The earth is always exposed to light and heat from the sun.  However, from our point of view, we see light and dark every 24 hours.  Plants have a adapted to light cycling.  Plants absorb carbon dioxide during the day and let out oxygen.  At night they consume the sugars collected through the day and use it to build more leaves and grow.  At this time they release carbon dioxide to the air

A biosphere needs to imitate day and night in the same way.  If plants are put into the dark for a time, they will not grow to produce food for inhabitants.  That means the lights cannot be kept on all the time.  To solved the problem, the biosphere is separated into two chambers.  One tank for day and other for night.  Each one is kept on 12 hours and then off for the remaining 12 hours.  The tank's lights are off by 180 degree cycle.  This means the tank with the lights "on" has to be developed enough to absorb the CO2 produced by the dark tank and vies-versa.  

Before this processes was developed, the CO2 level would stabilize to 2200 ppm.  This is with one tank and a 12 hour light cycle.  With both tanks connected together with air movement, it stabilizes around 540 ppm.  As the RH and temperature goes down, the CO2 also lowers.  As the tanks remains closed to outside air, the O2 hovers around 20% to 21%.  Other gasses like methane and NOX do not seem to change.  Before any animals are released into the biosphere, the CO2 needs to be consistently below 400 ppm.

Air movement

Wind is everything.  This is the device that mixes everything together.  CO2 is heavy and will settle to the bottom of the tank.  A fan moving air is needed to get gasses in contact with the leaves.  Just adding air movement reduces the CO2 by 500 ppm.

Below is a picture of a wind indicator coming out one of the pipe between both tanks.  It's a piece of plastic taped to the inside wall.  A small fan is attached to the in and out of the other tank.

Configuring a Mini-Spinning Earth

Both tanks need to be separated by a physical barrier but combined by air.  In this case you can see in the picture below the left tank is lit while the right one is off.  The two black pipes move air between each tank 24 hours.  The bottom pipe moves with from the left to the right.  The top one move air from right to left.  There are small fans in front of the pipes inside the left tank.  One point in and other point outwards.   Plants grow in both tanks.

The last pipe is attached to the marine tank.  Air will be pumped from the left tank and bubbled in the marine tank.  The returned air is piped back into the middle tank.  Please see the picture below.  This is not operational at this time.  The current test is to see how the plants can perform and reduce CO2 without the ocean.

There are two separate times for the lights.  While one tank is on, the other is off.  The marine tank is on its own schedule.

Other sources of Carbon Dioxide

The walls of the tank are a biosphere enemy; when it comes to controlling CO2.  Bacteria grows on the glass walls consuming light and moisture from the air.  The higher the moisture, the higher the CO2.  For instance, if all the walls are wiped down with isopropyl alcohol, the CO2 level goes down to around 100 ppm.  If water is left in a glass in an enclosed tank, the CO2 level will slowly increase to easily 10000 ppm.  It will take longer if the walls are wiped beforehand.  

If the walls are not wiped down, the CO2 shoots up.

If a plant in a pot is left in an enclosed tank, the CO2 will jump abruptly.  It will peak around 700 to 800 ppm and then settle to 600 ppm.  Slowly (over a few weeks) the CO2 will rise over 2000 ppm.  This is will a light source turned on and off every 12 hours.  If the plant is recently watered, the CO2 level climbs faster.

Future

This experiment is nowhere close to being completed.  I got CO2 to be consist around 550 ppm. It needs to be lowered.  As the tanks dry out and water is removed from the air, the CO2 has gone down under 500 ppm.  There are three dehumidifies in both tanks.  They are pulling out water from the air and watering the plants.  You can see in the right tank that is full of sifted soil.  Water is being pulled out of the air and dumping into the dry soil.  

There is also no soil in the left tank.  There are stilts holding up a screen where pots are supported.  Water dips to the bottom and create a load of CO2.  Once that is changed, I expect the CO2 to drop around 200 ppm.  This is the equilibrium achieved in earlier biospheres.

Stilts in tank

Sifted soil