The temporarily or permanently wet margins of rivers, lakes, and ponds and the peaty substrates of bogs and fens are home to many freshwater algae that do not thrive far from water. These may be referred to as the hydroterrestrial algae. However, the diverse soils of temperate and tropical agriculture, grasslands, forests, arctic and alpine tundra, and even deserts also harbor a surprisingly diverse and active flora of both eukaryotic algae and cyanobacteria. These are referred to as terrestrial algae, or sometimes as "edaphic algae." Cryptobiotic crusts (formed by eukaryotic algae together with filamentous fungi, yeasts, and cyanobacteria) are termed "epedaphic." General discussion or conclusions regarding the algal flora of soil and its functioning are hampered by historic taxonomic confusion and by literature up to the present day in which the numbers and activities of cyanobac-teria are included together with those of eukaryotic algae. The roles of these two groups in the soil nitrogen cycle are vastly different, since common soil cyanobac-teria such as Nostoc, Anabaena, Tolypothrix, Scytonema, and Cylindrospermum are highly active in nitrogen fixation, with rates often 20-30kg ha-1 year-1 (Zackrisson et al., 2004); rates in desert soil crusts are lower (1.4-9kg ha-1 year-1) and dependent upon taxonomic composition, moisture, and suitable temperatures (Belnap, 2002). There is room for a great deal of research to tease out the particular roles and quantitative significance of the various eukaryotic algae in soil.
However, in general, it can be said that they play an important role in net primary production and the incorporation of organic carbon and nitrogen into soil, crucial during primary succession on land created by volcanic activity or bared by the retreat of glaciers. Upon their death, much of this organic matter is generally readily accessible to microbial decomposition and enters the actively cycling nutrient pool. A small part, made up of cell wall materials and exudates, may become com-plexed with soil phenolics of plant origin and form more stable humic substances that help to provide soil structure. Together with soil fungi and some other microbes, terrestrial algae may also be responsible for liberation of nutrients from insoluble sources in the mineral fraction or horizon of soil (Metting, 1981; Jongmans et al., 1997). Finally, in many dune or desert environments where vascular plant growth is restricted by permanent or seasonal drought, eukaryotic algae alone or in mixed cryptobiotic crusts play a vital role in stabilizing the "soil" surface, gradually adding soil organic matter that holds soil moisture and provides nutrients for plant establishment.
Soil is a common habitat for nonmotile green algae. Both filamentous and coccoid forms occur, the latter more common in desert soils. Green algae tend to dominate the algal flora of acid soils, and some are observed only following enrichment culturing. Frequent genera include Actinochloris, Ankistrodesmus, Bracteacoccus, Characium, Chlorella, Chlorococcum, Chlorosarcinopsis, Fernandinella, Hormotilla, Keratococcus, Muriella, Protosiphon, Stichococcus, Tetracystis, Apatococcus, Desmococcus, Klebsormidium, and Ulothrix. Most are limited to the surface of wet soils, and some are epiphytic on algae or mosses. Trentepohlia forms long, brilliant orange filaments on soil-free rocks and bark, often not recognized as a green alga even by those who notice it. In this and many other Chlorophyta, pho-toprotective carotenoids mask the "typical" grass green color.
Red algae are infrequent in typical soils. Species of Cyanidium are found in and around acidic hot springs and species of Porphyridium grow as reddish gelatinous masses encrusting polluted, ammonium-rich soils in shaded areas and on wet, well-decayed logs. Both are spherical unicells with a mucilaginous matrix.
Species of Euglena (fusoid, uniflagellate cells with red eyespot and with or without chloroplasts) are abundant where moisture, organic matter, and often ammonium are high, such as wet footprints in paddocks, puddles, ditches, and farm ponds. Facultative photoautotrophs, they can subsist as achlorophyllous heterotrophs. The nonphotosynthetic, phagotrophic Peranema also frequents similar habitats.
Members of the golden-green algae (Xanthophyta) are usually found on the surface of moist soils. Vegetative cells are nonmotile and exhibit a variety of growth forms, from unicellular and globose or cylindrical to colonial and coenocytic, or filamentous. Genera reported from soil include Botrydiopsis, Botrydium, Bumilleria, Bumilleriopsis, Heterococcus, Vaucheria, and Xanthonema (=Heterothrix).
Diatoms form beautiful exoskeletons (frustules) of silica; deposits of millennia form diatomaceous earth, which can be used as a natural insecticide. Marine diatoms are probably the single largest primary producers in the world, and diatoms are also abundant in freshwater ecosystems. In terrestrial ecosystems, diatoms occur primarily in neutral to slightly alkaline soils, where their populations may reach 105 cells per gram dry weight of soil (Berard et al., 2004). Members of the genera Achnanthes, Cymbella, Fragilaria, Hantzschia, Navicula, Pinnularia, Stauroneis, and Surirella (many of which are admittedly artificial) are frequently reported. These are organisms of moist surfaces, including plants, other algae, litter, and bare mineral or organic soil.
For reviews of terrestrial algae, see Metting (1981) and Hoffmann (1989); useful taxonomic references include Foged (1978), Dodd (1987), Entwisle et al. (1997), Ling and Tyler (2000), John et al. (2002), and Wehr and Sheath (2003). As with much of biology, much more is known about these organisms in temperate (especially north temperate) regions of the world, but the biological roles of terrestrial algae everywhere deserve considerably more study.
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