Iron limitation may thwart the carbon dioxide removal potential of macroalgae cultivation

Carbon dioxide removal (CDR) is a critical component of climate change mitigation strategies, and ocean afforestation via macroalgae cultivation has been touted as a promising CDR approach due to its high productivity and favourable carbon-to-nutrient ratio. However, global CDR models largely overlook iron limitation, a potential bottleneck for sustainable macroalgae cultivation. Iron is an essential micronutrient for macroalgal growth and metabolism and also limits phytoplankton production in many ocean regions. Large-scale macroalgae cultivation could intensify nutrient competition, affecting both macroalgal CDR potential and broader marine ecosystems. Additionally, nutrient stoichiometry and uptake affinities influence whether macroalgae can outcompete phytoplankton in carbon fixation, determining the benefit of replacing phytoplankton production with macroalgae. However, these dynamics are often neglected in global macroalgal cultivation models, leaving significant gaps in our understanding of macroalgae’s CDR potential.

Our study is the first to assess the combined effects of nutrient demand, uptake affinities, and iron limitation on macroalgal CDR efficiency. Using an ocean biogeochemical model, we simulated 25 years of macroalgae cultivation followed by 50 years of cessation in the Exclusive Economic Zones (EEZs) of the global ocean under a high-mitigation scenario. The impact on macroalgal production potential and CDR efficiency was assessed using an ensemble of simulations that integrate published values of macroalgae stoichiometry and nutrient affinities.

Our results reveal that iron limitation reduces macroalgal CDR potential, decreasing the production potential three-fold after accounting for N and P limitation. CDR efficiency declined by 40% after 25 years of cultivation, with regions previously showing high CDR potential under N and P limitations exhibiting negative CDR when iron limitation is additionally accounted for. In these regions, cultivation reduced ocean carbon uptake rather than enhancement. Iron limitation also amplified ecosystem impacts, reducing phytoplankton production by up to 81% in iron-limited areas such as the Southern Ocean and upwelling zones. This reduction in primary production could weaken the biological pump, diminishing the overall CDR outcome and impacting broader ecosystems. Additionally, variability in macroalgal stoichiometry and nutrient uptake affinities contributed to substantial uncertainty in projections of CDR efficiency, with global CDR efficiency ranging from -43% to +53%.

Our findings underscore the need for iron demand and affinities to be included in projections of ocean afforestation. Failing to account for such nutrient dynamics may result in overestimating the efficacy of macroalgae-based CDR as a mitigation strategy.