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The role of macroalgae (seaweed) as a global contributor to carbon drawdown within marine sediments - termed 'blue carbon' - remains uncertain and controversial. While studies are needed to validate the potential for macroalgal‑carbon sequestration in marine and coastal sediments, fundamental questions regarding the fate of dislodged macroalgal biomass need to be addressed. Evidence suggests macroalgal biomass may be advected and deposited within other vegetated coastal ecosystems and down to the deep ocean; however, contributions to near-shore sediments within coastal waters remain uncertain. In this study a combination of eDNA metabarcoding and surficial sediment sampling informed by seabed mapping from different physical environments was used to test for the presence of macroalgal carbon in near-shore coastal sediments in south-eastern Australia, and the physical factors influencing patterns of macroalgal transport and deposition. DNA products for a total of 68 macroalgal taxa, representing all major macroalgal groups (Phaeophyceae, Rhodophyta, and Chlorophyta) were successfully detected at 112 near-shore locations. These findings confirm the potential for macroalgal biomass to be exported into near-shore sediments and suggest macroalgal carbon donors could be both speciose and diverse. Modelling suggested that macroalgal transport and deposition, and total organic carbon (TOC), are influenced by complex interactions between several physical environmental factors including water depth, sediment grain size, wave orbital velocity, current speed, current direction, and the extent of the infralittoral zone around depositional areas. Extrapolation of the optimised model was used to predict spatial patterns of macroalgal deposition and TOC across the coastline and to identify potentially important carbon sinks. This study builds on recent studies providing empirical evidence for macroalgal biomass deposits in near-shore sediments, and a framework for predicting the spatial distribution of potential carbon sinks and informing future surveys aimed at determining the potential for long-term macroalgal carbon sequestration in marine sediments., Competing Interests: Declaration of competing interest The authors declare no conflict of interest., (Copyright © 2023. Published by Elsevier B.V.)

Many marine ecosystems have the capacity for long-term storage of organic carbon (C) in what are termed "blue carbon" systems. While blue carbon systems (saltmarsh, mangrove, and seagrass) are efficient at long-term sequestration of organic carbon (C), much of their sequestered C may originate from other (allochthonous) habitats. Macroalgae, due to their high rates of production, fragmentation, and ability to be transported, would also appear to be able to make a significant contribution as C donors to blue C habitats. In order to assess the stability of macroalgal tissues and their likely contribution to long-term pools of C, we applied thermogravimetric analysis (TGA) to 14 taxa of marine macroalgae and coastal vascular plants. We assessed the structural complexity of multiple lineages of plant and tissue types with differing cell wall structures and found that decomposition dynamics varied significantly according to differences in cell wall structure and composition among taxonomic groups and tissue function (photosynthetic vs. attachment). Vascular plant tissues generally exhibited greater stability with a greater proportion of mass loss at temperatures > 300 degrees C (peak mass loss -320 degrees C) than macroalgae (peak mass loss between 175-300 degrees C), consistent with the lignocellulose matrix of vascular plants. Greater variation in thermogravimetric signatures within and among macroalgal taxa, relative to vascular plants, was also consistent with the diversity of cell wall structure and composition among groups. Significant degradation above 600 degrees C for some macroalgae, as well as some belowground seagrass tissues, is likely due to the presence of taxon-specific compounds. The results of this study highlight the importance of the lignocellulose matrix to the stability of vascular plant sources and the potentially significant role of refractory, taxon-specific compounds (carbonates, long-chain lipids, alginates, xylans, and sulfated polysaccharides) from macroalgae and seagrasses for their long-term sedimentary C storage. This study shows that marine macroalgae do contain refractory compounds and thus may be more valuable to long-term carbon sequestration than we previously have considered.

Kelp aquaculture is an emerging industry outside of Asia. To be successful, this industry requires a reliable production of seedstock, the optimisation of which greatly benefits from a detailed physiological understanding of the microscopic life-cycle stages of the cultured species. This study investigated the impact of six zoospore densities (10–278 mm−2) on the subsequent development of Ecklonia radiata gametophytes and sporophytes. The results showed that germination rates and sex ratio were unaffected by initial zoospore density, but there were significant effects on gametophyte size and sporophyte production. After two weeks, female gametophytes were largest at an initial zoospore density of 40 mm−2 while male gametophytes grew largest at densities below 40 mm−2, but after four weeks gametophyte size showed a negative relationship with initial zoospore density. Significantly more sporophytes developed at initial zoospore densities below 40 individuals mm−2 and no sporophytes were observed at the highest density (271 zoospores mm−2). These results clearly show the importance of initial zoospore density in optimising the nursery stage of kelp aquaculture. [ABSTRACT FROM AUTHOR]

[Display omitted] • Biomarkers to identify and quantify carbon contributors to sequestration are needed. • Use of complete sets of fatty acid (FA) profiles are novel to biomarker discovery. • XGBoost models of FA profiles discriminated seaweed carbon from other sources. • High predictive accuracies of this biomarker approach may facilitate quantification. • Quantifying seaweed contributions to blue carbon sequestration may soon be feasible. Differentiating between carbon contributors in marine environments is crucial to gaining a deeper understanding of marine carbon sequestration, and some efforts have been made through the application of various approaches. This study proposed a new approach through the use of fatty acid (FA) profiles of six marine macrophytes within three macroalgal lineages, and three coastal angiosperms (mangrove, saltmarsh, and seagrass). We compiled FA profiles (consisting of 84 individuals and 9 classes/groups of FAs) of 544 Australian coastal macrophyte species identified in published reports. The data were gradually screened into three different datasets (full-84FA, reduced-57FA, and reduced-48FA) for analysis to minimise the effects of imbalanced distributions of data on analysis. XGBoost (eXtreme Gradient Boosting) multiclass classification modelling with hyperparameter tuning was applied to reveal the specific FA signatures of each macrophyte lineage. The XGBoost models run across the three datasets generated high model-performance metrics including precision, recall, F-score, and multiclass-AUC, indicating similar performance between the three models with predictive accuracies of 94%, 85%, and 95%, respectively. At the class level, the three models also demonstrated high performance with precision, recall, and F-score values for each lineage above 0.95, except for Rhodophyta, which ranged from around 0.80 to 0.89. Overall, our findings suggest that the XGBoost classifier can reveal the lineage-specific patterns of FAs (carbon-based molecules) that can be implemented to predict and potentially quantify the carbon contributors to marine sediments, and more specifically, to discern macroalgal carbon contributions from those of other coastal macrophytes. [ABSTRACT FROM AUTHOR]

Large-scale seaweed aquaculture in the ocean is being pursued globally as a solution to many contemporary challenges, including climate change, food security, and ecosystem degradation. However, the required development and transformation of marine systems for farming may have unknown implications for sustainability objectives, such as those outlined in the United Nations Sustainable Development Goals (SDGs). The aim of this paper is to outline the opportunities for, and threats from, seaweed farming in the context of sustainability. We synthesise the perspectives of expert stakeholders from multiple sectors through a series of Australian workshops to catalogue the pathways through which seaweed farming may affect sustainability, giving specific focus to the SDGs. In doing so, this study illustrates that seaweed farming has the potential to influence, to some degree, the majority of SDGs, with both positive and negative influences. Indeed, seaweed farming is most likely to benefit progress towards achieving SDGs 2 (Zero Hunger), 8 (Decent Work and Economic Growth), 9 (Industry, Innovation, and Infrastructure), 12 (Sustainable Production and Consumption), and 15 (Life on Land). But expectations of seaweed farming may also fall short for supporting some goals if appropriate measures are not implemented to mitigate potential impacts, most notably SDG 14 (Life Below Water). We underscore that seaweed farming has the potential to contribute to sustainable development, and that this potential can only be realised with appropriate regulation and mitigation to avoid unwanted negative outcomes. Better identification and management of trade-offs between these potential positive and negative outcomes across sustainability domains, will be critical for realising the full potential of seaweed aquaculture for sustainable development. [ABSTRACT FROM AUTHOR]