Exploring Macroalgal Carbon Dynamics in a Changing Climate of Arctic Fjords

The rapidly warming Arctic, exceeding global averages, experiences heightened macroalgal growth in high Arctic fjords due to rising seawater temperatures and reduced sea ice. However, uncertainties surround the consequences of climate-induced changes in the carbon cycle resulting from extensive macroalgal growth and increased carbon flux dynamics in these fjords. This study examines the fate of macroalgal-derived fatty acids (Saturated Fatty Acid: SFA, Monounsaturated Fatty Acids: MUFA and Polyunsaturated Fatty Acids: PUFA) across Kongsfjorden and Krossfjorden (Ny-Alesund) in response to Arctic amplification. For this study, dominant brown, green, and red macroalgal species (n=20), along with sediment samples (n=18) across the fjords, were collected during summer 2022. Brown algae dominated with the highest average fatty acid concentration 435.72 ±534.14 μg/g, while red and green algae had lower concentrations 72.84 ±52.75 μg/g and 90.25 ± 84.67 μg/g, respectively. Brown algae exhibited a concentration trend of SFA>MUFA>PUFA, while green and red showed SFA>PUFA>MUFA. The primary PUFA in these algae were n-C₁₈ and n-C₂₀, and filamentous growth forms exhibited higher levels compared to thallus or short/dwarf forms in green and red algae. However, brown algae, except for the genus Chorda, did not exhibit clear trends for these compounds. The distinct phylogenetic position of brown algae from red and green algae likely accounts for these divergent patterns. The filamentous form having the highest concentration of fatty acids could result from increased resistance to degradation, attributed to their minimized surface-to-volume ratio. Macroalgal species outside their natural habitat (ex-situ) had higher PUFA, MUFA, and SFA concentrations, likely due to unfavourable conditions of growth in intertidal regions, suggesting enhanced adaptation for growth across the arctic fjords. While in the sediments, a significant (~50%) reduction in the PUFA and MUFA fraction concerning SFA was observed. The transport of the algal material was more towards the outer fjord and was possibly favoured by glacial melting and runoff activities. The decrease in fatty acids derived from algae, coupled with the presence of iso^- and anteiso^- branched-chain fatty acids, implies limited residence and faster turnover of algal matter into intermediate metabolites by microorganisms, possibly bacteria. Such observation suggests a potential release of carbon fluxes into the atmosphere through degradation of lipids, and contributing to a negative trend in the macroalgal-induced carbon storage in fjords.