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A recent theme in the study of microbial diversity has been the issue of whether and how the genetic and phenotypic variation of microorganisms is distributed along a geographic transect. Population genetic theory and the results of experimental evolution studies in the laboratory (e.g., Wright, 1931; Atwood et al., 1951; Bennett and Lenski, 1993) suggest that spatially structured microbial populations in nature should rapidly diverge from each other, provided that migratory gene flow among them is low, thereby creating geographic patterns of variation. Recent reports confirm that divergence of geographically isolated populations indeed outpaces the homogenization of genetic variation by migration, indicating the presence of dispersal barriers for microorganisms (e.g., Miller and Castenholz, 2000; Papke et al., 2003; Whitaker et al., 2003; Miller et al., 2006). These observations run counter to the longstanding idea that the abundance of a microorganism at a location is not limited by dispersal but is determined solely by environmental factors (Baas- Becking, 1934), a view that has been recently championed for eukaryotic microorganisms on the basis of morphological criteria (Finlay, 2002). Here, I will summarize our recent investigations of the biogeography of the moderately thermophilic, filamentous cyanobacterium, Mastigocladus (Fischerella) laminosus .

Specialized macro- and microorganisms inhabit the springs, each adapted to its habitat, and the cyanobacteria form a prominent part of these biota. Hot springs worldwide are inhabited by dense communities of cyanobacteria adapted to life at high temperatures. The most thermophilic microorganisms known thrive at temperatures above 110º C (Blöchl et al., 1997; Kashefi and Lovley, 2003). However, a temperature around 74º C appears to be the upper limit for photosynthesis (Castenholz, 1969; Brock, 1978). The cyanobacteria most tolerant to high temperatures are unicellular forms ( Thermosynechococcus ), which thrive in North America (e.g., the hot springs of Yellowstone), Japan and the eastern Mediterranean (Castenholz, 1969) but filamentous cyanobacteria also abound in hot springs worldwide (Copeland, 1936; Castenholz, 1969, 1984, 1996; Brock, 1978).

Iron is the fourth most abundant element in the Earth’s crust and the most abundant element in the Earth as a whole (Ehrilch, 2002). General agreement is that life evolved in the presence of soluble iron concentrations much greater than those typical today (MacLeod et al., 1994; Emerson and Moyer, 2002) and that life is based, therefore, on redox processes mediated by iron compounds (Beinert et al., 1997). The importance of cyanobacteria (CB) to the geophysical evolution of life on Earth cannot be overstated. In addition to contributions to Earth’s biomass and poikilotrophy (Levit et al., 1999), cyanobacterial phototrophic activity from the Archean to present times is thought to have contributed significantly to presentday oxygen and carbon dioxide concentrations of our atmosphere (ibid.), thus driving the evolution of modern oxygen-dependent organisms. Even though the evolutionary path of oxygenic photosynthesis is well studied (Xiong et al., 2000; Xiong and Bauer, 2002; Olson and Blankenship, 2004; Iverson, 2006), what triggered the transition from anoxygenic to oxygenic photosynthesis is still not clear. The answer to this question might be obtained by the search for coincidences between the remarkable events in Earth and life’s evolution.

Acid mine drainage (AMD) is a phenomenon commonly associated with mining activities throughout the world. This acidification is the consequence of sulfides in rock strata becoming exposed to water and oxygen (see Section 1.2). The low pH of AMD-contaminated water bodies may resemble that of some naturally occurring freshwater systems that harbor acidophiles. For example, regions of the West Coast of the South Island of New Zealand have streams and rivers with naturally low pH produced by leaching of fumic and fluvic acids from podocarp rainforests, as well as artificially low pH systems caused by AMD (Collier et al., 1990). Ancient low-pH environments generated by volcanism may have been crucial for the origin of life on Earth (e.g., Holm and Andersson, 2005; Phoenix et al., 2006), and thus habitats resembling AMD have probably existed for billions of years. Distinctly, however, extremely acidic habitats from anthropogenic sources are associated with a massive burden of spoil and heavy metals. AMD began during the industrial revolution, and now accounts for most of the extremely acidic habitats worldwide (Johnson, 1998).

A major question in microbial ecology is to identify the limits of life for growth and survival, and to understand the molecular mechanisms that define these limits. Our ongoing exploration of the Earth has led to continued discoveries of life in environments that have been previously considered uninhabitable. Thus, interest in the biodiversity and ecology of extreme environments has grown in recent years for several reasons: some of these are scientific and related to the idea that extreme environments are believed to reflect early Earth conditions; conditions that persisted for most of the time that life has been on the Earth and to which prokaryotes originally evolved and adapted (Schopf and Walter, 1982). Other reasons are more commercial, such as the use of the metabolic properties of some microorganisms for metal extraction.

The modern exploration of Cyanidium begins with T.D. Brock, who carried out a broad survey of more than 200 thermal areas in North and Central America, Europe (Iceland and Italy), Japan, Hawaii and New Zealand (Brock, 1978). The exploration covered aquatic and terrestrial stations of each thermal spring, documenting the presence of Cyanidium in both kinds of habitat and in all the thermal sites. According to Doemel and Brock (1971) the occurrence of C. caldarium was recorded in aquatic habitats between 20 C and 55 C, whereas on soils the temperature range was between 10ºC and 55–57ºC. In both habitats C. caldarium was always found at pH values below 5.0.

While members of the archae rule at the high end of the temperature spectrum of life, members of the bacteria and eukaryotes thrive in a wide range of extreme conditions, including low temperatures, high and low pH-values, high salinity, and desiccation. In this context, it is important to note that the definition of extreme (and thus extremophilic) is anthropocentric, defining those environments as extreme that are hostile to human life. Photosynthetic protists are particularly versatile when it comes to occupying extreme habitats and thriving under extreme conditions. Protists thrive in saturated salt solutions, in hot acid, in extreme cold, and at high pH. This chapter deals with a small group of thermo-acidophilic unicellular red algae, called the Cyanidiophyceae .

This chapter is intended as a brief introduction on specific, novel aspects of archaeal and eukaryotic biodiversity in two extreme marine environments, hydrothermal vents and deep subsurface sediments: deeply-branching, uncultured archaea occurring in both environments that in some cases do not fit into the well-established crenarchaeota-euryarchaeota dichotomy; the partial overlap in the archaeal community structure of hydrothermal vents and deep subsurface sediments; new developments to decode the physiology and carbon sources of subsurface and vent archaea; and the unexpected diversity of enigmatic protists at hydrothermal vents.

Deserts are defined in a classic paper by Noy-Meir (1973) as “water-controlled ecosystems with infrequent, discrete, and largely unpredictable water inputs.” They are found to a greater or lesser extent on all six continents (including Antarctica). Based on the moisture index system of Thornthwaite (1948), Meigs (1953) divided deserts into three categories: extremely arid (less than 60–100 mm mean annual precipitation), arid (60–100 to 150–250 mm mean annual precipitation), and semiarid (150–250 to 250–500 mm mean annual precipitation). Using this criterion, most of the deserts of North America would be considered semiarid or arid environments.

Fundamental problems in investigating on extinct or extant life on Mars, concern the presence of water and finding out to what extent living organisms may survive at its very low temperatures and atmosphere conditions. The collected data transmitted to Earth from the above-mentioned missions and the fascinating images of Mars, allowed a detailed reconstruction of the Red Planet surface; conjunctures were made about water as not an unknown element, at least in the past, on the planet. Scientists have now to ascertain if the astronomic and meteorological data about the actual conditions of Mars are compatible with the concept of life that we have on Earth (McKay et al., 1996). Once established that on Mars the actual conditions of water, temperature and atmosphere might be compatible with extreme life forms, which earthly organisms could colonize the Red Planet? About that, many other questions arise, such as life ever existed on Mars in the past or if some of the pre-existing organisms are still present somewhere; if terrestrial organisms could adapt to Mars conditions (Friedmann, 1986; Friedmann and Ocampo-Friedmann, 1995; Beaty et al., 2005).