Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

Abstract

The conservation, restoration, and improved management of terrestrial forests significantly contributes to mitigate climate change and its impacts, as well as providing numerous co‐benefits. The pressing need to reduce emissions and increase carbon removal from the atmosphere is now also leading to the development of natural climate solutions in the ocean. Interest in the carbon sequestration potential of underwater macroalgal forests is growing rapidly among policy, conservation, and corporate sectors. Yet, our understanding of whether carbon sequestration from macroalgal forests can lead to tangible climate change mitigation remains severely limited, hampering their inclusion in international policy or carbon finance frameworks. Here, we examine the results of over 180 publications to synthesise evidence regarding macroalgal forest carbon sequestration potential. We show that research efforts on macroalgae carbon sequestration are heavily skewed towards particulate organic carbon (POC) pathways (77% of data publications), and that carbon fixation is the most studied flux (55%). Fluxes leading directly to carbon sequestration (e.g. carbon export or burial in marine sediments) remain poorly resolved, likely hindering regional or country‐level assessments of carbon sequestration potential, which are only available from 17 of the 150 countries where macroalgal forests occur. To solve this issue, we present a framework to categorize coastlines according to their carbon sequestration potential. Finally, we review the multiple avenues through which this sequestration can translate into climate change mitigation capacity, which largely depends on whether management interventions can increase carbon removal above a natural baseline or avoid further carbon emissions. We find that conservation, restoration and afforestation interventions on macroalgal forests can potentially lead to carbon removal in the order of 10's of Tg C globally. Although this is lower than current estimates of natural sequestration value of all macroalgal habitats (61–268 Tg C year−1), it suggests that macroalgal forests could add to the total mitigation potential of coastal blue carbon ecosystems, and offer valuable mitigation opportunities in polar and temperate areas where blue carbon mitigation is currently low. Operationalizing that potential will necessitate the development of models that reliably estimate the proportion of production sequestered, improvements in macroalgae carbon fingerprinting techniques, and a rethinking of carbon accounting methodologies. The ocean provides major opportunities to mitigate and adapt to climate change, and the largest coastal vegetated habitat on Earth should not be ignored simply because it does not fit into existing frameworks.

Air‐sea carbon dioxide equilibrium: Will it be possible to use seaweeds for carbon removal offsets?

“…Figure 4 highlights the fates of both the parcel of water bearing the CO 2 deficit and seaweed biomass. There is no doubt that on a global scale, seaweeds form long‐term carbon storage pools in ocean sediments on timescales of 100 years or more, although their contribution is extremely small (<1%, 55 Tg C·y −1 ) compared to >99% from oceanic phytoplankton via the biological pump (5–15 Pg C·y −1 ; Boyd et al., 2019; Filbee‐Dexter et al., in revision; Pessarrodona et al., 2023). For any seaweed system (natural seaweed bed, nearshore or open ocean aquaculture) under consideration for carbon credits/offsets, two steps are required for MRV (Figure 4): (1) The parcel of seawater that bears the CO 2 deficit resulting from seaweed photosynthesis must be tracked over long timescales (weeks to months to over a year) to quantify the amount of atmospheric CO 2 that enters seawater, and (2) the amount of the organic carbon from the same seaweed system that enters long‐term storage (>100 years; as POC, DOC) must be quantified.…”

“…For example, the photochemical breakdown of DOC will likely increase in the optically brighter open oceans compared to turbid coastal waters (Wada et al., 2015). The contribution of seaweed carbon to the DOC pool is unknown, and we have little understanding of its transit time and fate offshore (Paine et al., 2021; Pessarrodona et al., 2023).…”

“…Various strategies are being proposed for CDR. Ocean-based CDR includes ocean alkalinity enhancement (Renforth & Henderson, 2017) and increasing the amount of seaweed-derived carbon storage on a global scale via the expansion of aquaculture, including into the open oceans (ocean afforestation; Ross et al, 2023), by restoring seaweed ecosystems that have been lost due to climate change (Smale et al, 2019;Wernberg et al, 2011) and conserving existing seaweed ecosystems (Pessarrodona et al, 2023).…”

“…They are being considered as a biological method of CDR and for eventual use in carbon trading as carbon removal offsets (Coleman et al, 2022;Cooley et al, 2022;Ross et al, 2023;Vanderklift et al, 2022;Figures S1-S7 in the Supporting Information): Indeed, some companies and NGOs are already selling seaweed carbon as offsets on the voluntary carbon market (Figures S2-S4). Ongoing projects include enhancing the scale of natural seaweed beds by restoring those that have been lost from climate change and local anthropogenic activities (Filbee-Dexter et al, 2022), protecting existing seaweed beds (Pessarrodona et al, 2023), and expanding the scale of seaweed aquaculture, including in the open ocean (N'Yeurt et al, 2012;Froehlich et al, 2019). Numerous start-up companies and NGOs are trialing growing seaweeds, particularly kelps (order Laminariales) but also pelagic Sargassum spp.…”

First mesophotic Ecklonia radiata (Laminariales) records within the iSimangaliso Wetland Park marine-protected area, east coast, South Africa

“…Kelp forests are also recognized as critical ecological habitats that provide several direct and indirect ecosystem services (Smale et al ., 2013; Vásquez et al ., 2014; Filbee-Dexter and Wernberg, 2020). These globally significant, highly productive ecosystems are however under threat from climate change, habitat destruction, and over-fishing (Harley et al ., 2012; Spalding et al ., 2019; Wernberg et al ., 2019a; Buglass et al ., 2022; Coleman et al ., 2022; Pessarrodona et al ., 2023), and require ongoing research to better understand and communicate their socio-ecological value, and to improve their conservation and management.…”