14 April 2012

Ensnared by a fungus

A single gram of soil can contain millions of different micro-organisms, including large amounts of fungi, protozoans, bacteria and nematodes (tiny, non-segmented worms). As would be expected, all of these different organisms interact with each other in complex food chains, which are often go unnoticed due to their small size and apparent insignificance. However, these interactions are in fact very important and have large effects on the decomposition of dead biological material and therefore, in the ecological recycling of nutrients.

Probably the most important fungi in recycling nutrients are the saprobic, wood-rotting species (see the earlier post 'Fungi: rotting civilisation from its very foundations' for more information), which are essential in allowing larger ecosystems, such as forests, to exist. However, they are not entirely responsible for recycling carbon and predatory fungi, belonging to the phyla Ascomycota, Basidiomycota and Mucoromycotina are also key contributors, by recycling the carbon from the micro-organisms that they kill.

The very rare predatory fungus Anthurus archeri.

There are over 200 identified species of predatory fungi and they can feed on a wide-range of micro-organisms including bacteria and other types of fungi, but predominantly feed off nematodes or other types of microscopic worm. Although it is not disputed that fungi can predate nematode worms, the finding was initially surprising as how can a relatively immobile fungus catch and eat an active worm?

Fungi have evolved many different techniques that enable them to do this, using both passive and active methods. Passive methods are fairly simple and such fungi produce a network of hyphae that permeates the surrounding soil. The hyphae act a bit like spider webs and have droplets of glue or hooks at regular intervals along their length. Upon contact with the glue or hook, a nematode becomes snared and cannot escape. The hyphae then 'blooms', extending hyphal tendrils that pierce the skin of the hapless worm and grow inside it. Once inside, the worm is digested and its nutrients are absorbed by the same tendrils and carried back to the central body (mycelium) of the fungus.

These passive methods are relatively simple methods of capture, probably being the first techniques to evolve as they are easy to produce, but are not the most efficient way of ensnaring prey. Thus, many predatory species have evolved more active methods of capturing prey that involved motion-triggered traps. These traps include the hour-glass shaped clamps of Nematoctonus, which latch onto unsuspecting nematodes as they swim past and hold them in a vice-like grip as the fungus extends its penetrative, invasive hyphae towards them; and the spectacular inflatable collar seen in Arthrobotrys species. The collar consists of 3 cells that inflate rapidly when a nematode swims through them, closing in less than 1/10th of a second that traps the nematode as would a clamp. The struggling worm often traps its tail in another collar, which are regularly spaced along the length of the hyphae, when thrashing around and becomes completely immobilised. As with the other species of predatory fungi, invasive hyphae bloom from these collars and penetrate the nematode; typically digesting it in 12-24 hours.

Active methods of capture: the hour-glass shaped clamps of the Basidiomycete Nematoctonus (left) and a nematode trapped in the inflating collar of the predatory fungus Arthrobotrys anchonia (right).

The presence of predatory fungi is significant for increasing the rate of carbon turnover in the biological Carbon Cycle and they also help to control the population size of nematodes, which can be damaging pests or parasites of plants, fungi and bacteria; all of which are ecologically important organisms.

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