Marine algae as a co₂ sink

Abstract

The most effective ways to reduce CO₂ emissions are to improve the energy efficiency of each economic sector and to reduce the cutting of tropical and temperate forests around the world. These options, however, may not fully reach their technical and economic potential due to various political and socioeconomic barriers. Other more innovative and less well developed mitigation measures therefore will be required. The most practical of these is to increase CO₂ sinks through photosynthesis in both standing tree biomass and in ocean primary producers. In this paper, the use of marine algae as CO₂ sinks is reviewed from a technical, engineering/economic, and environmental perspective. Two open ocean options are considered for large-scale CO₂ mitigation: the use of phytoplankton through Fe fertilization and macroalgal (kelp) farms, which can be used for both C sequestering and energy production. It has been estimated that these two approaches can sequester from 0.7 to 3 Gt C yr⁻¹ from the atmosphere at an estimated cost of $5 to 300 t⁻¹ C yr⁻¹. Other options currently under study are also mentioned. Numerous questions remain to be answered pertaining to the use of both microalgae and macroalgae for CO₂ assimilation before credible estimates of costs of C removal can be made for either system. In addition, there are several key environmental issues raised by the use of algae. A detailed discussion of these variables, including cost estimates, is presented.