Astrobiology by Unknown

Astrobiology by Unknown

Author:Unknown
Language: eng
Format: epub
ISBN: 9789811336393
Publisher: Springer Singapore


16.5 Discussion

16.5.1 Implications for Early Evolution of Photosynthesis

Depositional environments for Paleo- and Mesoarchean microfossil-bearing sedimentary rocks from the Kaapvaal and the Pilbara cratons are summarized in Tables 16.2 and 16.3. They show that microbes were already flourishing in a variety of environments well before 3.0 Ga (Van Kranendonk 2011), mostly in shallow to subaerial environments. This is consistent with records of non-microfossiliferous biosignatures, such as stromatolites and microbially induced sedimentary structures (e.g., Hofmann et al. 1999; Allwood et al. 2006; Djokic et al. 2017; Noffke et al. 2013). This may provide some hints about the metabolism of ancient microorganisms. An ozone shield was unlikely to have been present, considering the very low concentrations of free oxygen during the Archean (e.g., Holland 2006, Chap. 17). Shallow-water to subaerial habitats were probably radiated by harmful ultraviolet (UV) rays, although the lack of an ozone layer might have been compensated for by a UV-shielding organic haze (Wolf and Toon 2010), dispersed silica particles in water columns (Siever 1992; Stefurak et al. 2014), and/or the encrustation of cells with silica (Phoenix et al. 2001). In either case, one of the benefits from living in such environments would have been the utilization of light energy. This suggests that photosynthesis and even oxygenic photosynthesis might have already emerged in the Paleoarchean, as argued by or suggested from previous studies (Buick 2008; Nisbet and Sleep 2001; Tice and Lowe 2004, 2006a, b; Hoashi et al. 2009; Mukhopadhyay et al. 2014; Rosing and Frei 2004; Crowe et al. 2013; Planavsky et al. 2014; Lyons et al. 2014; Schirrmeister et al. 2016). The discovery of an axial zone in the large conical stromatolites in the Strelley Pool Formation at the Trendall Geoheritage Reserve (Hickman et al. 2011) may also support the idea that oxygenic photosynthesis was occurring locally, because modern motile cyanobacteria in hot springs have been shown to move toward light and in so doing to produce a similar axial-zone structure (Walter et al. 1976). Recently, Kremer and Kaźmierczak (2017) suggested that small spheroids in the 3.4 Ga Kromberg Formation represent fossilized coccoid cyanobacteria such as Microcystis. While such an early evolution of oxygenic photosynthesis is not always widely accepted and needs further studies, the emergence of anoxygenic photosynthesis is likely back to Paleoarchean (e.g., Tice and Lowe 2004).



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