This review looks at how silicification of bacteria allows preservation of the bacteria through fossilisation and also helps the bacteria to survive the hostile environment.

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Environmental Microbiology & Bioresources

The living condition in early Earth is very different from the modern world. Some bacteria found ways to survive through various strategies. This review looks at how silicification of bacteria allows preservation of the bacteria through fossilisation and also helps the bacteria to survive the hostile environment.

The habitable environment of early Earth is very different from what it is today. The early earth was significantly hotter, [1, 2] and the oxygen level was extremely low with less than 0.2% than that of present atmospheric levels. [3, 4] The early Earth was also strongly influenced by strong hydrothermally activity, leading to sea-water temperature to hit as high as 55oC. [5] Such hydrothermal influence can result in massive silicification of both sedimentary and volcanic units in early Archean terrains found in many regions such as Barberton greenstone belt in South Africa [6] and Pilbara Craton in northwest Australia. [7, 8] With such high concentration of silica, micro-organisms may be preserved in a fossil as silicified remnants. [9] The study of microbial silicification is important as they can provide information of microbe-silica interactions in places such as hot springs. Moreover, modern geothermal technology allow for contemporary analogues for conditions which Precambrian microorganisms are fossilized. [10] 

There are many researches that detail the silicification of microorganisms as preservation. For example, a thin microbial mat formed by 0.25-µm diameter filaments was silicified on the top of volcanic littoral silts, [11, 12] and similar silicified microbial mat remnants  was found in other parts of South Africa. [13] Further evidences include silicified colonies of small (<1 lm) coccoidal micro-organisms associated with 3.466 Ga-old volcanic sands and silts [14] and fields of small, silicified stromatolites with dissimilar morphologies were found in Australia. [15] Furthermore, microfossils with different morphologies and sizes within the same colony indicate that some sort of biodiversity. [14] 

In these studies, the silicification processes were often induced by using organosilicon solvents such as tetraethylorthosilicate [15], with a myriad of combination of various pressure and temperature. For example, Ferris and his colleagues [16] used low temperature of 70oC and low pressure of 1 bar whereas Oelher and Schopf [17] used high temperature of 100-300oC and high pressure of 1000-3000 bars. In other case, low temperature (4-25oC) and moderate pressure (1-500) was used. [18] Although many studies had provided significant insights into predominantly diagenetic related fossilization processes, they provide little in mechanisms controlling microbial silicification in environments such as ancient oceans or the modern hot springs.

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Microscopic observations of many fossil samples revealed several notions of the silicification process. Firstly, it indicated that silicification of microbes is a rapid process. [19] The speed of silicification also suggested that there are inorganic driving forces controlling the process. Further microscopic studies also suggested that microbes may have acted as nucleation sites for silica precipitation. [20] The initial step in such biomineralization process comprises of chemical interactions with mineralising ions in the bulk aqueous phase, [21] and that it is well known that bacterial surfaces possess many reactive binding sites. [22, 23]

However, beyond the bacterial surfaces, the controlling factors ...

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