Fungi and Eukaryotic Algae

R. Greg Thorn Michael D. J. Lynch

Introduction

Classification, Characteristics, and Ecological Roles in Soil References and Suggested Reading introduction

Fungi bind soil together, both literally and figuratively, by their filamentous form, their exudates, and their trophic interactions with all other groups of soil organisms. Soil is fundamentally an aquatic habitat, since water films and water-filled soil pores occur, at least ephemerally, even in dry deserts. For this reason, the eukaryotic algae will be treated together with fungi and fungus-like organisms in this chapter (Fig. 6.1); in addition, both groups share a range of morphologies from filamentous to cellular. Colonial algae and seaweeds with differentiated multicellular organs do not occur in soil and are beyond the scope of this chapter. Cyanobacteria (formerly known as blue-green algae) are prokaryotic members of the Domain Bacteria (=Eubacteria). To treat cyanobacteria as "algae" is as inaccurate and misleading as treating actinomycetes (prokaryotic, GC-rich members of the gram-positive Bacteria) as fungi. The distinction is of particular importance in soil ecology, since many cyanobacteria and some actinomycetes are important in fixation of nitrogen, a capacity not found in any eukaryotes. Both groups are discussed in Chap. 5. The chloroplasts of plants and other photosynthetic eukaryotes have their origins in Cyanobacteria, originally engulfed by an ancestor of the plant lineage (see Fig. 6.1), then transferred by secondary endosymbiosis of an ancestral red alga into the het-erokont and alveolate lineage (brown algae through dinoflagellates in Fig. 6.1) or by secondary endosymbiosis of an ancestral green alga into the excavate (euglenoids)

Chloroplast via cyanobacterial endosymbiosis

T"

Land plants Charophytes Green Algae Red Algae

Excavates

Secondary endosymbiosis I of red algae (a) or green I algae (b)

Cercozoa

Glaucophyta Euglenids a

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