Introduction

In this lab we will study two groups of organisms that have converged evolutionarily on similar ways of life. They are simple organisms but both live as absorbtive heterotrophs. One group, the Chytridiomycota, is related to the remainder of the true fungi, while the other group, the Oömycota, is unrelated to the fungi, and is instead more closely related to the diatoms and brown algae. The fungi and the Oömycota are extremely important to the economy of nature as decomposers of dead material, a role they fill with heterotrophic bacteria and some other organisms. Decomposition releases carbon dioxide into the atmosphere and returns minerals such as nitrogen to the soil in a form usable to plants. This process is as necessary as photosynthesis to the continued existence of the biological world. Without this cycling of minerals, many essential nutrients would remain bound up in organic matter and remain unavailable for use by growing organisms. The top 20 cm of fertile soil may contain nearly 5 metric tons of fungi and bacteria per hectare (2.47 acres). Many fungi and oömycetes are also important as parasites, both of plants and animals.

(A) Anatomy and Growth rates

Fungi are mainly filamentous in structure, (there are a few unicellular species). As with most organisms, structure relates to function. Most fungi obtain their nutrition by secreting exoenzymes that digest organic matter around the fungal cells, and the products of this digestion are then absorbed by the fungus. The filamentous growth form of fungi enhances this process by providing a tremendously large surface area for contact with organic matter. The filaments are also capable of rapid growth, allowing the fungus to reach new sources of food rapidly. Fungal filaments are known as hyphae, and a mass of hyphae, the vegetative body of the fungus, is called a mycelium. Growth of the hyphae occurs at the tips and materials are moved throughout the mycelium by cytoplasmic streaming. Growth can be extremely fast under the certain conditions with the length of a mycelium increasing at the rate of up to 1 km/day.

(B) From athletes foot to indispensable mutualisms

Some fungi are involved in other types of activities, such as parasitism or mutualism. Athlete's foot is caused by a parasitic fungus. Lichens represent mutualistic symbioses between fungi and algae or cyanobacteria. Another very common symbiosis is the intimate association of certain fungi with the roots of higher plants. These mycorrhizae ("fungus-roots") are essential to the growth of many plants on certain types of soils. Foresters were not able to grow a number of conifers in some countries until they discovered mycorrhizal associations and learned to move soil containing fungal spores to the new sites.

(C) Taxonomy

The placement of the fungi into a kingdom has been confusing. They have traditionally been placed with plants, but with the 5 kingdom system designed to reflect evolutionary trends, the Kingdom Fungi was erected. Both the Oömycota and the Chytridiomycota have in the past been placed with the Fungi, and were referred to as the “lower” fungi, and they have both been removed from the fungi, to be placed with the Protista. It now appears that the Oömycota are unrelated to the Fungi proper, and instead belong to the same provisional kingdom as the brown algae and the diatoms (the Kingdom Stramenopila). The Chytridiomycota were once removed from the fungi, but now appear to be part of the Kingdom Fungi after all. The Oömycota have cellulose in their cell walls while the chytrids and the higher fungi have cell walls made of chitin, a structural polysaccharide that differs from cellulose. Some Chytrids display an alternation of generations in their life cycle while the water molds (Oömycota) are haploid only at the gamete stage. The remaining three divisions (phyla) of the Fungi, the Zygomycota, Ascomycota, and Basidiomycota, possess haploid mycelia.

(D) The Chytridiomycota and the Oömycota

The Chytridiomycota and the Oömycota are primarily aquatic, the former often referred to as Chytrids (and sometimes as water molds), and the latter as oömycetes or water molds. Both groups are generally saprobes (decomposers), but parasites of algae, fish, and other aquatic organisms are also common. Some well-known species of these groups include Coelomomyces (Chytrid) which is being investigated as a biological control of mosquitos, and the oömycete Phytophthora, which was responsible for the Irish potato famine.

• Division Chytridiomycota
• Division Oömycota