The importance of waste

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In the previous chapter I argued that the key idea at the heart of ecology is that organisms must use their environment both as a source of energy and as a dumping ground for their waste products. Also that any process which organisms use to extract this energy cannot be 100% efficient. I described how these ecological ideas are linked to crucial concepts from the physical sciences such as the second law of thermodynamics and entropy. These arguments can be summarized as:

As such, there is still potentially free energy available in the waste products of organisms: as illustrated by the smaller font size used for the energy in waste products in this schematic representation. These waste products can potentially have important effects on other species, as illustrated by the problems caused by 'pollution'; that is, human waste products.

One approach to trying to visualize the quantity and importance of waste products produced by the Earth's biota is to think about the temperate deciduous forests that dominated much of Europe, eastern North America, China, and Japan, before being reduced by human actions. Every year, at the end of summer, the leaves on the trees are transformed from the greens of chlorophyll to the reds and yellows of anthocyanins and carotenoids, before falling to the forest floor. These leaves, along with the other plant and animal debris produced over the year amount to approximately 5 tonnes of biological waste per hectare per year. The figure for highly productive tropical forests can be an order of magnitude higher, at approximately 60 tonnes per hectare (Spooner and Roberts, 2005). It is common observation that something must happen to get rid of this waste, since when we enter a forest we are not up to our necks in leaf litter!

Organism

^ some energy used ^ Energy (waste)

The main strategy of this book is to use astrobiological thought experiments to give a wider perspective to thinking about Earth-bound ecology. As such it is worth considering the simplest possible biosphere, a hypothetical planet inhabited by only a single species; over time this species would convert its resources into both copies of itself and waste products. As these individuals died they would also be added to the waste product pool. Eventually such an ecological system would run down as the species ran out of crucial resources, which had become locked up in waste products. As Tyler Volk (1998, p. 52) has written, 'A monoculture planet is therefore a thought experiment with no place in reality.' I will return to thinking about this hypothetical planet in more detail in Chapter 9.

Clearly in any successful ecological system it is necessary for material in waste products to be recycled to make them available for reuse (Fig. 3.1). Any ecological system that produced an intractable waste product which couldn't be broken down would, on 'geological' time scales, be in trouble. A classic example of a biochemical product which is unusually difficult to break down is lignin (Box 3.1). Shortage of lignin-degrading guilds in the later Palaeozoic may explain the large coal deposits formed at this time. This build up of plant material in sediments would have been exacerbated by the fact that the large plants of

Fig. 3.1: This beach on the Spanish island of Mallorca shows what happens when a substance which is hard for organisms to break down (plastic) is added to a system over time. Plastic waste can now be found in quantity even on very remote beaches around the world (Barnes, 2002), as it is constantly being added to the marine system but only slowly broken down.

Fig. 3.1: This beach on the Spanish island of Mallorca shows what happens when a substance which is hard for organisms to break down (plastic) is added to a system over time. Plastic waste can now be found in quantity even on very remote beaches around the world (Barnes, 2002), as it is constantly being added to the marine system but only slowly broken down.

Box 3.1: Lignin and why its so hard to biodegrade

Lignins are phenylpropanoids and are the second most abundant polymer in most plants (after cellulose). They provide strengthening for cell walls and xylem vessels. They are also very important compounds in the Earth system, it is estimated that phenylpropanoids contain 30% of all organic carbon in the biosphere, most of this in the form of lignin (Davies, 2004). The burial of such compounds (e.g. in the Carboniferous coal deposits) has implications for the carbon and oxygen cycles which are discussed in Chapters 7 and 8.

There are three main reasons why lignin is difficult for organisms to break down (Robinson, 1990).

1. Molecules of lignin are insoluble in water and are too heterogeneous to be easily disassembled by specific enzymes.

2. Many lignin degradation products are themselves difficult to break down and/or are toxic to many organisms.

3. The C : N ratios in lignins are so high that few organisms could live on lignin even if they could break it down into usable compounds. Some of the fungi which can break down lignin get round this problem by trapping and digesting nematode worms using special hyphae, so partly subsidizing their nitrogen intake. Examples of fungi which use this strategy include the Oyster Cap fungi Pleurotus spp., which specialize in growing on wood (Spooner and Roberts, 2005).

the time (e.g. Club Mosses and Ferns) used more lignin in their structure than later woody plants (Robinson, 1990). The key members of the modern lignin degrading guild are fungi, particularly Basidiomyetes, many of which produce the familiar 'mushroom' fruiting bodies. These 'white rot' fungi break down both lignin and cellulose, their food source is the cellulose, however, they need to break down the lignin enzymatically to get at it (Fig. 3.2). This process leaves the remaining wood looking 'stringy, bleached and soft' (Spooner and Roberts, 2005). Had lignin consuming organisms not evolved this could have proved problematic for abundant life on Earth, with increasing amounts of resources entombed in organic deposits and unavailable to life. While simple microbial systems may be able to continue in such conditions, it is difficult to see how an active and complex biosphere could be supported with such a lack of recycling.

Fig. 3.2: The Stinkhorn Phallus impudicus is a common species of 'white rot' fungus in Britain, mainly colonizing dead wood after it has fallen to the forest floor (Spooner and Roberts, 2005). Such lignin degrading organisms are crucial in the breakdown of woody plant remains. In addition to its wood-rotting ability the fruiting body of the fungus is famed for its phallic shape, celebrated in its scientific name, and its obnoxious smell which attracts flies and slugs to aid its spore dispersal (Ramsbottom, 1953). The example in the photograph has just finished spore dispersal, the blackish liquefied spore mass having been largely removed from its head by visiting invertebrates.

Fig. 3.2: The Stinkhorn Phallus impudicus is a common species of 'white rot' fungus in Britain, mainly colonizing dead wood after it has fallen to the forest floor (Spooner and Roberts, 2005). Such lignin degrading organisms are crucial in the breakdown of woody plant remains. In addition to its wood-rotting ability the fruiting body of the fungus is famed for its phallic shape, celebrated in its scientific name, and its obnoxious smell which attracts flies and slugs to aid its spore dispersal (Ramsbottom, 1953). The example in the photograph has just finished spore dispersal, the blackish liquefied spore mass having been largely removed from its head by visiting invertebrates.

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