[Twisell]

Also algae could be neat allies. But there we are playing with fire, maybe GMO algae can be engineered to be supper efficient, but at the risk of compromising marine ecosystems.

[knob]

The fern Azolla already did this once! linky --> https://en.wikipedia.org/wiki/Azolla_event

[twic]

There's a lower-tech way to get algae on board - feed them the micronutrients they're limited by in the ocean, which is mostly iron:

https://en.wikipedia.org/wiki/Ocean_fertilization

https://en.wikipedia.org/wiki/Iron_fertilization