Comparison of marine macrophytes for their contributions to blue carbon sequestration

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... [ view full abstract ]

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 v attachment). Vascular plant tissues generally exhibited greater stability with a greater proportion of mass loss at temperatures >300°C (peak mass loss ~320°C) than macroalgae (peak mass loss between 175 –300 °C), consistent with the lignocellulose matrix of vascular plants. Greater variation in thermogravimetic signatures within and among macroalgal taxa, relative to vascular plants, was also consistent with the diversity of cell wall structure and composition amongst groups. Significant degradation above 600°C for some macroalgae, as well as some below-ground 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 sulphated 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.