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Oxygenic phototrophic microorganisms are abundantly found in environmental extremes of temperature, pH, salt concentration, and radiation. These extremophilic phototrophs include both prokaryotes (cyanobacteria) and eukaryotes (different types of algae). The prokaryotic cyanobacteria, belonging to the eubacterial domain, do not possess a defined nucleus and have no intracellular membrane-surrounded organelles, while the eukaryotic algae have nucleated, more complex and larger cells that contain chloroplasts and other organelles. It is nowadays well accepted that the prokaryotic anucleated cells have evolved into more developed eukaryotic organisms in a process termed eukaryogenesis. In this chapter we present a survey of the occurrence of oxygenic phototrophs in extreme environments, exploring the existence of phototrophic thermophiles (lovers of high temperature), psychrophiles (cold-loving organisms), halophiles (high salt-loving organisms), acidophiles (cells thriving at low pH), alkaliphiles (cells living at high pH), and radiation-resistant phototrophs. We then compare the performance of the prokaryotic and the eukaryotic phototrophs in each of the environmental extremes, and discuss the findings in the light of the evolutionary ideas relating to the formation of the eukaryotic cell.

Enhanced solar ultraviolet radiation (UVR) due to stratospheric ozone depletion is a major stress factor for many phototrophic organisms in aquatic and terrestrial ecosystems (Franklin and Forster, 1997). UVR includes the wavelengths below those visible to the human eye. According to the CIE (Commission Internationale de l’Eclairage), the spectral range is divided into three wavebands: 315–400 nm UVA, 280–315 nm UVB and 190–280 nm UVC. UVA is not attenuated by ozone, and hence its fluence rate will be unaffected by any ozone layer reduction reaching aquatic and terrestrial organisms. Increases in UVB have been particularly reported in Antarctica (McKenzie et al., 2003) and the adjacent geographic regions (southern parts of South America and Australia) (Buchdahl, 2002; Deschamps et al., 2004), as well as in more recent years in the Arctic region (Knudsen et al., 2005). UVB exposure is potentially harmful to all living organisms, but especially to photosynthetic organisms due to their requirement for light. UVB represents less than 1% of the total solar radiation reaching the earth’s surface, because it is absorbed partly by the ozone layer. It is particularly this waveband, which is influenced by changing stratospheric ozone concentrations caused by anthropogenic emissions of greenhouse gases, such as chlorinated fluorocarbons (Fraser et al., 1992).

The term “algae” designates a most diverse and ancient group of organisms that is polyphyletic by evolution and artificial by taxonomy. Its only common feature is the ability to perform aerobic photosynthesis. Algae range by size from tiny cyanobacterial cells of the picoplankton to the giant kelps dominating rocky coastlines. They settle most diverse aquatic habitats such as hot springs and Arctic ice, live on and in rocks and various organisms, travel by air currents for thousands of miles and can be found in groundwater. Algae have taken an important part in the evolution of Gaia and gave rise to embryophytes (plants: bryophytes and spermatophytes). In most textbooks, it is common wisdom that the success of algae is dependent on sufficient light available for net photosynthesis. However, there exists another world, hidden and without light, offering a plethora of aphotic habitats. That is the world underground, which is not only settled by bacteria, fungi and the terrestrial fauna, but also by algae. The following is meant to give an introduction into what we know and suggest on the life of algae underground, that is on algae not living, as crust forming algae do, on but in soil substrates. The leitmotiv shall be to look for lessons they can tell us about the tremendous potential of life to adapt to habitats, that – at first glance – appear to be rather hostile and strange. For practical reasons all kinds of both pro- and eukaryotic algae living on and within soil substrates will be designated as “soil algae.”

The meromictic lake is, in effect, two different habitats separated by a distinct vertical gradient (“the chemocline”, Hutchinson, 1937). By passing through the chemocline, the conditions with regard to dissolved salts and organic matter, particulates, gases and pH/eH may be altered significantly. Contrary to holomictic lakes where gradients may be established seasonally by temperature differences, the chemical gradient in truly meromictic lakes is sufficiently robust to withstand seasonal mixis. While the meromictic condition in temperate lakes may last for several to thousands or more years, some basins of marine origin may have conserved a gradient towards anoxia for millions of years, if unperturbed by glaciations. The origin, terminology, properties and significance of land-locked waters (fjords being isolated from the sea by postglacial isostatic equilibration) and various aspects of meromixis have repeatedly been discussed in papers and reviews (e.g., Strøm, 1936; Findenegg, 1937; Hutchinson, 1937; Kjensmo, 1967; Walker and Likens, 1975; Degens and Stoffers, 1976; Hakala, 2004) and textbooks (e.g., Ruttner, 1940; Hutchinson, 1957; Wetzel, 2001; Kalff, 2002). A review of biological implications at the chemocline level has been written by Tyler and Vyverman (1995). More information is hidden in the literature, and emerging from further studies of meromictic lakes of different origin (cf. Hakala, 2004). Here, information about protists inhabiting the chemocline and possibly the monimolimnion (or anoxic water of isolated fjords) are presented from unpublished observations and from literature. Since these authors’ experience is mainly from Norway and Sweden, examples will be chosen from the geo-diversity of fjords and lakes here. This presentation is a sequel and extension to the review by Tyler and Vyverman (1995) and the treatise on meromictic lakes in Finland by Hakala (2004).

Phototrophs (photolithoautotrophs) are organisms that use light as their energy source to synthesize organic compounds. These organisms include some bacteria, cyanobacteria, algae, and plants. They harvest light by various pigments, the main of these being chlorophylls, and its energy is transferred to the photosynthetic reaction centers. Even though phototrophs depend on light for their survival, some of these grow under very low light. In general, the terrestrial light flux, even under the most intense sunlight is too low for single chlorophyll molecules to sustain photosynthesis, since the arrival of photons would be so slow that the S states (Kok et al., 1970, Falkowski and Raven, 1997) would decay spontaneously, not allowing generation of oxygen or carbon reduction. In reality, light is harvested in the photosynthetic apparatus by “antennae,” consisting of hundreds of pigment molecules embedded in the thylakoids or similar membranes. The antennae have a far larger cross section,σ, or probability of intercepting a photon than single pigment molecules. The energy intercepted by the antennae migrates as excitation energy to the few chlorophyll molecules in the photosynthetic reaction centers.

Oxygenic phototrophs (cyanobacteria, algae and higher plants) primarily absorb solar energy in the visible spectral (400–700 nm) region by use of various chlorophylls, while anoxygenic phototrophs are bacteria, which can absorb infrared wavelengths (>700–1100 nm) by use of different bacteriochlorophylls (Overmann and Garcia-Pichel, 2004). Each of the groups also has a variety of characteristic antenna pigments and other accessory pigments that can enhance light capture and/or provide protection against excess actinic light and UV-radiation in specific habitats. However, amongst these broadly defined groups there are outlier organisms exhibiting atypical photopigmentation. Amongst the oxygenic phototrophs, the most conspicuous are: 1. The endolithic green alga Ostreobium sp. that inhabits coral skeletons and thrives under extreme shade below the coral tissue due to possession of a special Chl a antenna absorbing in the far-red region around 700–730 nm (Halldal, 1968; Fork and Larkum, 1989; Koehne et al., 1999), that is a region of the solar spectrum which is not absorbed by the overlying coral tissue (Magnusson et al., 2007); 2. The prochlorophytes present three independent lineages of cyanobacteria, that is the genera Prochlorococcus , Prochlorothrix and Prochloron . Prochlorococcus contains unique divinyl-Chl a and divinyl-Chl b photopigments and only minor amounts of phycobiliprotein (PBP) pigment, while Prochlorothrix and Prochloron are the only prokaryotes containing Chl b (Partensky and Garczarek, 2003).

Dinoflagellates are well-known and readily recognized protists, consisting of ~4,000 named extant and fossil species (Fensome et al., 1993). Dinoflagellates exhibit great diversity in many ecological parameters such as niche exploitation, and show extreme idiosyncrasy in their ultrastructural and molecular biological characteristics. The human impact of sudden dinoflagellate proliferation, in the form of harmful algal blooms (HABs), as well as the impact of the sudden loss or senescence of dinoflagellates, in the form of coral bleaching, have become an increasing focus of concern in recent years. For these reasons, studies of the molecular evolution, ecology, diversity and physiology of dinoflagellates have increased dramatically, and have revealed ever more interesting features. This review will explore the significance of recent findings in the phylogenetics, evolution, molecular biology and ecology of this intriguing group.

Diatoms (Bacillariophyta) are diploid eukaryotic unicellular algae with a wide range of regular and decorative shapes (Plate 1) that are now placed in the Heterokontophytes by botanists or the Stramenopiles by zoologists (Medlin et al., 1997). Using oxygenated carotenoids as light-harvesting pigments, they are generally photo-autotrophic, although there are N-heterotrophic species. Diatoms occur in very large populations in both freshwater and marine environments, in all climatic zones. They have been a very successful group from an early moment in their history. While claims of observations in Permian or even in Carboniferous deposits (Zanon, 1930) were spurious, the group’s origin is generally placed in the Jurassic, as in Round et al. (1990). The oldest fossil records are of marine species – some hardly different from modern ones, others without surviving relatives – and in deposits from the Cretaceous both species diversity and number of specimens can already be impressive (Harwood and Nikolaev, 1995).

From the upper reaches of the intertidal zone to the beginnings of terrestrial vegetation is a region of shoreline that is often sparsely inhabited by algae, and typically includes conspicuous expanses of bare rock. Inspection of the habitat reveals scattered or even abundant lichens, and often extremely patchy to extensive populations of macroscopic algae. The physiological ecology of photosynthetic algae in this part of the intertidal zone comprises the primary theme of this chapter. The organisms discussed here typically have extensive populations above mean high water neap tide (see Lüning, 1990; Lobban and Harrison, 1994; Little and Kitching, 1996, for introduction to tides and zonation). In general, these organisms are found exposed on bare rock and not in the rock pools where greater species richness occurs and less stringent environmental conditions are imposed. In terms of physiological constraints, the high intertidal and adjacent supratidal zone is among the most stressful encountered by organisms in general (Tomanek and Helmuth, 2002), and by marine macroalgae in particular (Davison and Pearson, 1996). First, the rigors of the environment are explored, and then the various adaptive strategies of algae to survive and thrive in this habitat are discussed. Davison and Pearson (1996) reviewed stress tolerance in intertidal seaweeds as a whole; however, here the focus is on the upper intertidal zone and on disruptive stresses that cause damage or limit growth.

Over millions of years of evolution, marine macroalgae (commonly referred to as seaweeds) have remained within a narrow and restricted niche, compared to the extensive area covered by oceans and seas. This narrow fringe is the intertidal zone, in which seaweeds are intermittently exposed to harsh conditions such as high irradiance, desiccation and high temperatures. What were the adaptive strategies and physiological needs of these plants to thrive and complete their life cycles over millions of years in these harsh environments? Seaweed’s first records date at least 300 million years, and within this period of time they went through several episodes of environmental change. Today, marine macroalgae comprise about 20,000 species of which a large number can be found within the intertidal zone. During evolution macroalgae diverged into three major categories or divisions: green (Chlorophyta), brown (Phaeophyta) and red (Rhodophyta) seaweeds. The present Mediterranean flora has a history of about five million years. After the isolation of the Mediterranean from the Atlantic, biota surviving the late cooling Miocene re-colonized the vacant basin and established the early Pliocene biota. Then, the Mediterranean Sea lost its coral reefs and its tropical character in general (Luning, 1990). The dramatic climate changes (glacial periods), which took place in this area in the Pleistocene, may have allowed a number of cold-temperate species to invade the area and to form disjunctive populations in cooler parts of the Mediterranean after the glaciations (Hoek and Breeman, 1990). Empty niche space and the climate changes in the late Pliocene and the Pleistocene may have promoted speciation and origin of endemic species. Today, the Mediterranean coasts are inhabited by a rich seaweed flora, including endemic, tropical, warm and cold-temperate species (Orfanidis, 1992).