Life in earth

Forest and Bird, Issue 303, 1 February 2002, Page 36

Life in

earth

Dr PETER MADDISON

finds the soil teeming with life

forms, but has concerns about genetic engineering.

Le in the soil is complex and abundant. It’s not something we generally think about, but what goes on underground has been a particular interest of mine during the debate on genetic engineering. It’s one of the reasons I’ve been advocating a precautionary approach to genetically modifying organisms, in case we disturb this web of life. There are some surprising facts about the soil ecosystem, unseen as it generally is. Underground ecosystems are extensive and include more biodiversity than the moreobvious systems above ground. The web of life in these environments includes complex associations between the roots of plants, fungi, and bacteria in legume roots. In fact there is an extensive network of fungal mycelia underground, part of a large inter-related food web relating to the breakdown of organic matter, all crucial to the carbon, nitrogen and oxygen cycles. Amongst the other organisms involved in these

processes are earthworms, insects, mites, nematodes, protozoa and bacteria. The network of cavities and holes left by animals moving below ground is very important too, as general habitat for many animals, from earthworms to insects and lizards. There is also an aquatic network which, as a thin film of water on the soil particles, allows for the existence of another large assembly of fauna, such as protozoa, nematodes, rotifers and tardigrades. It is evident that this vast web of life is crucial to the growth of plants. It is also inherently part of the basic elemental cycles which circulate and renew physical life — the carbon, nitrogen, sulphur, phosphorus and oxygen cycles.

Coupled with the understanding of these soil networks is the need to know about of some of the component players — the life forms which exist . be

underground. They include: BACTERIA AND ARCHAEA These two groups are so significant they are now both classified as kingdoms (giving them equivalent biological status to the kingdoms of Plantae and Animalia — plants and animals). They are comprised of many diverse ‘simple’ organisms. Bacteria cells, for example, lack nuclei and their DNA exists in a single chromosome. Because of this ‘simplicity’, bacteria have been much used in genetic engineering — Escherichia coli (the E. coli of many gut-rot episodes!) in particular. The numbers of bacteria are truly staggering. Perhaps the most graphic example is to note that there are more bacteria in one human mouth than there are people on this Earth. Coupled with these unimaginably vast ee numbers

is the fact that bacteria are very small — less than one micron in length — and are therefore not directly observable without a powerful microscope. Bacteria reproduce by binary fission (cells which simply divide) and can rapidly reproduce colonies of millions. Perhaps the most well-known examples of bacterial reproduction are those associated with disease — the quick colonisation of a cut by septicaemia, or the speed with which a ‘strep throat’ develops. Bacteria are also highly adaptable. They shuffle DNA between their cells by processes such as conjugation, transduction and transformation. In conjugation, fragments of plasmid DNA are transferred between donor and recipient bacteria. In transduction, DNA fragments are transferred between bacteria by a bacteriophage (a virus that infects bacteria). In transformation, living bacteria can take up DNA fragments from decomposing bacteria, released into water or soil.

Consequently there exists the possibility for bacteria to change and incorporate new DNA, a form of natural genetic

engineering. They can be hard to kill too. Bacteria can form resistant cysts that are able to survive a variety of extreme environmental conditions. Such cysts may remain dormant in the soil (or elsewhere) for years.

PROTOZOA These singlecelled organisms or Protista are > also now classified

as a separate biological kingdom. Like the bacteria,

protozoa are a very diverse group of small organisms, many of which live in the soil. Protozoa also have the ability to multiply rapidly and to survive (encyst) through periods of unfavourable conditions. EARTHWORMS Earthworms are key components of the soil fauna. They are important in incorporating organic materia! into the soil and thereby improving soil fertility and soil structure. New Zealand has a large endemic fauna of worms, some reaching over a metre in length.

NEMATODES These small worms are both free-living and parasitic on plant roots or fungi. Some are involved in the breakdown of organic matter. ARTHROPODS A vast range of Arthropoda occur in the soil environment — from Copepoda (tiny shrimp-like creatures) through insects, mites, millipedes to isopods bh (slaters or woodlice) and centipedes. The 4 ; importance of these |

animals in the al breaking down of organic matter cannot be overstressed. Along with the other fauna in the soil they can be part of the process of breaking down the dead bodies of animals and plants, and their waste products, into simple elements which are thereby made available for recycling. FUNGI Fungi grow underground, forming large threads through the soil. These penetrate decaying material and are important in decomposition. They also make links with the roots of plants. The associations so formed are crucial to the growth of many plants, particularly trees and orchids, to which the fungi attach themselves. There has been very little study done (or even considered) regarding the likely effects of genetically modified organisms on soil organisms. Soil

fauna and flora form very complex associations and communities. They are vital to the survival of all terrestrial plants and animals, and to human activities such as agriculture and horticulture. Guarding the soil against the possibilities of contamination is fundamental to conservation. DR PETER MADDISON is an ecological consultant, and a member of Forest and Bird’s national

executive. His Ph.D dissertation was on the effects of pesticides on the interactions of soil fauna.