Diversity, Distribution and Ecology of Green Algae and Cyanobacteria in Urban Habitats

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 (SO₂, 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.