Fucoid brown algae inject fucoidan carbon into the ocean

Abstract

Brown algae annually convert gigatons of carbon dioxide into carbohydrates, including the complex extracellular matrix polysaccharide fucoidan. Due to its persistence in the environment, fucoidan is potentially a pathway for marine carbon sequestration. Rates of fucoidan secretion by brown algae remain unknown due to the challenge of identifying and quantifying complex polysaccharides in seawater. We adapted the techniques of anion exchange chromatography, enzyme-linked immunosorbent assay, and biocatalytic enzyme-based assay for detection and quantification of fucoidan. We found the brown alga Fucus vesiculosus at the Baltic Sea coast of south-west Finland to secrete 0.3% of their biomass as fucoidan per day. Dissolved fucoidan concentrations in seawater adjacent to algae reached up to 0.48 mg L −1 . Fucoidan accumulated during incubations of F. vesiculosus , significantly more in light than in darkness. Maximum estimation by acid hydrolysis indicated fucoidan secretion at a rate of 28 to 40 mg C kg −1 h −1 , accounting for 44 to 50% of all exuded dissolved organic carbon. Composed only of carbon, oxygen, hydrogen, and sulfur, fucoidan secretion does not consume nutrients enabling carbon sequestration independent of algal growth. Extrapolated over a year, the algae sequester more carbon into secreted fucoidan than their biomass. The global utility of fucoidan secretion is an alternative pathway for carbon dioxide removal by brown algae without the need to harvest or bury algal biomass.

Structural and functional characterization of the novel endo-α(1,4)-fucoidanase Mef1 from the marine bacterium Muricauda eckloniae

“…Indeed, brown macroalgae polysaccharide biomass is considered to be a likely CO 2 carbon sink. Recent data on fucoidan exudate of F. vesiculosus, which appears to form an external mucilage barrier layer on this brown seaweed species, strongly suggest that this secreted fucoidan sequesters a significant amount of carbon, and thus that such extracellular fucoidan may contribute significantly to removal of CO 2 from the atmosphere (Buck-Wiese et al, 2023).…”

Carbon sequestration and climate change mitigation using macroalgae: a state of knowledge review

“…Finally, the chemical composition of DOC itself also affects its lability, and this composition is not well understood for most macroalgae. Macroalgae DOC contains high quantities of phenolic and humic‐like compounds, which are typically more recalcitrant than other molecules (Wada et al ., 2008; Buck‐Wiese et al ., 2022; Feng et al ., 2022).…”

“…Decay during transport may be slower in cold waters (Filbee‐Dexter et al ., 2022 a ), while most carbon appears to be respired in warmer tropical regions (Haas et al ., 2013). The presence of refractory molecules such phlorotannins, fucans, xylans, long‐chain lipids, sulfated polysaccharides, aromatic and lignin‐like compounds can delay degradation times (Wada et al ., 2007; Trevathan‐Tackett et al ., 2015; Buck‐Wiese et al ., 2022; Feng et al ., 2022).…”

“…Advances in numerous fingerprinting techniques offer promising prospects in that regard. For example, eDNA analysis can be used to verify the presence of macroalgae carbon in sediments or deep‐water samples (Ortega et al ., 2019; Frigstad et al ., 2021; Ørberg et al ., 2022), while the increasingly widespread use of Fourier‐transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) or antibody‐based techniques can enable the detection of specific recalcitrant molecular species derived from macroalgae (Buck‐Wiese et al ., 2022; Feng et al ., 2022; Li et al ., 2022). Ultimately, verifying sequestration of macroalgae carbon will require dating of sediments or water masses, whose sampling and analysis is often costly.…”

Phosphate starvation stops bacteria digesting algal fucan that sequesters carbon

“… AbstractAlgae provide a solar powered pathway to capture and sequester carbon by injecting stable fucan made from carbon dioxide into the ocean 1–4 . Stability of the pathway is at odds with the presence of marine bacteria with genes of enzymes that can digest fucan and release the carbon dioxide 5 .…”