Symbiosis in the Rainforest

Skyrail Nature Diary: March 2013

This month we’ll be looking at symbiosis in the rainforest. Symbiosis is the phenomenon of two or more unrelated organisms living together in cooperation. Many people associate symbiosis with animals living together. The clown fish and the sea anemone is a well known example. Most symbioses involve at least one microorganism. Several of these symbioses are vitally important to the survival of whole ecosystems. For those interested in the Great Barrier Reef, it’s worth mentioning the symbiosis between coral polyps and tiny dinoflagellate algae that live together in what is essentially a wet desert. Without this symbiosis, the coral reef could not survive. How tight a symbiosis is varies considerably depending on the needs of the partners involved. Often it’s hard to know whether or not the relationship is actually beneficial to both partners. There is also the problem of reproducing while still maintaining the symbiosis. Lichens have solved this problem by simply breaking off pieces of their bodies that contain both partners which grow up to become new lichens. Others have to get together by various means after spores or seeds have germinated on their own. Various chemicals have been shown to attract partners to each other. For fungi this is a simple matter of growing towards the source of the attractant and making the connection. Algae often have mobile spores that can move towards their partner as long as they have water to move in.

Mycorrhizae are perhaps the single most important association in the tropical rainforest. All trees seem to have a close relationship between their roots and a fungus. The term mycorrhiza actually means “fungus root”. Fungi are generally much better at collecting minerals than tree roots. The main reason for this is their unique body shape as a simple mass of very long threads. Many fungi are also able to break down compounds as most other organisms are unable to get at the locked up nutrients. However, rainforest floors generally have very little organic material available. This is where the trees come in. Trees are amazing producers of organic material via the use of photosynthesis, the process of using sunlight as an energy source to turn simple carbon dioxide into food. In some rare cases the mineral collectors are a special kind of fungal like bacteria known as actinomycetes. European Alders have actinomycets on their roots. The fungal partners are of three basic types. The ectomycorrhizae attach to the tree roots as an external sheath. Most conifers have this kind of mycorrhiza. There are two types of fungi that penetrate the insides of the tree roots. One simply drills into the roots while maintaining its thread shape. The Vesicular-Arbuscular Mycorrhizae form into bladder and bush like structures that interact very closely with the roots. This latter type of mycorrhiza is by far the most common in nature.

Nitrogen fixation is an important process in nutrient poor soils. By far the most common source of nitrogen is in the atmosphere in the form of gaseous nitrogen. This molecule is very difficult to break down and only bacteria have been shown to be able to do it successfully. The process of turning this gaseous nitrogen into ammonia is known as nitrogen fixation. The process is interestingly disabled by the presence of oxygen, yet the most common nitrogen fixers need oxygen to live! Cyanobacteria have devised an ingenious way to circumvent this problem. They simply make special cells known as heterocysts where oxygen is completely removed. All food is imported from neighbouring cells. The Azolla ferns and the majority of cycads have entered into a symbiotic relationship with these cyanobacteria. Azolla ferns have special chambers inside their tiny leaves for the cyanobacteria while cycads have special surface roots called coralloid roots to house them. All legumes, on the other hand, cooperate with special bacteria known as rhizobia. The rhizobia are housed in special organs on the roots known as nodules. These nodules are formed by the presence of these special bacteria and contain large amounts of a special protein very similar to our own blood hemoglobin, leghemoglobin. This protein is purple and has a high affinity for oxygen. Its purpose is to mop up all the oxygen in the nodules so that nitrogen fixation can occur!

by Tore Lien Linde