based on my research into phytoplanktons and cyanobacteria-based climate change mitigation for the purposes of commercialization, my take on this is that planting trees is probably a significantly easier approach in terms of the amount of effort involved on an ongoing basis.

in terms of efficacy, i'd say it depends on how productive your phyto/cyano farm is versus planting trees over a 10-year course per dollar. phyto/cyano farms will be super dense in terms of sequestration, capacity but you'll be investing hundreds of dollars per unit.