It's either keeping mars sterile or seeding a diverse and balanced biosphere. If we seed mars with a small biased microbiology we could accidentally skew it too far from earths biology and make it unlivable for humans. Maybe. Nobody knows for sure.
Too bad that I will probably perish before I witness such earth-shattering changes
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That's true, but the first step is just finding stuff that can survive at all, so that an ecology can build out around it.
Even if the result is unlivable for humans, other outcomes could still be valuable. For instance, whatever can survive will be converting compounds in the surface into gases, which will increase atmospheric pressure and one of the big problems with Mars is its low atmospheric pressure.
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