**2. Photosynthetic extremophiles**

When we think of extremophiles, prokaryotes come to mind first. Thomas Brock's pioneering studies of extremophiles carried out in Yellowstone's hydrothermal environments, set the focus of life in extreme environments on prokaryotes and their metabolisms (Brock, 1978). However, eukaryotic microbial life may be found actively growing in almost any extreme condition where there is a source of energy to sustain it, with the only exception of high temperature (>70ºC) and the deep subsurface biosphere (Roberts, 1999). The development of molecular technologies and their application to microbial ecology has increased our knowledge of eukaryotic diversity in many different environments (Caron et al., 2004). This is particularly relevant in extreme environments, generally more difficult to replicate in the laboratory.

Recent studies based on molecular ecology have demonstrated that eukaryotic organisms are exceedingly adaptable and not notably less so than the prokaryotes, although most habitats have not been sufficiently well explored for sound generalizations to be made. In fact, molecular analysis has also revealed novel protist genetic diversity in different extreme environments (Roberts, 1999).

Temperature is one of the main factors determining the distribution and abundance of species due to its effects on enzymatic activities (Alexandrof, 1977). Most extremophiles that survive at high temperatures (95-115ºC) are microorganisms from the archaeal or bacterial domains. On the contrary, for eukaryotic microorganisms, the highest temperature reported is 62ºC, and most of the metazoans are unable to grow above 50ºC (Rothschild & Mancinelli, 2001). Surprisingly, photosynthetic prokaryotes, such as cyanobacteria, have never been found in hot acidic aquatic systems (Brock, 1973). Instead, these ecological niches are usually profusely colonized by species of the order Cyanidiales, red unicellular algae (Brock, 1973). Thus, species from the genera *Galdieria* and *Cyanidium* have been isolated from hot sulfur springs, showing an optimal growth temperature of 45ºC and a maximum growth temperature of 57ºC (Seckbach, 1994; Ciniglia et al., 2004). These extreme hot springs are usually acidic (pH 0.05-4) and frequently characterized by high concentrations of metals such as cadmium, nickel, iron or arsenic, that are highly toxic to almost all known organisms.

Additionally, phototrophic eukaryotic microorganisms have colonized environments characterized by temperatures at or below 0ºC. Some algal species bloom at the snow surface during spring (Fujii et al., 2010), and complex microbial communities have been found on glaciers, probably the most widely studies environments after marine ice habitats. Aplanospores of *Chlamydomonas nivalis* are frequently found in high-altitude, persistent

Therefore, this "co-evolution" between organisms and their environment is an intrinsic feature of living systems. Life survives and sometimes thrives under what seem to be harsh conditions on Earth. For example, some microbes thrive at temperatures of 113°C. Others exist only in highly acidic environments or survive exposures to intense radiation. While all organisms are composed of nearly identical macromolecules, evolution has enabled such microbes to cope with a broad range of physical and chemical conditions. What are the features that enable some microbes to thrive under extreme conditions that are lethal to many others? An understanding of the tenacity and versatility of life on Earth, as well as an understanding of the molecular systems that some organisms utilize to survive such extremes, will provide a critical foundation that will help us to understand the molecular

When we think of extremophiles, prokaryotes come to mind first. Thomas Brock's pioneering studies of extremophiles carried out in Yellowstone's hydrothermal environments, set the focus of life in extreme environments on prokaryotes and their metabolisms (Brock, 1978). However, eukaryotic microbial life may be found actively growing in almost any extreme condition where there is a source of energy to sustain it, with the only exception of high temperature (>70ºC) and the deep subsurface biosphere (Roberts, 1999). The development of molecular technologies and their application to microbial ecology has increased our knowledge of eukaryotic diversity in many different environments (Caron et al., 2004). This is particularly relevant in extreme environments,

Recent studies based on molecular ecology have demonstrated that eukaryotic organisms are exceedingly adaptable and not notably less so than the prokaryotes, although most habitats have not been sufficiently well explored for sound generalizations to be made. In fact, molecular analysis has also revealed novel protist genetic diversity in different extreme

Temperature is one of the main factors determining the distribution and abundance of species due to its effects on enzymatic activities (Alexandrof, 1977). Most extremophiles that survive at high temperatures (95-115ºC) are microorganisms from the archaeal or bacterial domains. On the contrary, for eukaryotic microorganisms, the highest temperature reported is 62ºC, and most of the metazoans are unable to grow above 50ºC (Rothschild & Mancinelli, 2001). Surprisingly, photosynthetic prokaryotes, such as cyanobacteria, have never been found in hot acidic aquatic systems (Brock, 1973). Instead, these ecological niches are usually profusely colonized by species of the order Cyanidiales, red unicellular algae (Brock, 1973). Thus, species from the genera *Galdieria* and *Cyanidium* have been isolated from hot sulfur springs, showing an optimal growth temperature of 45ºC and a maximum growth temperature of 57ºC (Seckbach, 1994; Ciniglia et al., 2004). These extreme hot springs are usually acidic (pH 0.05-4) and frequently characterized by high concentrations of metals such as cadmium, nickel, iron or

Additionally, phototrophic eukaryotic microorganisms have colonized environments characterized by temperatures at or below 0ºC. Some algal species bloom at the snow surface during spring (Fujii et al., 2010), and complex microbial communities have been found on glaciers, probably the most widely studies environments after marine ice habitats. Aplanospores of *Chlamydomonas nivalis* are frequently found in high-altitude, persistent

adaptations that define the physical and chemical limits for life on Earth.

**2. Photosynthetic extremophiles** 

environments (Roberts, 1999).

generally more difficult to replicate in the laboratory.

arsenic, that are highly toxic to almost all known organisms.

Fig. 1. Some examples of eukaryotic photosynthetic extremophiles. A.- *Cyanidium caldarium*, B.- *Dunaliella salina*, C.- *Euglena mutabilis, D.- Klebsormidium* sp.

snowfields where they are photosynthetically active despite cold temperatures and high levels of ultraviolet radiation (Stibal et al., 2006). Distinct microbial communities composed of psychrophilic bacteria, microalgae and protozoa colonize and grow in melt pools on the ice surface, or in brine channels in the sub-ice platelet in the Arctic even during winter, at extremely low temperatures of -20ºC (Garrison & Close, 1993).

Non aquatic environments, where desiccation is common and water is a scarce resource, are also colonized by photosynthetic microorganisms. In these ecosystems, open spaces are usually covered by biological soil crusts, a highly specialized community of cyanobacteria, mosses and lichens. Without a doubt, the most colorful coatings on rocks are produced by lichens, a remarkable symbiotic relationship between microscopic algal cells and fungal filaments. Although lichens can also withstand extreme environmental conditions, they generally cannot survive as well in the dry, sun-baked deserts (Garthy, 1999). The most recurrent species of lichens found in deserts are the large colonies of the lime-green map lichen *Rhizocarpon geographicum*, the ashy-gray *Aspicilia cinerea* and the orange *Caloplaca saxicola.* They might be thousands of years old. In fact, the colorful chartreuse rock lichen *Acarospora chlorophana* may grow only a few millimeters per century.

Photosynthesis in Extreme Environments 275

Filamentous, mucilage-producing Oscillatoriales are responsible for much of the biomass and three-dimensional structure of these polar mat consortia. They have been shown to tolerate a wide range of conditions and to maintain slow net growth despite the frigid ambient temperatures (Tang et al., 1997; Jungblut, 2010). In addition, other cyanobacterial taxonomic groups are also present in cold environments. Species of the genus *Phormidium*, *Pseudanabaena or Leptolyngbya* are also wide distributed in the polar regions (Fernández-Valiente, 2007) (Fig.2). Usually, the photosynthetic rates per unit of chlorophyll a of the microbial mats formed by these species are low compared with other cyanobacterial

Hot springs are other extreme environment in which photosynthetic cyanobacteria can also be found. These springs are well-isolated habitats occurring as clusters in globally distant regions, and the microorganisms that inhabit them are extremophiles adapted to conditions quite different from the ambient. Thus, in well studied North American hot springs, such cyanobacterial mats are formed by rod-shaped unicellular cyanobacteria of the genus *Synechococcus* with an upper temperature limit of 72 ºC (Papke et al., 2003) as well as the green nonsulfur bacterium *Chloroflexus*. *Chloroflexus* is a thermophilic filamentous anoxygenic phototrophic bacterium, and can grow phototrophically under anaerobic conditions or chemotrophically under aerobic and dark conditions. According to 16S rRNA analysis, Chloroflexi species are the earliest branching bacteria capable of photosynthesis, and they have been long regarded as a key organism to resolve the obscurity of the origin and early evolution of photosynthesis. Chloroflexi species contains a chimeric photosystem that comprises some characters of green sulfur bacteria and purple photosynthetic bacteria, and also has some unique electron transport proteins compared to other photosynthetic

Highly acidic environments are relatively scarce worldwide and are generally associated with volcanic activity and mining operation (Baffico et al., 2004). The natural oxidation and dissolution of the sulfidic minerals exposed to oxygen and water results in acid production, and the process can be greatly enhanced by microbial metabolism (Nordstrom & Southam, 1997; González-Toril et al., 2003). At the same time, low pH facilitates metal solubility in water, particularly cationic metals (such as aluminum and many heavy metals), and therefore acidic water tends to have high concentrations of heavy metals (Johnson, 1998). Although low pH and high metals concentrations are restrictive to most aquatic life, large phototrophic biofilms and mats composed of filamentous green algae such *Zygnemopsis* or phototrophic protists such *Euglena* are often observed to thrive in extreme acidic environments (reviewed by Das et al., 2009a,b). Thus, acidic ecosystems are frequently colonized by organisms at an oligotrophic or mesotrophic level with dominant alga taxa belonging to the Crysophyceae, Chlorophyta and Bacillariophyta. These taxa are sometimes found in considerable quantities indicating a remarkable potential for primary production (Amaral-Zettler et al., 2002). Besides, presence of *Klebsormidium* sp. and *Euglena mutabilis* in a given environment is an ecological indicator of low pH and high level of metals (Valente &

Thus, since extreme acidic environments are often the consequence of anthropogenic influences (e.g., mining activity or acid rain), most ecological studies of acidic waters have

communities from more temperate environments (Ariosa et al., 2006).

bacteria (Tang et al., 2011).

Gomes, 2007).

**3. Photosynthesis in extreme environments** 

**3.1 Acidic environments. The Río Tinto (SW, Spain) case** 

Besides photosynthetic eukaryotes, in extreme environments, phototrophic microorganisms such as cyanobacteria are also frequently found forming thick microbial mats. Cyanobacterial mats are found in a broad range of environments, some of which can be considered extreme, such as hypersaline ponds and lakes, thermal springs, dry and hot deserts and the cold environment of polar regions (Stal, 2000; Zakhia et al., 2007). These communities often dominate total ecosystem biomass and productivity, and must contend with persistent low temperatures, repeated freeze–thaw cycles and highly variable light, nutrient and osmotic regimes (Vincent, 2000). These extreme habitats typically exhibit spatial gradients of chemical and physical factors, including extreme variations in temperature or salinity over relatively short distances that may influence local community structure (Miller & Castenholz, 2000; Nübel et al., 2001). In photosynthetic mats, daily shifts in oxygen production contribute to significant variation in depth-related chemical structure, including periodic hyperoxia near the surface and highly variable oxygen penetration (Des Marais, 2003).

**A B** 

Fig. 2. Some examples of prokaryotic photosynthetic extremophiles. A.- *Pseudoanabaena* sp*.*, B.- *Chloroflexus* sp*.*, C.- *Synechococcus* sp*., D.- Phormidium* sp.

Besides photosynthetic eukaryotes, in extreme environments, phototrophic microorganisms such as cyanobacteria are also frequently found forming thick microbial mats. Cyanobacterial mats are found in a broad range of environments, some of which can be considered extreme, such as hypersaline ponds and lakes, thermal springs, dry and hot deserts and the cold environment of polar regions (Stal, 2000; Zakhia et al., 2007). These communities often dominate total ecosystem biomass and productivity, and must contend with persistent low temperatures, repeated freeze–thaw cycles and highly variable light, nutrient and osmotic regimes (Vincent, 2000). These extreme habitats typically exhibit spatial gradients of chemical and physical factors, including extreme variations in temperature or salinity over relatively short distances that may influence local community structure (Miller & Castenholz, 2000; Nübel et al., 2001). In photosynthetic mats, daily shifts in oxygen production contribute to significant variation in depth-related chemical structure, including periodic hyperoxia near the surface and highly variable oxygen penetration (Des

**A B** 

10 m 50 m

**C D**  Fig. 2. Some examples of prokaryotic photosynthetic extremophiles. A.- *Pseudoanabaena* sp*.*,

5 m

10 m

B.- *Chloroflexus* sp*.*, C.- *Synechococcus* sp*., D.- Phormidium* sp.

Marais, 2003).

Filamentous, mucilage-producing Oscillatoriales are responsible for much of the biomass and three-dimensional structure of these polar mat consortia. They have been shown to tolerate a wide range of conditions and to maintain slow net growth despite the frigid ambient temperatures (Tang et al., 1997; Jungblut, 2010). In addition, other cyanobacterial taxonomic groups are also present in cold environments. Species of the genus *Phormidium*, *Pseudanabaena or Leptolyngbya* are also wide distributed in the polar regions (Fernández-Valiente, 2007) (Fig.2). Usually, the photosynthetic rates per unit of chlorophyll a of the microbial mats formed by these species are low compared with other cyanobacterial communities from more temperate environments (Ariosa et al., 2006).

Hot springs are other extreme environment in which photosynthetic cyanobacteria can also be found. These springs are well-isolated habitats occurring as clusters in globally distant regions, and the microorganisms that inhabit them are extremophiles adapted to conditions quite different from the ambient. Thus, in well studied North American hot springs, such cyanobacterial mats are formed by rod-shaped unicellular cyanobacteria of the genus *Synechococcus* with an upper temperature limit of 72 ºC (Papke et al., 2003) as well as the green nonsulfur bacterium *Chloroflexus*. *Chloroflexus* is a thermophilic filamentous anoxygenic phototrophic bacterium, and can grow phototrophically under anaerobic conditions or chemotrophically under aerobic and dark conditions. According to 16S rRNA analysis, Chloroflexi species are the earliest branching bacteria capable of photosynthesis, and they have been long regarded as a key organism to resolve the obscurity of the origin and early evolution of photosynthesis. Chloroflexi species contains a chimeric photosystem that comprises some characters of green sulfur bacteria and purple photosynthetic bacteria, and also has some unique electron transport proteins compared to other photosynthetic bacteria (Tang et al., 2011).
