15The role of macroalgae in Blue Carbon assessments has been controversial, partially due to 16 uncertainties on the fate of exported macroalgae. Available evidence suggests that macroalgae is 17 exported to reach the open ocean and the deep-sea. Nevertheless, this evidence lack of systematic 18 assessment. Here, we provide robust evidence of macroalgal export beyond coastal habitats. We 19 used metagenomes and metabarcodes from the global expeditions Tara Oceans and Malaspina 20 2010 Circumnavigation. We discovered macroalgae worldwide at up to 5,000 km from coastal 21 areas. We found 24 orders, most of them belong to Rhodophyta. Diversity of macroalgae was 22 similar across oceanic regions, although the assemblage composition differed. The South 45 macroalgae are few 10 . This evidence imbalance could be related to lineage-specific features of 46 the macroalgae cell wall composition and differences in cell-degradation rates 11 . Furthermore, 47 most calculations of macroalgal primary production suggest that macroalgal carbon is exported 48 as dissolved and particulate organic carbon (DOC and POC) 12,13 , which are not visually 49detectable. An inclusive method, such as the identification of macroalgal environmental DNA 50 (eDNA), could provide evidence of macroalgal carbon export in the ocean, and may allow the 51 required systematic and consistent assessments. eDNA is the DNA left behind by organisms in 52 the surrounding environment including degraded cell tissues, gametes, animal feces, etc. As 53 DNA comprises approximately 3% of cellular organic carbon 14 , the presence of macroalgal DNA 54 in waters beyond macroalgal habitats is both an indicator of the presence of the species and 55 evidence (not necessarily quantitative) of the export of macroalgal carbon. 56Here, we examined the presence and relative abundance of Rhodophyta, Phaeophyta, and 57Chlorophyta macroalgal eDNA sequences in the ocean. The sequences were derived from 58 hundreds of metagenomes generated by two global expeditions: Tara Oceans 15 and Malaspina 59 2010 Circumnavigation 16 . These expeditions surveyed the global ocean from surface to 4,000 m 60 depth, and sequenced the particulate material present in environmental water samples 17,18 (see 61 Methods). Although the expeditions primarily assessed the microbial and planktonic diversity, 62 they also generated a global DNA resource that allows identification of multicellular eukaryotes. 63We exploited the potential of this eukaryotic eDNA resource to explore the presence of 64 macroalgae in the global ocean. This holistic approach has not been attempted before, but is 65 semi-quantitative and consistent for evaluating the hypothesis that macroalgal material is broadly 66 exported across the global ocean. 67We identified macroalgae using two global ocean datasets. The first one included 163 68 metabarcodes of amplicon 18S rDNA from Tara Oceans 19 . The second one included 417 69 metagenomes pooled from the Tara Oceans 20 and Malaspina 21 expeditions (see Methods). We 70 used two differe...

Blue carbon is the organic carbon in oceanic and coastal ecosystems that is captured on centennial to millennial timescales. Maintaining and increasing blue carbon is an integral component of strategies to mitigate global warming. Marine vegetated ecosystems (especially seagrass meadows, mangrove forests, and tidal marshes) are blue carbon hotspots and their degradation and loss worldwide have reduced organic carbon stocks and increased CO 2 emissions. Carbon markets, and conservation and restoration schemes aimed at enhancing blue carbon sequestration and avoiding greenhouse gas emissions, will be aided by knowing the provenance and fate of blue carbon. We review and critique current methods and the potential of nascent methods to track the provenance and fate of organic carbon, including: bulk isotopes, compound-specific isotopes, biomarkers, molecular properties, and environmental DNA (eDNA). We find that most studies to date have used bulk isotopes to determine provenance, but this approach often cannot distinguish the contribution of different primary producers to organic carbon in depositional marine environments. Based on our assessment, we recommend application of multiple complementary methods. In particular, the use of carbon and nitrogen isotopes of lipids along with eDNA have a great potential to identify the source and quantify the contribution of different primary producers to sedimentary organic carbon in marine ecosystems. Despite the promising potential of these new techniques, further research is needed to validate them. This critical overview can inform future research to help underpin methodologies for the implementation of blue carbon focused climate change mitigation schemes.

Estimation of marine macrophyte contribution to coastal sediments is key to understand carbon sequestration dynamics. Nevertheless, identification of macrophyte carbon is challenging. We propose environmental DNA (eDNA) metabarcoding as a new approach for identification of sediment contributors, and compared this approach against stable isotopes-the traditional approach. eDNA metabarcoding allowed high-resolution identification of 48 macroalgae, seagrasses, and mangroves from coastal habitats. The relative eDNA contributions of macrophytes were similar to their contributions of organic carbon based on stable isotopes; however, isotopes were unreliable for taxonomical discrimination among macrophyte sources. Additionally, we experimentally found that eDNA abundance in the sediment correlates with both the DNA (84%, R 2 = 0.71, p = 0.001) and the organic carbon content (76%, R 2 = 0.58, p = 0.006) per macrophyte lineage. These results demonstrate the unparallel resolution of eDNA as a method for estimation of the organic carbon contribution of marine macrophytes to blue carbon stocks.

Marine macrophytes, especially macroalgae, form highly productive ecosystems, with a global net primary production of about 2 Pg C year −1 spread across 4 million km 2 in coastal areas (Duarte, 2017). Marine macrophytes export most of their net production to the global ocean and seafloor (Duarte & Cebrián, 1996;

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