Small nitpick, but you can have a 'single fitness function' represent an entire environment. That's the entire concept of a fitness function: to simplify the merit of solution within an complex environment (or set of constraints/inputs/etc.) into a single figure. Obviously nature has a much complex fitness function than the efficiency of this one CO2 process. I think its more correct to say a "simpler fitness function".
Actually, there is only one evolutionary fitness function, and that is reproductive fitness. The environment is the only variable. If your environment contains an intelligent agent that values the production of (say) alcohol, and the power to influence the selection of alcohol-producting organisms then an organism that produces alcohol can thrive. But this is true of anything, not just alcohol. Substitute "kitten photos" (or anything else) for "alcohol" and you will still have a true statement.
