Questions raised by sinking seaweed: A Life Cycle Assessment of Macroalgae Cultivation and Sinking for Carbon Dioxide Removal

Abstract Purpose: Among the portfolio of emerging marine carbon dioxide removal approaches, macroalgae cultivation and sinking (MCS) has attracted growing scientific, commercial, and policy interest; yet its environmental implications remain poorly understood. This study presents the first comprehensive cradle-to-grave life cycle assessment of the ‘grow nearshore, sink offshore’ MCS strategy with Saccharina latissima in Northern Europe. Beyond climate mitigation potential, it identifies environmental trade-offs and hotspots across three areas of protection, and evaluates key parameters influencing environmental performance. Methods: An attributional cradle-to-grave LCA was conducted, covering the complete MCS product system from onshore hatchery operations and nearshore cultivation to offshore transportation and deep-sea deposition. A functional unit of one tonne of atmospheric carbon dioxide (CO 2 ) stored for > 100 years via the intentional deep-sea deposition of cultivated macroalgae below the deep-ocean carbon sequestration horizon (> 1 000 m) was adopted. Using ReCiPe 2016, environmental impacts were assessed across climate change, ecosystem quality, human health, and natural resource scarcity, accounting for carbon (C) and nutrient uptake and leakage. Contribution analyses were performed to identify environmental hotspots, while the influence of seven key parameters (biomass yield, transportation distance, C/nutrient leakage, line seeding method, longline material, future background conditions, and LCIA method family) were examined via sensitivity analyses. Results and discussion: Under the modelled conditions, MCS achieves a net climate benefit of 227 kg CO 2 -eq/t of CO 2 stored. This is, however, accompanied by net adverse impacts on ecosystem quality (2.03 x 10 − 6 species.yr), human health (1.40 x 10 − 3 DALYs), and natural resource scarcity (83.9 USD 2013). Offshore transportation, C/nutrient leakage, cultivation infrastructure, and hatchery electricity consumption emerge as the main environmental hotspots. Climate change impacts are the most susceptible to sensitivity, with the largest effects caused by biomass yield and sequestration durability variability, with a 50% reduction in yield or increase in C/nutrient leakage, shifting MCS from net carbon removal technology to net-emitter. Alternative longline materials, future background conditions, and LCIA method families also each substantially affect the environmental outcomes. Conclusions: Although MCS could potentially achieve net carbon removal under the assumed conditions, sequestration efficiency is limited (23%), and its broader environmental trade-offs and remaining uncertainties raise concerns regarding large-scale deployment. Improving biomass yields, reducing transportation, optimising cultivation and deposition infrastructure, and adopting energy-efficient, renewably powered hatchery operations could markedly enhance environmental performance. Nevertheless, further research is essential to quantify local post-deposition C/nutrient pathways and develop marine-specific impact assessment methods before large-scale implementation can be justified.