The carbon absorbed in the photosynthesis does not just go free to burn again in oxidation, it stays as a building material for the living creature itself. Considering a positive process, in which more organisms are created (reproduced through mitosis) than those that die, in time the raw quantity of CO₂ should decrease. I'm not sure what to do afterwards with that mass of algae though.
The carbon does not go free because it is bound up in some organic compound, almost always a hydrocarbon (which of course requires Hydrogen) and would still combust at 460 Celsius. Perhaps if this life-form lived exclusively in the upper atmosphere? Then it would just have the problem of getting anything but carbon, oxygen and nitrogen.
