Re: The answer is more complicated than that
"Carbon dioxide isn't a pollutant like nitrous oxide or other emissions. It is an essential component of virtually all life on Earth. Other than bizarre ecosystems like around the deep ocean vents, every animal on Earth inhales oxygen and exhales carbon dioxide. Conversely, plants absorb CO2 and emit oxygen."
True enough, but in terms of biomass and probably metabolic activity- the
"bizarre ecosystems" shouldn't be thought of as bizarre, meaning odd/freakish/or even loosely as rare. Though as far as human expectations as to "how things should be" bizarre ecosystem is a apt description.
As I mentioned before, I would think that Mark (who I know) has had some interest in biosphere II, would have learnt something from their failed experiment.