text: Seaweed grow in shallow and coastal areas, and capture significant amounts of carbon that can be transported to the deep ocean by oceanic mechanisms; seaweed reaching the deep ocean sequester carbon and prevent it from exchanging with the atmosphere over millennia. Growing seaweed offshore with the purpose of sinking the seaweed in the depths of the sea to sequester carbon has been suggested. In addition, seaweed grows very fast and can theoretically be harvested and processed to generate biomethane, via anaerobic digestion to generate electricity, via cogeneration/CHP or as a replacement for natural gas. One study suggested that if seaweed farms covered 9% of the ocean they could produce enough biomethane to supply Earths equivalent demand for fossil fuel energy, remove 53 gigatonnes of  per year from the atmosphere and sustainably produce 200 kg per year of fish, per person, for 10 billion people. Ideal species for such farming and conversion include Laminaria digitata, Fucus serratus and Saccharina latissima'.
  Both macroalgae and microalgae are being investigated as possible means of carbon sequestration. Marine phytoplankton perform half of the global photosynthetic CO fixation (net global primary production of ~50 Pg C per year) and half of the oxygen production despite amounting to only ~1% of global plant biomass.
  Because algae lack the complex lignin associated with terrestrial plants, the carbon in algae is released into the atmosphere more rapidly than carbon captured on land. Algae have been proposed as a short-term storage pool of carbon that can be used as a feedstock for the production of various biogenic fuels.
  Large-scale seaweed farming (called "ocean afforestation") could sequester huge amounts of carbon. Wild seaweed will sequester large amount of carbon through dissolved particles of organic matter being transported to deep ocean seafloors where it will become buried and remain for long periods of time. Currently seaweed farming is carried out to provide food, medicine and biofuel. In respect to carbon farming, the potential growth of seaweed for carbon farming would see the harvested seaweed transported to the deep ocean for long-term burial. Seaweed farming has gathered attention given the limited terrestrial space available for carbon farming practices. Currently seaweed farming occurs mostly in the Asian Pacific coastal areas where it has been a rapidly increasing market. The IPCC Special Report on the Ocean and Cryosphere in a Changing Climate recommends "further research attention" on seaweed farming as a mitigation tactic. 
  However, seaweed farming, and carbon farming in general,  only keeps the carbon within the fast carbon cycle, in intimate contact with the ocean and atmosphere, and once in equilibrium with the ecology, cannot be expected to hold additional carbon.