[BLanen]

Algea is only a co2 battery unless you can capture it and store the algea without it fermenting. If you just grow it in the Ocean, you need to increase the Ocean's algea carrying capacity and form it thus into a battery because dead algea gets fermented by bacteria into co2 and methane again. Also the ocean's carrying capacity for algea is currently ever decreasing due to oceanic acidity increasing from co2.

Artificially growing algea in pools and using that to make fuel/ pump it underground so the fermentation products don't get released is not nearly efficient enough to be viable. Ironically this usually also uses fertilizers which are dependent on the fossil fuel industry.