Blue carbon sequestration: An unintended driving force for marine pollution

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Global climate change driven by anthropogenic CO₂ emissions has intensified efforts to achieve carbon neutrality by 2050. Blue carbon ecosystems, including mangroves, seagrasses, tidal flats, and offshore systems, collectively serve as critical sinks that store 26% of global CO₂ emissions from 2014 to 2023.¹ In recognition of their potential in mitigating climate impacts, blue carbon sequestration has been deemed a critical natural solution to achieve negative emissions and carbon neutrality by scientists and policymakers. To this end, a new UN ocean decade program on marine negative emission technologies (NETs) has been proposed: the Global Ocean Negative Carbon Emissions Program (ONCE, https://www.global-once.org),² which encompasses ocean alkalization, fertilization, coastal wetland restoration, and sustainable mariculture. Although most of these activities remain conceptual, model-based, or pilot studies, these approaches are expected to become nonnegligible anthropogenic intervention forces in marine biogeochemical cycles.

Synergistic pollutant biogeochemical cycling with marine NETs

Considering that most marine pollutants can be considered anthropogenic organic carbon, carbon sequestration processes in marine NETs show a high degree of overlap with multiphase transport and the biogeochemical cycling and biological responses of marine pollutants (Figure 1).

Figure 1.

Synergistic effects of NETs on marine pollution and pathways toward dual-win carbon and pollution management

Ocean fertilization (OF) technology, typically implemented in the open ocean (offshore), aims to accelerate phytoplankton growth (atmospheric carbon fixation) and biological carbon pumps (BCPs) (vertical carbon transport to the deep sea) through the addition of micronutrients such as iron, nitrogen, or phosphorus. The BCP sequesters atmospheric CO₂ and releases particulate organic carbon (POC) as well as dissolved organic carbon (DOC) into seawater, where the microbial carbon pump involved can transform DOC into recalcitrant DOC (RDOC). Notably, POC and RDOC, which include structural polymers such as polysaccharides, humic substances, and carboxyl-rich alicyclic molecules, have a strong ability to bind organic pollutants and metal ions. Pollutants absorbed or aggregated by POC and RDOC can remain in the water column for long periods of time and thereby be simultaneously sequestered from surface water, which is referred to as the “pollutant biological pump.”³ Therefore, the increased anti-biodegradable forms of POC and RDOC in OF deployment not only serve as a significant long-term carbon sequestration mechanism but also strongly enhance the air-to-water flux and vertical transport of pollutants.

Ocean alkalinity enhancement (OAE), another offshore intervention technology, aims to store atmospheric CO₂ in the form of carbonate and bicarbonate in the ocean via the addition of alkaline minerals. OAEs can modify the physicochemical conditions of seawater, thereby affecting the speciation and bioavailability of marine pollutants. For example, calcite carbonate can serve as an effective ballast to sink low-density plastic debris to ocean sediments.⁴ In addition, the major minerals used in OAEs, such as limestone, olivine, brucite, forsterite, or calcite, have very high metal contents (e.g., Ca, Mg, Fe, Ni, Cr, and other heavy metals). In the OAE, the annual mining, grinding, distribution, and deployment of gigatons of rock are accompanied by an additional and effective land-to-sea transport route for metal ions.

Coastal wetlands, including salt marshes, mangroves, and seagrass beds, can achieve exceptional carbon sequestration through three synergistic mechanisms: (1) high-productivity vegetation captures atmospheric CO₂, (2) tidal pumping facilitates rapid sediment accretion, and (3) sulfidic conditions inhibit methanogenesis. Coastal wetlands not only facilitate carbon capture through vertical burial and tidal-induced lateral transport but also provide natural processes for pollutant removal, such as biodegradation, photodegradation, phytoremediation, and sedimentation.

Sustainable mariculture relies primarily on algae cultivation and three-dimensional aquaculture of benthic shellfish to increase carbon sequestration. This approach utilizes photosynthesis to convert dissolved CO₂ into recalcitrant organic carbon, thereby increasing the seawater pH and thus promoting further atmospheric CO₂ absorption in aquaculture areas. However, the aquaculture equipment used in such systems (e.g., fishing rafts) may introduce plastic-related pollutants into coastal areas.⁵ Artificial upwelling to transport nutrients from the bottom to the upper water layer in aquaculture areas could also increase the immobilization, accumulation, and degradation of benthic pollutants. Moreover, the “deep-ocean dumping” of harvested seaweed and shells as a “carbon sequestration” approach by various commercial and research initiatives (e.g., OceanNETs [https://www.ocean-nets.eu] and Undaria [https://bluecarbon.co.nz]) could lead to direct pollutant input into deep-sea environments, which is highly controversial.

Unintended consequences and knowledge gaps of NETs in the context of marine pollution

In the context of the marine environment as a global reservoir and final sink of pollutants, NETs inevitably have wide-ranging unintended consequences for marine pollution patterns and ecosystem impacts (Figure 1).

NETs can provide multiple benefits in mitigating marine pollution. The large-scale protection and restoration of coastal wetlands can strengthen both the stabilization of land-based pollutants and the trapping of pollutants experiencing lateral advection via tidal and oceanic currents. Enhanced air-to-water and water-to-sediment fluxes of marine pollutants by artificial upwelling and ocean enhancement technologies may slow the long-range transport of pollutants in both the atmosphere and seawater.³ However, critical gaps remain in elucidating the mechanisms governing pollutant input, transport, and fate under climate intervention scenarios, as well as in quantifying the co-benefits of NETs in mitigating global marine pollution.

On the other hand, NETs mechanistically alter the occurrence of pollutants in seawater and increase pollutant storage in deep-sea environments. Owing to the hydrophobicity of most organic pollutants, NET-introduced carriers such as RDOC, POC, and phytoplankton can cause more particulate-adsorbed pollutants (rather than in the dissolved phase) and prolonged retention times in seawater. Consequently, the modified presence pattern of pollutants allows for a greater effective pollutant dose to marine organisms, leading to elevated bioaccumulation and ultimately greater toxic effects on marine ecosystems. Another major concern is that ocean enhancement and deep-ocean dumping processes introduce additional pollutants into the deep-sea environment, resulting in potential pollutant hotspots.⁵ Once the physical and chemical conditions in the deep-sea change, the degradation of refractory organic matter can trigger the release of pollutants into the marine environment, possibly causing detrimental effects on deep-sea ecosystems. Key research priorities include understanding the long-term ecological consequences for NETs and assessing the formation and stability of deep-sea pollutant hotspots.

Furthermore, NETs may have other indirect impacts on marine pollution and its ecological effects. The goal of marine NETs to increase organic carbon sequestration is incompatible with pollutant degradation. The partitioning and bonding of pollutants in POC (or RDOC) decreases their metabolic, oxygenation, and remineralization rates in the water column. In addition, the massive deposition or transfer of POC and biological debris into the deep sea can alter the chemical properties of water and sediments, which may promote oxygen consumption and the release of additional hydrogen sulfide and methane.⁵ This likely influences microbially driven pollutant transformation in sediments because their reduction competes with pollutants for electron acceptors. Marine NETs can also restructure pelagic, mesopelagic, and benthic food chains and reorganize ecosystems, including nutrient partitioning, population assemblages, and community interactions, thereby indirectly altering the ultimate ecological impacts of pollutants. It is also essential to evaluate how the indirect effects of NETs alter the environmental fate, bioavailability, food web transfer, and ecological toxicity of marine pollutants.

Recommendations for a win-win situation involving pollution mitigation and carbon sequestration

The complex interactions of pollutant cycling and carbon sequestration contribute to uncertainties in the prediction of the environmental outcomes of NETs. To address such limitations, future research should focus on four key areas to enable informed decision-making (Figure 1).

• •Deepening mechanistic research: interdisciplinary field studies and systematic observations are necessary to elucidate the complex interactions between carbon sequestration processes and pollutant dynamics among different spheres of the marine system. This includes investigating the co-transport pathways and source-sink relationships of pollutants and organic carbon, as well as the underlying microbial, physicochemical, and hydrodynamic mechanisms.
• •Empirical evaluations: during the implementation of marine NETs, it is essential to assess and validate pollutant cycling outcomes through multiscale empirical studies, including simple laboratory simulations, mesocosm experiments, pilot studies, and demonstration zone observations, to comprehensively evaluate the ecological impacts of disturbed marine pollution.
• •Development of predictive models: advanced modeling tools should be developed to predict the long-term impacts of marine NETs on pollutant distributions and ecological risks. Ideally, such models should incorporate a more robust description of the fate and ecological effects of marine pollutants in remote and deep-sea areas.
• •Integrating ecological impacts into NET evaluation frameworks: an integrated framework is essential for obtaining a holistic understanding of NETs’ overall consequences. This requires establishing assessment systems that grant marine pollution and ecological effects parity with carbon sequestration potential, socioeconomic impacts, and regulatory feasibility, thereby enabling informed decision-making.

Marine NETs offer a promising pathway toward carbon neutrality, but their implementation must be carefully managed to consider unintended impacts on pollutant behavior and ecological effects. There is an urgent need for sustained multidisciplinary collaboration and technological innovation. A critical priority is to establish an integrated research paradigm, assessment framework, and implementation plan that balances carbon sequestration benefits with marine pollution risk, ensuring that marine NETs are deployed in a win-win and environmentally sustainable manner.

Funding and acknowledgments

The authors acknowledge support from the National Natural Science Foundation of China (grant nos. 42325603, 42449303, and 42494884) and the Guangdong Basic and Applied Basic Research Foundation of China (grant no. 2024B1515020074). The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.

Declaration of interests

The authors declare no competing interests.

Published Online: January 8, 2026