The basic misunderstanding/lie in the "seaweed will save us" line is this: unlike on land, plants only play a relatively small part in the food chain. Why? Because seaweed can only grow in the tiny fraction of ocean where the water is both calm and shallow enough for light to reach the bottom. The vast bulk of life in the ocean is instead ultimately dependent on free-floating, microscopic plankton. Yes, phytoplankton photosynthesize using CO2, but the majority of them are also dependent on calcium carbonate to form exoskeletons. We mess with this at our peril.
