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The unambiguous evidence for the presence of life in the Archean is only limited by the preservation potential of sedimentary rocks. Throughout Earth’ preserved sedimentary deposits, prokaryotic bodily fossils and geochemical fossils, for example, products of the Calvin-cycle dependent carbon isotopic fractionation, can be found. Nevertheless, irreproducible analyses in organic geochemistry, misinterpretations of artifacts from sample preparation and of organic contaminants, and uncertainties on the age and nature of the Archean rock formations are copious in evaluation of the earliest traces of life. The understanding of geological processes strongly influence discussions of the ancient, supposed biological relicts from c. 3.8 billion years old (3.8 Ga) metasedimentary rocks. The evidence for prokaryotic bodily preserved microfossils of the Neoarchean, at 2.7 to 2.5 Ga is by orders of magnitude stronger, as rocks of this age are abundant and better preserved.

The more than 40 chapters in this book provide an updated overview of our current understanding of the life of oxygenic phototrophs – from simple prokaryotic cyanobacteria to multicellular eukaryotic macroalgae – in a wide variety of extreme biotopes in which they are exposed to diverse forms of stress: high and low pH, high and low temperatures, including below zero temperatures, excessively high and extremely low light intensities, salt concentrations up to saturation, and xeric environments. We should not forget situations, e.g., the intertidal where conditions, especially related to water availability, fluctuate on a daily basis. In addition, many contributions deal with “polyextremophilic” phototrophs, which are simultaneously adapted to multiple forms of environmental stress. The unicellular red alga Cyanidium caldarium and its relatives Galdieria sulphuraria and Cyanidioschyzon merolae are among the best known examples of such polyextremophiles, being adapted to life in hot thermal pools at temperatures in the range of 45–56ºC and pH values as low as <2–4.