MTB are versatile also with respect to nitrogen metabolism, the fixation of molecular nitrogen (Bazylinski and Blakemore, 1983; Bazylinski et al., 2000) and denitrification being performed by different strains. There are two types of deni-trification, assimilatory and disassimilatory nitrate reduction, both being important for terraformation. Assimilatory nitrate reduction allows cells to use extracellular nitrate as a source of nitrogen for biosynthetic purposes whereas disassimilatory nitrate reduction will eliminate nitrogen gas in the atmosphere. The latter can be done by some MTB as well as by other bacteria such as Pseudomonas denitrificans, Thiobacillus denitrificans etc., which can be an important factor for terraformation. The use of nitrate as the terminal electron acceptor in anaerobic respiration (nitrate respiration) by some MTB could enable them to act, together with other denitrifying bacteria, for terraformation by increasing the atmospheric N2 level. It seems probable that Mars has significant nitrate reserves (equivalent to as much as 300 mbar of gaseous N2) which could be mobilized by denitrification. Denitrification should be very intense during the first phases of terraformation, until the atmospheric oxygen level would increase enough to inhibit the process at aerobic sites and forcing nitrate-respiring bacteria to withdraw to anaerobic niches.
In a more advanced stage of terraformation, when atmospheric nitrogen level would surpass a certain limit (5 mbar), nitrogen fixing bacteria including cyanobacteria and some MTB (Bazylinski and Blakemore, 1983; Bazylinski et al.,
2000) could begin fixing nitrogen, thus making it available for various organisms and trophic chains.
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