peroxidases, researchers resurrected ancestral lignolytic peroxidases from the Polyporales, a basidiomycete order that emerged 150mya, and analyzed the lineage from that ancestor to the modern ''P. chrysosporium''. One of the major findings was that ancestral versatile peroxidase (AVP) was not capable of functioning efficiently at low pH, a characteristic associated with modern LiPs. Findings also suggested that AVP possessed a much wider substrate specificity, the loss of which being an evolutionary cost of developing further specificity. Early peroxidases were unable to directly degrade lignin and relied on metal cations to separate phenol groups. Only later would peroxidases acquire the ability use a tryptophanyl radical, interacting with a bulky polymer at the surface of the peroxidase, to attack non-phenolic lignin. These findings highlight the importance of taking plant evolution into account when analyzing the evolution of white-rot fungus. Researchers note that plant cell walls have been steadily increasing and show evidence of convergent evolution. White-rot PODs also demonstrated convergent evolution. As plant cell walls have become more efficient, so have the peroxidases that destroy them. Researchers attempted to further understand the evolutionary development of white-rot fungi by using bioinformatics. They analyzed sixty-two genomes of Agaricomycetes of white-rot , brown-rot, ECM and other nutritional modes. Given that both white-rot and brown-rot share the ability to cleave cellulose and hemicellulose, they suggest that PODs developed after cellulolytic enzymes and that white-rot mechanisms were an elaboration based on the already existing saprotrophic model, not just on the utilization of PODs. Understanding the evolutionary development of white-rot fungi provides insight onto a variety of potential uses.

Current and future applications

White-rot fungi have historically been valued as food, but in recent years exploration of their enzymatic capabilities has revealed white-rot fungi’s potential in depollution. White-rot fungi have long since been staples of human diet and remain an important source of nutrition for people around the world. White-rot fungi are commercially grown as a source of food – for example the

shiitake mushroom The shiitake (alternate form shitake) (; ''Lentinula edodes'') is an edible mushroom native to East Asia, which is now cultivated and consumed around the globe. It is considered a medicinal mushroom in some forms of traditional medicine. Ta ...

, which in 2003 constituted approximately 25% of total mushroom production.Vane, C. H. (2003). "Monitoring Decay of Black Gum Wood (''Nyssa sylvatica'') During Growth of the Shiitake Mushroom (''Lentinula edodes'') Using Diffuse Reflectance Infrared Spectroscopy." Applied Spectroscopy 57(5): 514–517. Due to white-rot fungi’s important ability to degrade lignin, they have been increasingly explored as potential sources in mycoremediation applications, applications focused on removing organic pollutants from the environment. All three enzyme types of lignin decomposition (LiPs, MnP, and Laccase) have been explored. White-rot fungi have been determined to degrade PAHs, chlorinated aromatic hydrocarbons (CAHs), DDT, lindane, polycyclic aromatics, PCBs, PCP, polychlorinated dibenzo(p)dioxins, and azo dyes when studied in ''Phanerochaete chrysosporium, Trametes versicolor, Bjerkandere adusta,'' and ''Pleurotus ostreatus''. Noted limitations of white-rot fungi as pollutant cleaners is due to difficulty establishing the fungi in non-natural conditions. Other applications include biosorption, a process where biomass is utilized to remove solute wastes preventing pollution. Researchers studied the effect white-rot fungi could have on absorbing heavy metal ions via alginic acid, a linear polysaccharide composed of 1,4-linked Beta-D-mannuronic and alpha-L-guluronic acid. The findings from the study indicated that ''Fungalia trogii'' was capable of biosorption of Hg2+, Cd2+, and Zn2+ in low pH environments. The potential establishment of white-rot fungi as a stable mycoremediator remains an important future discovery. White-rot fungi remain an important source of great unrealized potential.

Induced fungal decay

A special way of giving grown wood an unusual structure is to infect it with a

fungus by storing it in a humid environment (''fungal decay''). The fungus penetrates the layers of the wood and changes the nature of the

cells Cell most often refers to: * Cell (biology), the functional basic unit of life Cell may also refer to: Locations * Monastic cell, a small room, hut, or cave in which a religious recluse lives, alternatively the small precursor of a monastery w ...

. This process creates individual patterns and shades of colour. The wood treated in this way is then excellently suited for the production of all kinds of design objects. In order to stabilise the wood structure weakened by the fungus,

resin In polymer chemistry and materials science, resin is a solid or highly viscous substance of plant or synthetic origin that is typically convertible into polymers. Resins are usually mixtures of organic compounds. This article focuses on nat ...

s or plastics are usually introduced into the material by special

vacuum A vacuum is a space devoid of matter. The word is derived from the Latin adjective ''vacuus'' for "vacant" or " void". An approximation to such vacuum is a region with a gaseous pressure much less than atmospheric pressure. Physicists often ...

processes. A special icing process applied to

beech Beech (''Fagus'') is a genus of deciduous trees in the family Fagaceae, native to temperate Europe, Asia, and North America. Recent classifications recognize 10 to 13 species in two distinct subgenera, ''Engleriana'' and ''Fagus''. The ''Engl ...

wood leads to results similar to those obtained with fungal decay. After the wood has been soaked, it is iced and then dried. The result is a very light wood with an almost black

grain A grain is a small, hard, dry fruit ( caryopsis) – with or without an attached hull layer – harvested for human or animal consumption. A grain crop is a grain-producing plant. The two main types of commercial grain crops are cereals and legum ...

. This result, which also occurs very rarely in nature, is called ''ice-beech''.

Natural durability

Natural durability is the inherent capability of wood to tolerate and resist fungal decay and

insect Insects (from Latin ') are pancrustacean hexapod invertebrates of the class Insecta. They are the largest group within the arthropod phylum. Insects have a chitinous exoskeleton, a three-part body ( head, thorax and abdomen), three pa ...

attacks, such as

woodboring beetle The term woodboring beetle encompasses many species and families of beetles whose larval or adult forms eat and destroy wood (i.e., are xylophagous). In the woodworking industry, larval stages of some are sometimes referred to as woodworms. The ...

s and

termite Termites are small insects that live in colonies and have distinct castes ( eusocial) and feed on wood or other dead plant matter. Termites comprise the infraorder Isoptera, or alternatively the epifamily Termitoidae, within the order Blat ...

s, and

marine organisms Marine life, sea life, or ocean life is the plants, animals and other organisms that live in the salt water of seas or oceans, or the brackish water of coastal estuaries. At a fundamental level, marine life affects the nature of the planet. ...

. This protective feature is attributable to specific biological compounds, called ''extractives'' that are toxic to wood-destroying organisms. Along with the tree’s growth, the sapwood converts into

heartwood Wood is a porous and fibrous structural tissue found in the stems and roots of trees and other woody plants. It is an organic materiala natural composite of cellulose fibers that are strong in tension and embedded in a matrix of lignin t ...

and this brings physical and chemical changes to the wood. As a result, the permeability decreases while the natural durability increases. Thus, the extractives responsible for natural durability are mainly present in the heartwood, although they may also be contained in small amounts in the sapwood. Different chemicals have been isolated from the heartwood of naturally rot-resistant trees and have shown to be protectants, including

polyphenol Polyphenols () are a large family of naturally occurring organic compounds characterized by multiples of phenol units. They are abundant in plants and structurally diverse. Polyphenols include flavonoids, tannic acid, and ellagitannin, some ...

s, lignans (e.g. gmelinol,

plicatic acid Plicatic acid is a carboxylic acid from the resin acid group. It is naturally found in ''Thuja'' and cypress resin. It is the main irritant and contact allergen present in thuja wood. (''Cf.'' pine, where the primary irritant is abietic acid.) ...

flavonoid Flavonoids (or bioflavonoids; from the Latin word ''flavus'', meaning yellow, their color in nature) are a class of polyphenolic secondary metabolites found in plants, and thus commonly consumed in the diets of humans. Chemically, flavonoids ...

s (e.g. mesquitol), tropolones (e.g. hinokitiol and other thujaplicins), sesquiterpenoids (e.g. α-cadinol). The natural durability varies between tree species, geographic regions, environmental conditions, growth stage, and increases with the age. Thereby, some trees are more resistant to fungal diseases and insects and their

lasts longer than other trees. Notably, the timber of these trees remain durable for a long-time period, even around a century, thereby they have been used as a reliable building material for centuries. Since the young trees do not produce enough protecting chemicals, some trees grow with a hollow, rotten trunk at an early age. However, the stands of old-growth trees are more naturally durable than second-growth stands. Tree species that have significant natural durability include '' Lagarostrobos franklinii'' (Huon pine), ''

Intsia bijuga ''Intsia bijuga'', commonly known as Borneo teak, Johnstone River teak, Kwila, Moluccan ironwood, Pacific teak, scrub mahogany and vesi, is a species of flowering tree in the family Fabaceae, native to the Indo-Pacific. It ranges from Tanzania ...

'' (Ipil), ''

Eucalyptus ''Eucalyptus'' () is a genus of over seven hundred species of flowering trees, shrubs or mallees in the myrtle family, Myrtaceae. Along with several other genera in the tribe Eucalypteae, including '' Corymbia'', they are commonly known as ...

'' species ( Ironbark), '' Podocarpus totara'' (Totara), ''

Vitex lucens ''Vitex lucens'', or pūriri, is an evergreen tree endemic to New Zealand. History Pūriri was first collected (by Europeans) at Tolaga Bay by Banks and Solander during Cook's first visit in 1769. The plant was excellently described by Soland ...

'' (Puriri), ''

Agathis australis ''Agathis australis'', commonly known by its Māori name kauri (), is a coniferous tree in the family '' Araucariaceae'', found north of 38°S in the northern regions of New Zealand's North Island. It is the largest (by volume) but not t ...

'' (Kauri), trees of the ''

Cupressaceae Cupressaceae is a conifer family, the cypress family, with worldwide distribution. The family includes 27–30 genera (17 monotypic), which include the junipers and redwoods, with about 130–140 species in total. They are monoecious, subdioeci ...

'' family, such as ''

Chamaecyparis obtusa ''Chamaecyparis obtusa'' (Japanese cypress, hinoki cypress or hinoki; ja, 檜 or , ) is a species of cypress native to central Japan in East Asia, and widely cultivated in the temperate northern hemisphere for its high-quality timber and or ...

'' (Hinoki cypress), ''

Thuja plicata ''Thuja plicata'' is an evergreen coniferous tree in the cypress family Cupressaceae, native to western North America. Its common name is western redcedar (western red cedar in the UK), and it is also called Pacific redcedar, giant arborvitae, w ...

'' (Western red cedar), '' Thujopsis dolabrata'' (Hinoki asunaro), '' Juniperus cedrus'' (Canary Islands juniper), '' Cedrus atlantica'' (Atlas cedar), ''

Chamaecyparis taiwanensis ''Chamaecyparis taiwanensis'' (Taiwan cypress; ) is a species of cypress, native to the mountains of Taiwan, where it grows at altitudes of 1300–2800 m.Rushforth, K. (1987). ''Conifers''. Helm . Description It is a slow-growing coniferous tre ...

'' (Taiwan cypress), and many other species belonging to this family. According to th
EN 350:2016 standards
by the

APA – The Engineered Wood Association APA – The Engineered Wood Association is a nonprofit A nonprofit organization (NPO) or non-profit organisation, also known as a non-business entity, not-for-profit organization, or nonprofit institution, is a legal entity organized and ...

, the durability of wood and wood-based products to fungal decay can be classified into five categories: very durable (DC1); durable (DC2); moderately durable (DC3); slightly durable (DC4); and not durable (DC5). And the durability to insect attacks can be categorized as durable (DC D); moderately durable (DC M); and not durable (DC S). Generally, the heartwood of durable tree species is considered as very durable, whereas the sapwood of all tree species is considered as not durable and is the most vulnerable.

Wood preservation

A wide selection of timber preservation has been developed to give the wood an improved durability and to protect it from decay. The wood can be treated according to the purpose (biological protection, e.g. fungi, insects, marine organisms) and the environment (interior, exterior, above ground, in ground, in water) of its use. Timber preservatives include

chromated copper arsenate Chromated copper arsenate (CCA) is a wood preservative containing compounds of chromium, copper, and arsenic, in various proportions. It is used to impregnate timber and other wood products, especially those intended for outdoor use, in order to p ...

(CCA),

alkaline copper quaternary Alkaline copper quaternary, usually abbreviated ACQ, is a type of water-based wood preservative product containing a soluble copper(II) complex and quaternary ammonium alkyl- or aryl-substituted compounds ("quats"). Thus the product was original ...

(ACQ), copper azole (CuAz), borates,

sodium Sodium is a chemical element with the symbol Na (from Latin ''natrium'') and atomic number 11. It is a soft, silvery-white, highly reactive metal. Sodium is an alkali metal, being in group 1 of the periodic table. Its only stable ...

and

potassium silicate Potassium silicate is the name for a family of inorganic compounds. The most common potassium silicate has the formula K2SiO3, samples of which contain varying amounts of water. These are white solids or colorless solutions.Gerard Lagaly, Werner ...

, oil-based preservatives, such as

creosote Creosote is a category of carbonaceous chemicals formed by the distillation of various tars and pyrolysis of plant-derived material, such as wood or fossil fuel. They are typically used as preservatives or antiseptics. Some creosote types were ...

and

pentachlorophenol Pentachlorophenol (PCP) is an organochlorine compound used as a pesticide and a disinfectant. First produced in the 1930s, it is marketed under many trade names. It can be found as pure PCP, or as the sodium salt of PCP, the latter of which dis ...

, light organic solvent preservatives (LOSP),

propiconazole Propiconazole is a triazole fungicide, also known as a DMI, or demethylation inhibiting fungicide due to its binding with and inhibiting the 14-alpha demethylase enzyme from demethylating a precursor to ergosterol. Without this demethylation ste ...

- tebuconazole-

imidacloprid Imidacloprid is a systemic insecticide belonging to a class of chemicals called the neonicotinoids which act on the central nervous system of insects. The chemical works by interfering with the transmission of stimuli in the insect nervous system ...

epoxy Epoxy is the family of basic components or cured end products of epoxy resins. Epoxy resins, also known as polyepoxides, are a class of reactive prepolymers and polymers which contain epoxide groups. The epoxide functional group is also co ...

resins, wood acetylation, natural or biological preservation, such as treatment with heat (

thermally modified wood Thermally modified wood is wood that has been modified by a controlled pyrolysis process of wood being heated in absence of oxygen inducing some chemical changes to the chemical structures of cell wall components (lignin, cellulose and hemicellu ...

), mud,

tung oil Tung oil or China wood oil is a drying oil obtained by pressing the seed from the nut of the tung tree (''Vernicia fordii''). The oil and its use are believed to have originated in ancient China and appear in the writings of Confucius from ab ...

, impregnation using biopolymers from agricultural waste ( biological modified timber), covering wood with copper sheathes, etc. Treatment of timber with natural extractives derived from rot-resistant trees, such as hinokitiol, tannins, and tree extracts, is another promising environmentally-friendly wood preservation method. The more permeable is the wood, the easier is it to treat. According to th
EN 350:2016 standards
the treatability of woods can be categorized in four levels: (1) easy to treat; (2) moderately easy to treat; (3) difficult to treat; and (4) extremely difficult to treat.

Safety

Over the years a lot of concerns have arisen regarding the

arsenic Arsenic is a chemical element with the symbol As and atomic number 33. Arsenic occurs in many minerals, usually in combination with sulfur and metals, but also as a pure elemental crystal. Arsenic is a metalloid. It has various allotropes, b ...

and

chromium Chromium is a chemical element with the symbol Cr and atomic number 24. It is the first element in group 6. It is a steely-grey, lustrous, hard, and brittle transition metal. Chromium metal is valued for its high corrosion resistance and hard ...

contents of the CCA. In 1986, the U.S.

Environmental Protection Agency A biophysical environment is a biotic and abiotic surrounding of an organism or population, and consequently includes the factors that have an influence in their survival, development, and evolution. A biophysical environment can vary in scale ...

(EPA) recognized arsenic as a human carcinogen. Water contamination with arsenic and its compounds is a serious public health issue, and their release to the environment and soil pollution is another environmental problem. Different regulatory interventions have been undertaken worldwide to restrict their use in the wood industry, especially in timber for residential use. By the end of 2003, the U.S EPA and the wood industry agreed to discontinue the use of CCA in treating timber for residential use. Its use is also prohibited in

Canada Canada is a country in North America. Its ten provinces and three territories extend from the Atlantic Ocean to the Pacific Ocean and northward into the Arctic Ocean, covering over , making it the world's second-largest country by to ...

Australia Australia, officially the Commonwealth of Australia, is a sovereign country comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands. With an area of , Australia is the largest country by ...

, and the

European Union The European Union (EU) is a supranational union, supranational political union, political and economic union of Member state of the European Union, member states that are located primarily in Europe, Europe. The union has a total area of ...