They naturally extract CO2 from the air and use it as feed. The more CO2 plants absorb, the less CO2 remains trapped in the atmosphere.
If left alone, plants are eaten by other organisms, releasing the carbon back to the carbon cycle within months.
In anoxic waters, plants decompose extremely slowly, effectively storing the carbon much longer.
It is the largest anoxic body of water on earth, 2km deep, surrounded by fertile lands. The Black Sea is the optimal environment allowing affordable, environmentally safe, gigaton scale carbon removal in this decade.