Closed Loop Life Support Systems

> my understanding is that many plants switch to using O2 instead of
> CO2 when there is minimum sunlight... a surprising but important
> consideration.

Don't they all? But what does it matter: the amount of carbon in a
plant is the net amount it removed from the air as CO2, and the amount
going back and forth in one day is a very small fraction of that so
that it's not that different from a continual release of O2 and
absorption of CO2. And in a space habitat, you would probably stagger
the dark and light periods of different batches of plants so that you
would be using the capacity of your mirrors or PV all the time.