Maintaining very small ecosystems

I have been toying with an idea now, for several weeks. I have a friend at
the University of Surrey, who recently graduated and wrote his thesis in
plant biology. Together, we have been examining the problems associated
with small-enclosed ecosystems, primarily the build-up of organic toxins
and the instability of the atmosphere. The biosphere-2 experiments found
that the balance of gases within a small ecosystem is unstable. Oxygen
levels can fall to intolerably low levels. In the case of the Russian
BIOS-3 experiment, the plants produced too much oxygen. The atmosphere was
effectively emptied of CO2 and many plants could not photosynthesise. The
atmospheric balance of biosphere's tends to swing between the two extremes.
The human inhabitants of small biosphere's could compensate for
varying atmospheric gas concentrations, if given enough time. If the
atmosphere were found to be depleted in oxygen, more flora could be
grown. If the atmosphere were found to be too rich, a portion of the
flora would be removed.
What is needed most of all, is a mechanism that buffers the effect of
ecosystem changes and allows the crew time, in order to react to the
changing needs of the habitat. A mechanical atmospheric processing plant
is required. If high levels of CO2 are registered within the atmosphere,
the efficiency of photosynthetic organisms that convert it into O2 and
sugar will increase. The huge CO2 swings within a biosphere can easily
swamp this ability. A mechanical atmospheric processing device, equipped
with both a CO2 and O2 gas storage tanks, could be used to buffer the
changes. If too much CO2 is produced, the system will remove it from the
atmosphere and liquefy it. If too much oxygen and not enough CO2 are
present, Oxygen would be removed and stored and CO2 released. In order
to compensate for high levels of either gas, rapidly growing algae could
be farmed within large tanks of water. Algae provide a very effective
mechanism for storing unwanted carbon within a compact space.