How to generate process of photosynthesis with out trees?

The natural process by which green house gases are removed from the atmosphere is the photosynthesis. But due to rapid deforestation and increasing green house gases (CO2) there could be shortage of oxygen in future. Atmospheric percentage of oxygen is only 21%, still sufficient, due to oxygen cycle. The process of photosynthesis requires CO2, water and a tree. But while percentage of trees going down and down per density of population shortage of oxygen likely. Then how to carry out process of photosynthesis without trees. Is there any method or can it be developed in future?

One comment

1. Alastair says:

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   Why not just use trees to remove CO2? If they were buried in an anoxic site: muddy delta, coal mine, or deep ocean, that would remove CO2.