**1. Introduction**

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Our ongoing exploration of Earth has led to continued discoveries of life in environments that have been previously considered uninhabitable. For example, we find thriving communities in the boiling hot springs of Yellowstone, the frozen deserts of Antarctica, the concentrated sulfuric acid in acid-mine drainages, and the ionizing radiation fields in nuclear reactors (González-Toril et al., 2003; Lebedinsky et al., 2007; Pointing et al., 2009). We find some microbes that grow only in brine and require saturated salts to live, and we find others that grow in the deepest parts of the oceans and require 500 to 1000 bars of hydrostatic pressure (Horikoshi, 1998; Ma et al., 2010). Life has evolved strategies that allow it to survive even beyond the daunting physical and chemical limits to which it has adapted to grow. To survive, organisms can assume forms that enable them to withstand freezing, complete desiccation, starvation, high levels of radiation exposure, and other physical or chemical challenges. Furthermore, they can survive exposure to such conditions for weeks, months, years, or even centuries. We need to identify the limits for growth and survival and to understand the molecular mechanisms that define these limits.

Biochemical studies will also reveal inherent features of biomolecules and biopolymers that define the physico-chemical limits of life under extreme conditions. Broadening our knowledge both of the range of environments on Earth that are inhabitable by microbes and of their adaptation to these habitats will be critical for understanding how life might have established itself and survived.

The diversity of life on Earth today is a result of the dynamic interplay between genetic opportunity, metabolic capability, and environmental change. For most of their existence, Earth's habitable environments have been dominated by microorganisms and subjected to their metabolism and evolution. As a consequence of geological, climatologic, and microbial processes acting across geological time scales, the physical-chemical environments on Earth have been changing, thereby determining the path of evolution of subsequent life. For example, the release of molecular oxygen by cyanobacteria as a by product of photosynthesis as well as the colonization of Earth's surface by metazoan life contributed to fundamental, global environmental changes. The altered environments, in turn, posed novel evolutionary opportunities to the organisms present, which ultimately led to the formation of our planet's major animal and plant species.

Photosynthesis in Extreme Environments 273

**A B** 

10 m 10 m

30 m 50 m

**C D**  Fig. 1. Some examples of eukaryotic photosynthetic extremophiles. A.- *Cyanidium caldarium*,

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

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

B.- *Dunaliella salina*, C.- *Euglena mutabilis, D.- Klebsormidium* sp.

extremely low temperatures of -20ºC (Garrison & Close, 1993).

*Acarospora chlorophana* may grow only a few millimeters per century.

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