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Familiar examples of green algae (Chlorophyta) on land include those that participate in symbiotic associations with fungi, forming lichens (e.g., Coccomyxa , Myrmecia , Stichococcus , Trebouxia , Ahmadjian, 1958; Friedl, 1997), and taxa that grow richly on natural and man-made surfaces or on leaves of citrus and magnolia trees (e.g., Prasiola , Trentepohlia , Cephaleuros , Rindi and Guiry, 2004; Rindi et al., 2005). Besides these examples, green algae can occur in rock (endolithic), or at the surface (epidaphic), or just below the surface (endedaphic) of soil (Friedmann et al., 1967; Bell, 1993). Green algae are components of desert soil communities known as biological soil crusts or cryptogamic crusts (Evans and Johansen, 1999; Belnap and Lange, 2001). Crust communities are found on all continents on Earth, in arid and semi-arid habitats, where soil moisture is limiting and vascular plant cover is sparse (e.g., Johansen, 1993; Evans and Johansen, 1999; Green and Broady, 2001). Along with cyanobacteria, fungi, lichens, diatoms, and bryophytes, desert green algae form water-stable soil aggregates that have important ecological roles in nutrient cycling, water retention, and stabilization of soils (Evans and Johansen, 1999). The fragile nature of desert crust communities makes them highly susceptible to disturbance by trampling and fire, and has lead to numerous studies on the recovery of crusts after disturbance (Belnap and Eldridge, 2001; Nagy et al., 2005). Reviews of the ecology of crusts can be found in West (1990), Eldridge and Greene (1994), Evans and Johansen (1999), and Belnap and Lange (2001). This paper provides background information about the taxonomy of green algae from arid soil communities, and highlights recent studies that address the fine scale distribution, evolutionary relationships, diversification, and origins of Chlorophyta on land.

Aeroterrestrial phototrophic microorganisms typically form conspicuous biofilms in all climatic zones at the interface between any type of solid substratum and the atmosphere. In temperate regions such as North-Western Europe, eukaryotic green microalgae (Chlorophyta) are the most abundant aeroterrestrial organisms (see also Rindi, this volume), whereas cyanobacteria dominate warm-temperate to tropical regions (Ortega-Calvo et al., 1995; Tomaselli et al., 2000). Aeroterrestrial green microalgae grow epiphytically and epilithically on natural surfaces such as tree bark, soil and rock, and are known to be the photobionts of lichens (Ettl and Gärtner, 1995). These organisms also occur in urban areas on anthropogenic surfaces such as roof tiles, concrete, building facades and other artificial surfaces where they cause aesthetically unacceptable discolouration known as patinas and incrustations (Gaylarde and Morton, 1999; Tomaselli et al., 2000).

Over the past millions of years the land on our planet has been the testing ground for many experiments or, more dramatically, the battleground for many invasions. A myriad of ancestral plant forms came from the sea and lakes to exploit the terrestrial environment. Those life forms were algae, simple photoautotrophic organisms that eventually prepared the land for the terrestrial flora and fauna that were to follow. They successfully conquered the land in terms of making it a useable new habitat for themselves and developed new forms and processes to adapt. Those plant “invaders” or “conquerors” are represented today by algae living among us populating soils and other terrestrial habitats. Most of the photosynthetic organisms that occur nowadays in aquatic habitats belong to this heterogeneous category generally called algae. These organisms are phylogenetically unrelated, or only distantly related, and differ enormously in terms of gross morphology, ultrastructure, biochemical traits and many other important features. Several lineages of algae successfully colonized terrestrial environments. Although from the ecological point of view the most important conquest of land was that of the green algae of the streptophytan lineage (those that gave rise to land plants), several other groups did succeed in becoming terrestrial. Representatives of these lineages are presently commonly found in terrestrial environments and unlike land plants, have maintained a very similar morphology to that of their aquatic relatives. The Charophyta and the Chlorophyta sensu stricto are the two groups of eukaryotic algae that along with the prokaryotic Cyanobacteria (blue-green algae) have been most successful in colonizing terrestrial environments.

Eukaryotic algae and cyanobacteria occur virtually in every terrestrial habitat on our planet. Organisms belonging to these groups are present even in some of the most extreme terrestrial environments, such as rocks in hot and cold deserts (Friedmann and Ocampo-Friedmann, 1984), Antarctic soils (Broady, 1996) and highly acidic post-mining sites (Lukešová, 2001). As early as the beginning of the nineteenth century, it was realized that microalgae occur also on walls, masonry and other man-made substrata (e.g. Dillwyn, 1809; Agardh, 1824); however, very little attention has been devoted to this type of algal communities until recently. Cities are artificial environments in which artificial substrata (such as concrete, asphalt, glass and metal) provide the largest part of the surfaces available for the colonization of microorganisms. The surfaces of many urban buildings are exposed to full sunlight; organisms growing on such surfaces are therefore frequently subjected to extremely high light irradiance, high levels of UV radiation and extreme dehydration (Crispim and Gaylarde, 2004; Karsten et al., 2005). The temperature of walls and roofs is subjected to a high range of variation and, in tropical regions, can reach 60–70ºC (Tripathi et al., 1990). Most urban habitats are also affected by large amounts of pollutants, such as gases (SO_2, CO, NO_X, hydrocarbons, ozone), aerosols, dusts and heavy metals (Seaward, 1979; John, 1988). Due to such a negative combination of factors for organisms of aquatic origin, for microalgae and cyanobacteria cities can be certainly considered extreme environments. Reports on algae and cyanobacteria from urban habitats have gradually appeared in the last few decades. Most studies on this subject concern European, Asiatic and South American cities; at present, there is almost no information published for other continents. In general, the knowledge of the diversity and ecology of these communities is still rudimentary, because most studies have focused much more on the biodeterioration operated by these organisms on artificial surfaces than on their biology. In this chapter, the information currently available on cyanobacteria and green algae of urban environments is summarized. General aspects of the diversity and distribution of these organisms in urban habitats are discussed, and the composition and ecology of the most common algal assemblages in these environments are described in detail.

The world ocean, where most aquatic oxygenic phototrophs, cyanobacteria as well as algae, are found, contains around 35 g/l of salt. As a consequence, marine phytoplankton has to be adapted to life in a saline environment. Many representatives of the cyanobacteria as well as of the algae have developed adaptations to life at far higher salt concentrations as well, and some can even be found in saltsaturated environments, such as the Dead Sea, the Great Salt Lake, Utah, and saltern crystallizer ponds in which the total salt concentration exceeds 300 g/l (Oren, 2002). Species of the unicellular green alga Dunaliella are among the most prominent inhabitants of such hypersaline environments.

Bacteria have inhabited Earth for 3.8 billion years and life on our planet was microbial for 3.2 billion years (Schopf, 1994). During this long period, microorganisms have evolved an incredible diversity, although a major part of this diversity may have already existed in the Archean. Cyanobacteria and, hence, oxygenic photosynthesis evolved 2.7–2.2 billion years ago and had therefore ample time to diversify and adapt to newly evolving niches that emerged on Earth (Schopf et al., 2002; Blank, 2004; Tice and Lowe, 2004). Through the advent of oxygenic photosynthesis (Blankenship, 1992), cyanobacteria were responsible for the oxygenation of the Earth’s atmosphere (Buick, 1992), thereby allowing the evolution of plants and animals 0.6 billion years ago and eventually were shaping the present biosphere. Cyanobacteria combine the fixation of CO_2 and N_2, the two most important biogeochemical processes on Earth. They are globally important primary producers and contribute greatly to the global nitrogen budget (Karl et al., 2002). Cyanobacteria are essential players in the Earth’s present and past ecosystems. For any understanding of the evolution of life and of the biogeochemical cycles on Earth, knowledge about the ecology and evolution of the cyanobacteria is a prerequisite. Cyanobacteria colonized successfully almost any illuminated environment on Earth, many of which are considered to be hostile for life. Cyanobacteria play a prominent role in many of these extreme environments. This chapter attempts to find clues explaining the evolutionary and ecological success of cyanobacteria.

The Great Salt Plains (GSP) spans approximately 65 km^2 in northwestern Oklahoma, USA. Although soil on the flats consistently retains about 10–20% water by weight, the flats are largely devoid of macroscopic plants due to seepage and evaporation of subterranean NaCl-dominated Permian brine at 15–25% salinity. Except following infrequent heavy rain, a thin (<1–10 mm) variable salt crust persists over much of the flats, and the interstitial water is often near NaCl saturation (>25%). However, several intermittent and a few permanent freshwater streams traverse the flats, providing localized lower salinity niches. Episodic heavy direct rainfall and associated flooding of these streams inundates vast areas of the flats with fresh or low salinity water, which quickly recedes. The flats then return to the more typical salt crust over a period of days to weeks. Other than the GSP, virtually nothing is known about algae of hypersaline intermittent lakes, even in Australia which has countless such lakes (Timms, in press). Algae are ubiquitous on the GSP flats, although chlorophyll biomass is typically very low, and diversity appears to be restricted to a small subset of genera in the divisions Cyanophyta, Chlorophyta, and Bacillariophyta (Major et al., 2005; Kirkwood and Henley, 2006). Chlorophyll biomass is correlated with interstitial dissolved inorganic nitrogen (DIN), particularly ammonium, but not soluble reactive phosphorus (Kirkwood and Henley, 2006). However, given the dynamic salinity and temperature conditions on the flats surface (Major et al., 2005; Kirkwood and Henley, 2006), nutrient availability may be a comparatively trivial problem for physically stressed algae and cyanobacteria. Indeed, most GSP algal isolates are unable to grow in the laboratory at the high interstitial salinities typically found in situ, yet direct soil samples invariably yield viable algae when exposed to reduced salinities in the laboratory (Major et al., 2005; Kirkwood and Henley, 2006). Thus, most or all of the algae of the GSP must tolerate high salinities that will not support growth.

Earth is a planet that records its own history, including aspects of its biological history (Knoll, 2003). The long-term evolution of life is recorded by fossils, molecular biomarkers, biogeochemically informative isotopic abundances, especially of carbon and sulfur, and sedimentary textures that reflect biological activity. One might reasonably expect that other planets have recorded their histories as well, encrypting evidence of past tectonics, climate, atmospheric composition, and, if present, life in the physical and chemical properties of sedimentary rocks. For most of Earth history, the biota was microbial, and it is likely that microbial signatures will be the astrobiological targets of any planetary sediment we will be privileged to examine close at hand.

Measuring the extent of eukaryotic microbial diversity is essential to our understanding of eukaryotic evolution and the structure and function of microbial food webs. In the past several years, molecular approaches have been used to address an increasing interest in the diversity of microbial eukaryotes, particularly that of protists from various marine environments. These have included pelagic environments (e.g., Moon-van der Staay et al., 2001; Massana et al., 2002), deep-sea environments (López-García, 2001), the ocean surface (Díez et al., 2001; Moon-van der Staay et al., 2001), coastal environments (Massana et al., 2004), and a river (Berney et al., 2004), as well as extreme environments, including acidic and iron-rich rivers (Amaral Zettler et al., 2002), deep-sea hydrothermal vents (Edgcomb et al., 2002; López-García et al., 2003), microoxic (<10 μM oxygen) and anoxic waters and sediments in salt marshes (Stoeck and Epstein, 2003), permanently anoxic deep-sea waters (Stoeck et al., 2003, 2006), anoxic shallow sediments of marine and freshwater (Dawson and Pace, 2002; Bernhard et al., 2006). These studies have revealed an extraordinary diversity of previously undetected eukaryotic lineages based on small-subunit ribosomal RNA (SSU rRNA) sequences. For a recent overview of higher level classification of eukaryotes that emphasizes the protists, see Adl et al. (2005). Anoxic (lacking dissolved oxygen) environments have been present throughout Earth’s history, and sulfide-rich conditions are likely to have existed in the deep oceans into the late Proterozoic (Canfield, 1998; Shen et al., 2002), during the origin and early diversification of eukaryotes when atmospheric oxygen concentrations were about 1% of present day levels (Schopf and Klein, 1992).

Antarctica is the coldest, driest, and most isolated continent of our planet. The White Continent can be subdivided in several climatic zones (roughly sub- Antarctic, maritime Antarctic, and continental Antarctic) in which the possibility for life settlement strictly depends on the environmental conditions which gradually become harsher moving from maritime to continental Antarctica and, within the continental Antarctica, moving from the coast to the interior of the continent (Øvstedal and Lewis Smith, 2001). With only two phanerogams occurring at the edges of the continent, Antarctic terrestrial habitats are entirely dominated by lower organisms, including invertebrates, bryophytes, fungi, algae, and diverse prokaryotes. In continental Antarctica no vascular plants are present; the life of terrestrial ecosystems concentrates in the ice-free sites along the coastal areas where lichens, fungi, mosses, and algae grow abundantly; their occurrence decreases towards inland stations where isolated rocks occasionally present epilithic microorganisms, depending on the climate and the rock surface exposition and slope. In the ice-free areas of the McMurdo Dry Valleys (Southern Victoria Land), conditions become even more hostile. There, lichens occasionally colonize sheltered rock surfaces and life mostly withdraws inside porous rocks where milder nanoclimatic conditions are present. These life-forms, named cryptoendolithic, represent the predominant form of colonization of the Antarctic deserts (Friedmann and Ocampo, 1976; Friedmann, 1982; Wierzchos and Ascaso, 2002). The fissures and cracks of granitic rocks from this area are also colonized, by chasmoendolithic organisms (De los Ríos et al., 2004, 2005a, 2007). In these habitats, microbial life apparently meets in rather narrow niches and forms simple or more complex communities.