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‘Mangrove’ is an overall term to indicate a tropical or subtropical community of highly adapted trees and shrub species growing in intertidal estuarine and secluded marine areas. Mangroves act as physical barrier to mitigate the effects of coastal disasters like tsunami, hurricanes, and waves. Mangroves create unique niche that hosts rich agglomeration of species diversity. The submerged part of mangrove roots, trunks, and branches serve as islands of habitat that may attract rich epifloral and faunal communities including bacteria, fungi, macroalgae, and invertebrates. Despite low nutrient levels, mangroves grow efficiently in this environment (Sengupta and Chaudhuri, 1991; Alongi et al., 1993;Vazquez et al., 2000; Bashan and Holguin, 2002) through efficient recycling of available nutrients by the activity of microorganisms (Alongi et al., 1993; Kathiresan, 2000; Holguin et al., 2001; Bashan and Holguin, 2002). Even though mangrove ecosystem is one of the valuable ecosystems, this is the most threatened one at present (Farnsworth and Ellison, 1997;Kathiresan, 2000; Adeel and Pomeroy, 2002). Besides making awareness amongst society, policy-makers can also constitute by-laws for the conservation and restoration of mangrove ecosystem. Bacteria have significant role in the recycling of nitrogen in mangrove environments. Cyanobacteria, a group of photosynthetic prokaryotes, are vital component of the microbiota ranging from unicellular colonial to filamentous contribute a source of nitrogen in every mangrove ecosystems (Kathiresan and Bingham, 2001). This is one of the ignored groups where only a very few studies have been conducted. The studies on cyanobacteria associated with mangroves are very important not only because of their abundance, but also of their high capability for nitrogen fixation, which are natural candidates for future reforestation and rehabilitation of destroyed mangroves (Bashan et al., 1998). Hence, this chapter is aimed at depicting the present status of mangroves, their importance, and to analyze the pioneer articles on cyanobacteria inhabiting in mangrove ecosystems.

An intertidal sandy beach is a constantly changing habitat, and, in that sense, it could be regarded as an extreme environment. It alternates between a seabed and a land with every tidal transition, and this alternation changes physical conditions such as beach morphology, water level, nutrients, oxygen level, salinity, temperature, light intensity, etc. Sand is an unstable substratum. Tides and waves constantly move sands on the submerged shore face. Even a single rainfall during the low tide changes the physical conditions, and a one-night storm could change even the landscape of the shore resulting in a catastrophe for its microbial communities.

Theoretical geochemical considerations (Kempe and Degens, 1985) and field work at sites of active growth of in situ calcified cyanobacterial mats and biofilms (e.g., Kempe et al., 1991; Kempe and Kazmierczak, 1993; Kazmierczak and Kempe, 2006) convinced us that in the past the ocean must have been more alkaline than at present and that it was of higher CaCO_3 supersaturation (SI_Calcite > 0.8) (Kempe and Kazmierczak, 1990a, 1994; Kazmierczak et al., 2004). Two processes contributed to the higher alkalinity: (1) the slowly declining high primary alkalinity established in the Hadean ocean by binding large amounts of degassed and cometary CO_2 through silicate weathering as CO^2- _3 and HCO^- _3 in ocean waters (“Soda Ocean”) and (2) the effect of the export of excess alkalinity from sulfate reduction processes in stagnant marine basins (“Alkalinity Pump”) (Kempe, 1990; Kempe and Kazmierczak, 1994). The latter alkalinity source started to be effective only after enough sulfate became available in the ocean (probably during the last 1.0–0.8 Ga – for evaluation of sulfate level in the Precambrian ocean see e.g., Buick, 1992; Grotzinger and Kasting, 1993; Eriksson et al., 2005). It could be the single most important factor for short-term modifications of Phanerozoic ocean chemistry. Sudden, in geological terms, export of alkalinity from overturning anaerobic basins could cause high pH and Ca^2+ stress upon the marine biota (Kempe and Kazmierczak, 1994; Brennan et al. 2004; Kazmierczak and Kempe, 2004b), and is associated with negative δ^13C excursions in carbonate sequences at the Precambrian/Cambrian transition (e.g., Knoll et al., 1986; Magaritz, 1989; Magaritz et al., 1991), where biocalcification started in several phyla almost simultaneously (e.g., Lowenstam and Margulis, 1980; Lowenstam and Weiner, 1989).

Microscopic algae and cyanobacteria (the term micro-algae will be used in the text to cover both eukaryotic algae and prokaryotic cyanobacteria) are able to colonize almost all of the biotopes on earth. They are the most important primary producers in both sea and freshwater ecosystems. Their importance in terrestrial ecosystems increases further in extreme habitats because of the decreased competition of higher plants. For example, in the Antarctic, the role of algae as primary producers increases from the maritime to the continental areas where harsher conditions limit the development of mosses (Wynn-Williams, 1985). Algal mats and biological soil crusts are found worldwide in various extreme environments (Broady, 1979; Vincent, 1988; Cohen and Rosenberg, 1989; Belnap and Lange, 2001). As primary producers, micro-algae represent the bottom of the food webs, and serve as an important food source for a wide spectrum of animals. The aim of this chapter is to summarize recent knowledge about the role of terrestrial and freshwater micro-algae as a food source for invertebrates, with particular attention to extreme habitats.

Cyanobacteria are commonly thought of as microbial phototrophs that are characteristic of warm water environments such as hot springs (Steunou et al., 2006), stratified lakes during summer (Vazquez et al., 2005) and tropical oceans (Johnson et al., 2006). It is less widely known that many cyanobacterial taxa achieve their greatest ecological success at the opposite thermal extreme, in polar and alpine environments. One of the first discoveries of the prolific growth of cyanobacteria in the cryosphere (the ensemble of cold environments containing snow and ice) was by the Swedish-Finnish explorer Adolf Erik Nordenskiöld. In his expedition across the Greenland Ice Cap in 1870 his team discovered black sediment that he called ‘cryoconite’, cold rock dust collecting in melt holes (Leslie, 1879). On closer inspection they observed that this material was composed of not only inorganic sediments but also black-pigmented cyanobacteria, now known to be mostly the heterocystous species Calothrix parietina (Gerdel and Drouet, 1960). They concluded that because of its dark colouration, this cyanobacteria and its bound sediment absorbs radiation and hastens melting of the ice, a process more recently documented on glaciers (Takeuchi et al., 2001) and ice shelves in the Canadian High Arctic (Mueller and Vincent, 2006).

The Antarctic habitats are some of the driest and coldest ecosystems on the Earth. Earlier there was a general acceptance that polar deserts harbored little life (Priscu, 1999). But, recent studies have revealed the existence of microbes in: the snow near the South Pole (Carpenter et al., 2000), the 3.5 km deep in Vostok ice (Karl et al., 1999; Priscu et al., 1999a), exposed soils (Wall and Virginia, 1998), sandstones (Friedmann et al., 1993), meltwater ponds (Vincent, 1988), liquid water column of permanently ice-covered lakes (Priscu et al., 1999b), and the ice covers of permanent lake ice (Priscu et al., 1998; Psenner et al., 1999). Most of the microbes found in these habitats are prokaryotic (Vincent, 1988; Gordon et al., 2000; Brambilla et al., 2001). Among these microbes, one of the most important components is the photosynthetically active cyanobacteria. They provide for an adequate quantity of fixed carbon via photosynthesis to drive a well-developed ecosystem (Vincent, 1988). On the contrary, in those habitats where there is a lack of cyanobacteria, biomass production by the addition of new carbon and nitrogen is slowed. Thus, such habitats are poor in biodiversity and also poor in trophic levels. In Antarctic habitats the cyanobacteria are adapted and acclimated to their environment in terms of temperature, freeze/thaw survival photoprotection, as well as light acquisition for photosynthesis (Vincent et al., 1993a, b, c; Tang et al., 1997; Nadeau et al., 1999; Tang and Vincent, 1999; Nadeau and Castenholz, 2000). Though cyanobacteria play a significant role in ecosystem dynamics, only a few of them have been considered true psychrophiles (Tang et al., 1997; Fritsen and Priscu, 1998). They are classified as psychrotolerant or psychrotrophic due to their ability to metabolize near 0ºC and also because their temperature optima for growth are typically above 15ºC. Some of the cyanobacterial groups, for example, Leptolyngbya, Phormidium, Oscillatoria , and Nostoc are cosmopolitan and occur in highly divergent environmental extremes.

Cryoseston inhabits one of the most extreme environments in the Earth biosphere. The phototrophic components are composed exclusively from microorganisms, adapted to life conditions of melting snow. All species occurring in cryosestic assemblages evidently colonised the snowfields secondarily, their ancestors originating from other habitats. Cryosestic communities develop in snowfields and on the surface of glaciers, where the temperature surpasses 0ºC periodically (daily, or over variously long time periods), and the snow changes locally from solid to liquid state. It means, that the temperature adaptability of cryosestic species must allow to start the intense metabolic activities immediately after melting their cells accommodated in snow. Such adaptation also occurs in algae from other biotopes (in subaerophytic, endolithic and terrestrial habitats), but it is the conditio sine qua non in typical cryosestic algae. Another precondition is that the cryosestic microflora can develop only in snowfields and glaciers remaining and persisting in air temperatures above 0ºC over some periods, and under convenient irradiance conditions (cf. Hoham and Duval, 2001). This situation occurs mainly in mountains and polar and subpolar regions over the spring and summer periods.

Diatoms are unicellular microalgae that contribute to 20% of the global carbon fixation. This is as much as the carbon fixed by all tropical rainforests combined (Armbrust et al., 2004). Diatoms are found all over the globe, in freshwater and seawater, in hot and cold habitats. Their most distinctive feature is a silicified cell wall (termed frustule) made of hydrated amorphous silica and a small amount of organic material (sugar). The architecture of the frustule is based on silica patterns that are structured on a nano-to-micrometer scale. These nano-patterns can vary from species to species, creating unique morphotypes that are used as taxonomic keys

In vitro cryopreservation is the storage of viable cells at ultra-low temperatures (196ºC), usually in liquid nitrogen or its vapor phase. Under these conditions it is assumed that metabolism is arrested and cells are stable for indefinite periods, so long as liquid nitrogen supply is maintained. The fact that cells tolerate cryogenic temperatures is remarkable as survival after cryopreservation is common to a wide range of biodiversity. The in vitro cryobank is one of the most, if not the most extreme low-temperature environment that an organism, or component part thereof, will ever encounter on earth. It is fascinating to speculate how, with the aid of cryoprotection (Fuller, 2004) so many diverse life-forms survive such extreme cold. Cryopreservation has important applications for astrobiology and in vivo studies of extremophiles; as water and temperature are physical determinants of life, indeed water is a prerequisite for life. This chapter considers cryoconservation in a wider context, appraising the comparative utilities of both natural and artificial cryobanks as repositories and research tools that may be used to help understand how life survives extreme cold. Algae are the subject of choice as they are one of the oldest and most diverse groups of organisms; their ancestral, fossil remains have been found in strata dating from 1.4 billion to 2.1 billion years (Cloud et al., 1969; Han and Runnegar, 1992). Algae are ubiquitous primary producers and formidable extremophiles, yet, compared with other biological resources, their preservation in cryobanks (Day et al., 2005) and their utilization as a valuable economic resource remains limited.

An Italian scientist, Giuseppe Meneghini (1839) (Fig. 1) was the first to study and describe the thermo-acidophilic algae inhabiting the sulphur hot-spring of Acquasanta (Ascoli Piceno, Italy). He observed “small and very small globules” (0.2–2.0 µm) and he proposed the new species Coccochloris orsiniana (Cyanophyta) without any diagnosis. In the following 100 years, these algae, always considered as one species, captured the interest of many phycologists who tried to clarify their systematic position. Because of their simple morphology and the absence of sophisticated means of investigation, these algae were variously identified as Palmella orsiniana (Chlorophyta) (Kützing, 1849), Chroococcus varius (Tilden, 1898), Protococcus botryoides f. caldarium (Tilden, 1898), Pleurococcus sulphurarius (Chlorophyta) (Galdieri, 1899), Pleurocapsa caldaria (Collins et al., 1901), Palmellococcus thermalis (West, 1904), Pluto caldarius (Copeland, 1936), Cyanidium caldarium (Geitler and Ruttner, 1936), Dermocarpa caldaria (Drouet, 1943) and Rhodococcus caldarius (Hirose, 1958). By using morphometric analyses of field samples, De Luca and Taddei (1970) identified for the first time, two thermo-acidophilic algae (Fig. 2). These were provisionally named Cyanidium caldarium forma A and Cyanidium caldarium forma B. In the early 1980s the Neapolitan school used morphological, physiological and ultrastructural data to invalidate the previous descriptions, and made a definitive revision of the taxonomy of these algae. Two species were formally recognized: Cyanidium caldarium (Tilden) Geitler (Fig. 3) and Galdieria sulphuraria (Galdieri) Merola (Merola et al., 1981) (Fig. 4).