Editing IPCC:AR6/SROCC/Chapter-5 (section)

===== 5.5.1.2.3 Seaweeds (macroalgae) =====

Seaweeds do not directly transfer carbon to marine sediments, unlike the rooted coastal vegetation considered above (Howard et al., 2017 <sup>[[#fn:r1762|1762]]</sup> ). Nevertheless, seaweed detritus can deliver carbon to sedimentary sites (Hill et al., 2015 <sup>[[#fn:r1763|1763]]</sup> ) and may provide a source of refractory dissolved organic (Krause-Jensen and Duarte, 2016 <sup>[[#fn:r1764|1764]]</sup> ). Recent studies indicate that globally important amounts of carbon may be involved in these processes (Krause-Jensen and Duarte, 2016 <sup>[[#fn:r1765|1765]]</sup> ; Krause-Jensen et al., 2018 <sup>[[#fn:r1766|1766]]</sup> ; Smale et al., 2018 <sup>[[#fn:r1767|1767]]</sup> ). There is, however, currently ''low confidence'' that enhancement of natural seaweed production can provide a significant mitigation response, due to large uncertainties relating to sequestration duration and effectiveness. Such considerations relate to transport pathways, the fate of material transported to deeper water, and the timescales of its subsequent return to the atmosphere over decadal to century timescales.

Seaweed aquaculture is inherently more manageable as a mitigation response (N‘Yeurt et al., 2012 <sup>[[#fn:r1768|1768]]</sup> ; Chung et al., 2013 <sup>[[#fn:r1769|1769]]</sup> ; Chung et al., 2017 <sup>[[#fn:r1770|1770]]</sup> ; Duarte et al., 2017 <sup>[[#fn:r1771|1771]]</sup> ). If linked to biofuel or biogas production (N‘Yeurt and Iese, 2014 <sup>[[#fn:r1772|1772]]</sup> ; Moreira and Pires, 2016 <sup>[[#fn:r1773|1773]]</sup> ; Sondak et al., 2017 <sup>[[#fn:r1774|1774]]</sup> ), there would be potential to reduce emissions (as an alternative to fossil fuels); if also linked to carbon capture and storage (Hughes et al., 2012 <sup>[[#fn:r1775|1775]]</sup> ), it may be possible to achieve negative emissions (net CO 2 removal from the atmosphere). Full life cycle analyses are needed to assess the energy efficiency of such approaches, and the viability of scaling them up to climatically-important levels, taking account of associated environmental and socioeconomic implications.

A different mitigation option using seaweeds relates to their use as a dietary supplement for ruminants to suppress methane production. In vitro studies have given promising results (Dubois et al., 2013 <sup>[[#fn:r1776|1776]]</sup> ; Machado et al., 2016 <sup>[[#fn:r1777|1777]]</sup> ; Machado et al., 2018 <sup>[[#fn:r1778|1778]]</sup> ). However, because the potential scale of real-world benefits have yet to be quantified, there is ''low confidence'' in this approach as a mitigation option.

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