Microbial symbiosis can occur between microorganisms and between microorganisms and “higher” organisms such as plants, insects and animals. The relationship can be either mutualistic, commensalistic, parasitic or neutralistic. One organism may exist inside the cell or tissue of another (endosymbiosis) or live close to but outside the partner’s tissue (ectosymbiosis).

The advantages that the relationship confers to the partners tend to be either nutritional or for protection but sometimes it is not obvious. Nutritional relationships include the synthesis by one partner of a vitamin (or components to make the vitamin) which the other partner cannot synthesis on its own (ref/example).

Lichens are a symbiotic relationship (ectosymbiosis) between an algae and a fungus with both benefiting. The algae gains protection against the elements by living in close proximity to the fungus, whilst the fungus receives nutrients from the photosynthesis carried out by the algae.

Plants form a close relationship with both bacteria and fungi. Nodules found in leguminous plants, such as pea, clover, soybean, alfalfa are the result of the association between bacteria and the plant. It is estimated that around 15,000 legume species form this relationship (Sprent, 2001). Non-leguminous plants such as Casuarina and Alnus spp. may also form nodules on the root. In other cases, the bacteria inhabit specialised plant structures e.g. in Azolla or form nodules in other parts of the plants such as the stem.

Rhizobia bacteria invade the plant tissue and form structures on the root. Inside, the rhizobia bacteria “fix” nitrogen to a form which the plant can use. The plant benefits from the nitrogen, whilst the bacteria benefit by being protected against predation and an environment is created which is conducive to the correct functioning of nitrogenase, the enzyme involved in reducing the atmospheric nitrogen to ammonia. Nitrogenase is sensitive to the level of oxygen present in the nodule and to the level of ammonia. Its activity is controlled by leghaemoglobin, produced by the host but which may divert oxygen to the symbiont.  However, the formation of nodules and the activity of the rhizobia is a drain on the plants’ resources with around 1/3^rd of the material produced by photosynthesis being diverted to the nodule in young plants. Nodule formation is sensitive to pH, with a neutral-to-alkaline preferred, and the levels of available aluminium.

The zone surrounding the plant root (the “rhizosphere”) is home to a large number of microorganisms which live as symbiots on or around the root. Nutrients produced from photosynthesis is sent to the plant root and is “leaked” into the surrounding soil. Here it is utilised by the microbial flora. The plant gains by some microbial by-products and having a “sink” for excess material produced by photosynthesis.

Fungi form four symbiotic relationships with plants: ecto -, endo(arbuscular)-, ericaceous- and orchidaceous- mycorrhiza which are differentiated on the basis of their interaction with their host(s) and the species of host (Table 1).

In the ectomycorhiza, the fungal hyphae surrounds the plant root but does not penetrate into the root cells but instead grows in the spaces between the plant root cells to forma net around the root. This causes physical changes to the roots in that the plant no longer grows fine root hairs and the roots become thicker at the infection site (root tips) and may become a different colour. About 6000 tree species form this type of association including oaks, willow, poplar and pine. Plants may form more than one association with different species of fungi.

The other relationships are endotrophic in that the fungus invades the host tissue. In the arbuscular mycorrhiza a globular or arbuscule is formed within specific root cells. These fungi are found in grassland, savannah and tropical forests (Read & Perez-Moreno, 2003). This association is thought to occur in about 90% of all plants except members of the cabbage and mustard family. Similar to the arbuscular are those found in the orchids (orchidaceous mycorrhiza) and the Ericaceae (the ericoid mycorrhiza) (e.g. heather, rhododendron). In orchids, the fungus helps seed germination and early seedling growth. In some species, the association continues throughout the life of the plant with the fungus supplying nutrients such as phosphorous or nitrogen which would otherwise be unavailable to the plant.

These types of associations are essential in maintaining plant community health and can provide resistance to disease and drought as well as supplying a wide range of limiting nutrients (van der Heijden et al., 2008). In this way, fungi living below ground significantly influence the plants growing above in their productivity and vitality.

In a similar manner, bacteria may enter into the tissue of plants and fungi. Once thought to be weak pathogens, these endosymbiotic bacteria have been found in a number of plants (Lodewyckx et al., 2002) and can promote the growth of the plant through nitrogen fixation, the synthesis of hormones and enzymes or by producing anti-fungal agents (Hallmann et al., 1997). Endosymbiotic bacteria have been found in the basidiomycete fungus Laccaria bicolour (Bertaux et al., 2003) as well as several arbuscular mycorrihza fungi (Artursson et al., 2006). Endophytes may have originated from the bacterial communities living on the root or leaf surfaces and may pass from plant to plant via seeds or, in the case of fungi from spore to hyphae.

There are a large number of symbiotic associations with microorganisms and animals. Many microorganisms (bacteria, fungi & protozoa) inhabit the rumen or intestine of the animals including insects such as termites. In many cases, the microorganism is responsible for breaking down material such as cellulose which the host cannot or for supplying nutrients from simpler molecules (Leadbetter et al., 1999).

Table 1. Types of mycorrizhal associations (adapted from Ritz, 2005)

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