Saprophytes and symbionts

One of the most remarkable adaptations of plants to their environment is their method of obtaining food. Apart from the many autotrophic plants that manufacture food by photosynthesis there are some heterotrophic organisms that live off other organisms (as parasites), or off their decaying matter (as saprophytes) or in a mutually beneficial relationship (as symbionts). They are frequently found in inhospitable environments such as dark, dense forests, glacial polar and tundra regions, and even underground, but they also occur in our everyday environments.

Saprophytes are organisms that secure their food directly from the dead and decaying tissues of other organisms. They do so because they lack chlorophyll and cannot photo-synthesize or because they contain few green parts, which allow the manufacture of only a small amount of the nutrients that they need. They include bacteria, some algae, fungi, and some flowering plants.

Symbiosis means, literally, living together. Some biological definitions restrict the meaning to a permanent association of two different organisms with a movement of metabolites between the two in which each derives an advantage from the other. Many symbioses involve an alternation between parasitism and symbiosis. Symbioses may involve an association between plants and animals, such as the algal cells in the coelenterate Hydra, in sea squirts and mollusks, or between different plant types, to form lichens, root-nodules, and mycorrhiza.

The saprophytic orchid Indian pipe (Monotropa uniflora) has no chlorophyll and lives underground. It sends up shoots above ground only when it is about to flower, each shoot bearing a single bloom. The plant lives off the decaying leaf litter on the forest floor.

Saprophytes

True saprophytes secrete enzymes to break down the complex carbohydrates and proteins of food sources and then absorb the soluble foodstuffs into their cells. The most familiar saprophytes are fungi, particularly the basidio-mycete mushrooms, and shelf fungi. Their hy-phae form a meshed network called a mycelium, which spreads over the substrate and penetrates it. The hyphae secrete digestive enzymes that dissolve the solid components of the surrounding material; the solution is then absorbed through the membranes of the fungal body.

Most flowering saprophytes are heath species and are found in tropical Asia and Australia. They start their life as saprophytic tissue underground, nourished also by mycorrhizae; they later develop green stems and leaves. In some species, however, these parts may take several years to appear, and other species remain completely saprophytic throughout their life. In most of these species the plant lies underground except when flowering, for example Indian pipe (Monotropa uniflora), but several species even flower underground, like the Australian orchids Cryptanthemis slateri and Rhizanthella gardneri.

Lichens

A lichen is a permanent association between a fungus and an alga in which the two symbionts form a single thallus. The fungus parasitizes the algal cells, extracting the carbohydrates that are formed by algal photosynthesis. It also lives saprophytically on algal cells that die. In turn, the fungus protects the alga from high light intensity and provides it with a structure that is more resistant to desiccation than the algal cell walls are.

The algal symbiont is usually a cyanobacterium (blue-green alga) or green alga; the fungal symbiont is in most cases an ascomycete, although a few lichenized basidiomycetes are known. Each species of lichen is an association of a particular algal and fungal species. Most display one of three morphologies: leafy (foli-ose), encrusting (crustose), and erect and tufted (fruticose).

Lichens occur worldwide and can tolerate nutrient-poor, hostile environments. They grow on exposed rock in deserts and polar regions, on solidified lava, on the bark of trees and on leaf surfaces (especially in tropical rain forests), on gravestones, and on the asbestos roofs of buildings. The main deterrent to lichen growth is atmospheric pollution—many species are sensitive to such pollutants as sulfur dioxide.

Root-nodules

The symbiosis between certain soil bacteria and leguminous plants to form nitrogen-fixing root-nodules is of vital importance to modern agriculture. The root hairs of legumes are invaded by the aerobic bacterium Rhizobium spp., which penetrates the cortical tissue and multiplies there at the expense of the host cell nutrients and enzymes. The host cells divide and enlarge to form a nodule, the cells of which become densely filled with millions of bacteria. The bacteria cells fix atmospheric nitrogen and the legume digests the bacteria, thus obtaining the nitrogen compounds for food. The nodule finally dies, and large populations of undigested bacteria return to the soil. The root-nodules of alder [Alnus sp.) and other trees contain symbiotic actinomycetes, which are also capable of fixing nitrogen.

The roots of a plant without fungi (A) absorb water and what mineral nutrients they can from the surrounding soil. Many plants, however, have mycorrhizal fungi surrounding their roots (B). These fungi extract minerals more easily from the soil, which they pass to the roots through their hyphae that penetrate the root cortex. In turn, they absorb some of the carbon compounds manufactured by the plant through photosynthesis.

Mycorrhizae

A mycorrhiza is a symbiotic association between the hyphae of certain fungi and the roots of higher plants. The mycorrhizal fungus increases the solubility of soil minerals and improves the uptake of nitrogen, potassium, and phosphorus by the host plant, protects the host roots against pathogens, and produces plant growth hormones. In return, the fungus receives a carbohydrate food supply from the photosynthetic activity of the host.

Two main types of mycorrhizae occur. In endomycorrhiza the fungal hyphae live inside the host root between and inside the cells; the fungi are usually zygomycetes, although those found in the aerial roots of tropical orchids are basidiomycetes. In ectomycorrhizae a mantle of fungal hyphae covers the root externally with some hyphae growing among the cells of the cortex; these fungi are mainly gilled or pore basidiomycetes and include Boletus and some puffballs, although some are ascomy-cetes, such as truffles (Tuberales). The ectomycorrhizae are most common on tree roots rather than roots of herbs.

The encrusting lichen Xanthoria palietina is a common feature on gravestones.
A lichen (A) forms when fungal hyphae surround an , algal cell (B). Gradually the hyphae and algal cells multiply. The photosynthetic algal cells become trapped in the upper layers of the thickly intermeshed hyphae. Loosely interwoven hyphae form the middle layer of a mature lichen (C).