Comparison of marine macrophytes for their contributions to blue carbon sequestration

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 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.

Carbon dioxide mitigation potential of seaweed aquaculture beds (SABs)

“…They also have higher proportions of recalcitrant carbon in their tissues that are not easily broken down (Gao and McKinley 1994;Delille et al 2009;Trevathan-Tackett et al 2015). Seaweeds can transform DIC via photosynthesis, thereby decreasing the pCO 2 in seawater.…”

“…By removing a significant amount of carbon from the ocean at harvest time (Tang et al 2011), these life forms provide potential tools for biomass production as well as CO 2 sequestration (Duarte et al 2005). In addition, seaweeds acting as CO 2 sinks can sequester carbon within their biomass throughout their life spans (Chung et al 2013) and beyond (Delille et al 2009;Trevathan-Tackett et al 2015).…”

“…Seaweeds and SABs can potentially make effective contributions to CO 2 mitigation because some seaweeds have cell wall structures and composition that can store carbon over the long-term by becoming a carbon donor to other ecosystems (Hill et al 2015;Trevathan-Tackett et al 2015) and by converting the biomass into a range of bioenergy products from biogas to liquid and solid biofuels. We discuss these roles in more detail below.…”

“…turnover time in the Sargasso Sea is 10 to 100 years (Ramus 1992). A more recent study has found that not all seaweeds have short turnover times and some show potential for long-term carbon sequestration because they contain compounds that are very recalcitrant and are likely to break down slowly in sediments (Trevathan-Tackett et al 2015).…”

“…Some material though can be buried in sediments or transported to the deep ocean where, even if remineralization occurs, the resulting DIC can be retained in deep oceanic waters for hundreds of years (Harrold et al 1998;Dierssen et al 2009;Trevathan-Tackett et al 2015). Alternatively, if macroalgal biomass is used as a substitute for fossil fuels, this could potentially mitigate the rate of global climate change by reducing our reliance on the latter (Chung et al 2011;N'Yeurt et al 2012).…”