Daisyworld is a model Earth covered in black and white daisies, that either absorb or reflect light. In the model, black daisies need less light because they absorb more, increasing the Earth's heat absorption. White daisies absorb less so need more light, but also reflect more light back, increasing the surface albedo of the planet and lowering its ambient temperature.
Because of this, there's a feedback loop were even if the sun gets a little hotter or colder, the successful daisy would spread and either warm or cool the Earth in response, effectively acting as a stabilizer. Gaia theory suggests that the world is full of these stabilizing systems.
Anyone who ever played SimEarth in the 90s, it was based on this theory and even had a Daisyworld simulation built in.
In this case, the paper is suggesting that seaweed grows much faster in response to raised CO2, and then sequesters some of that carbon underwater.
One question I have is how sequestered the undersea carbon really is and whether it will have other, unknown effects on the deep see ecosystem.