Good question. Seems like they could -- although co2 might be toxic at given levels like o2 would be at 3-4x concentration for us. I don't know about what else they would need -- surely some other nutrients that aren't just floating around. Wouldn't it be awesome if we could just throw some single celled lighter-than-co2 algae in there that could float and produce oxygen and nitrogen. Wait 10,000/100,000/1,000,000? years and just fly around the newly atmosformed atmosphere.