19 March 2012

Fungi: rotting civilisation from its very foundations

Wood is one of the most useful natural resources to man due to its unique and remarkable mechanical properties and consequently, has been heavily used by humans throughout our history. In particular, wood has been heavily exploited for its use in building and furniture construction, mainly since it is readily available and is able to bear heavy loads at a relatively small diameter. Wood is also extremely important as the basis of paper, an industry that produces over 300 million metric tons of paper-based products every year.

The main reason that wood has such remarkable and desirable properties is due to the fact that each individual wood cell is very strong, with thick cell walls made from the dense wood matrix. This matrix is a complex mixture of polysaccharides (cellulose and hemicelluloses), lignin, starch, pectin and various other proteins; and although the relative quantities of lignin and hemicelluloses differ between hardwoods and softwoods, wood remains highly resilient to degradation and very few organisms are able to break it down.

The most abundant and arguably, most important, organisms that are able to break down wood are wood decaying fungi, which include white-rot, brown-rot and soft-rot species; and wood decolourising fungi, such as moulds and blue stain fungi. Such organisms are extremely common and break down enormous quantities of wood each year, having important roles in ecological nutrient recycling systems, such as the Carbon and Nitrogen Cycles, where they release the nutrients that are 'locked away' in dead biological material back into the environment.

Although wood decolourising fungi are not technically true wood decaying fungi because they mainly deteriorate the aesthetic properties of wood and do not weaken its strength, they are still highly destructive. This is mainly because they increase the permeability of wood cells by riddling them with holes, which makes them more vulnerable to colonisation by true wood-rotting fungi, discussed below, that actively degrade the wood. The main issue then, that humans have with wood decolourising fungi is that it can severely damage and even completely ruin pieces of artwork. This is mainly because such species of fungi are highly resilient and are able to digest both the paint dyes themselves and the canvas or paper that the artwork was created on. Thus, wood decolourising fungi are a nightmare for art galleries and museums, being extremely difficult to remove without further damaging the art. A further problem that wood decolourising fungi can cause is that many species produce volatile chemicals and mycotoxins that can cause allergic reactions in some people, or release spores that can cause respiratory diseases. A common example of this is Aspergillosis, caused by Aspergillus moulds.

This photograph shows a painting (left) that had been invaded by wood decolourising fungi and (right) the same painting after it had been treated for the fungi using a plant oil conservation technique. Note however, that although much of the painting has been restored, it is still irreversible damaged.

Although losing pieces of art to wood decolourising fungi is annoying and sometimes even heartbreaking, it is nothing compared to the damage that can be caused by true wood-rotting fungi. True wood decaying fungi degrade all of the different polymers in wood, which causes significant and irreversible structural damage that greatly reduces its overall strength - effects that are incredibly detrimental to wood used in the construction of buildings, ships and furniture! There are three main types of true wood decaying fungi: white-rot fungi, brown-rot fungi and soft-rot fungi, which each affect hardwoods and softwoods differently and have varying prominence among different climates.

Brown-rot fungi predominantly attack wood by degrading its constituent polysaccharides and cause little, if any, damage to lignin. Brown-rot decay mainly effects softwoods and is characterised by a rapid depolymerisation of cellulose that causes a reduction in the mechanical properties of wood. The breakdown of cellulose is believed to be accomplished via an oxidative process, which involves the production of a highly reactive compound called hydrogen peroxide when the fungi breaks down hemicelluloses. It is thought that these molecules of hydrogen peroxide then diffuse into the middle, or S2 layer, of wood cell walls and cause the general decay of cellulose; although scientists don't know for sure... However, whatever the mechanism by which brown-rot fungi works, it gives the wood a brown colouration (hence the rots name) and the wood usually displays a brick-like pattern of cracks and splits, which results from an uneven pattern of decay. Brown-rot is most commonly seen in buildings and species such as Antrodia and Gloeophyllum are frequently seen on furniture, the main products made using softwood.

The brown-rot Serpula lacrymans, commonly known as dry rot, attacking an abandoned house in the USA.

White-rot fungi differ from brown-rot species because they degrade both lignin (they are the only known organisms are are able to digest it completely) and polysaccharides. White-rot is more common in hardwoods, where they digest lignin by secreting non-specific lignin mineralising enzymes (LMEs) into the surrounding wood. The most important LME is laccase, which breaks lignin down into carbon dioxide. Lignin is a brown pigment and is responsible for giving wood its dark colouration. Thus, white-rot fungi 'bleaches' the wood that it has invaded and turns it paler over time, as it digests more and more  lignin for food - giving the rot its name.

Fomitopsis pinicola, a species of white-rot fungi that has invaded a broken tree trunk and  is completely stripping it of lignin. The digested wood is white and very brittle.

Soft-rot decay is characteristic of wood in wet or damp conditions, such as the wood in ships, fence posts and windowsills in buildings (if they condense frequently). Such decay mainly involves the degradation of polysaccharides and only very rarely the breakdown of lignin. Soft-rots attack hardwoods and softwoods non-preferentially, which they accomplish by extending long tube-like growths called hyphae, into individual wood cells. Their hyphae usually enter cells through pits on the cell's surface and once inside, they split into fine-penetration branches that grow through the inner layer of the wall and into the cellulose-rich S2 layer. Thus, they are able to digest the polysaccharides in the wood and cause havoc for humans because they can degrade wood incredibly quickly.

As mentioned before, wood decaying fungi are extremely important in nature and are essential organisms in food chains. The natural breakdown of wood and dead biological material is even useful to humans, particularly for farmers, as it helps to maintain the fertility of soils so that their crops grow better. The main problem of decaying fungi however, occurs when they invade wood that has an anthropogenic use because they are extremely destructive. This destruction has many impacts for humans as decaying fungi can undermine the structural stability of buildings so that they become unsafe and eventually, collapse; rot away any clothing that uses natural, cellulose-based fibres, such as cotton; and destroy books and paper-based products, which was a much bigger issue in the past since books were the primary way of recording information and knowledge.

Mould that has invaded a room and is degrading the walls from their base. Eventually, the wood will become too weak and degraded to hold the weight that it is supporting and the wall will collapse.

Fortunately, methods have been devised to control wood decaying fungi, although it is impossible to prevent their invasion forever. The most common way of protecting wood from rotting fungi is adding an anti-fungal preservative to it either during its production or onto the finished product, usually as an ingredient of a paint or varnish. Common preservatives include copper azole and chromated copper arsenate.

Thus, despite the high efficiency of wood decaying fungi and wood decolourising fungi in invading and degrading wood, the damage that they cause can be prevented with regular cleaning and by using appropriate anti-fungal chemicals. Therefore wood, one of the most important natural resources to man, can be effectively protected and as a result, the wood treatment industry is a huge and incredibly profitable business that is critical to the stability of our civilisation.

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