bacterial metabolic activities exchange
all major reactive gases with atmosphere :
N2, N2O, O2, CO2, CO, H2,
CH4, NH3, H2S and many others
several of these re-cyclings are restricted
to bacteria - notably N2
also central in cycling C, P, S
photosynthetic bacteria began the change
in character of earth's atmosphere ~3 b.y.a.
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