Algae form prolific coastal habitats and contribute the largest carbon dioxide (CO₂) flux globally. Most of the algal biomass is transported to the coastal ocean as particulate organic matter (OM), of which the contribution of carbon occurs mainly in depositional environments. Identifying the algal contribution alongside other major sources of organic carbon (OC) is crucial for determining allochthonous and autochthonous contributions in blue carbon habitats. In this study, we identified the algal contribution to OC storage in the seagrass meadow of Tanjung Adang Shoal (Johor, Malaysia) using dual-stable isotope measurements (δ¹³C and δ¹⁵N) and environmental DNA (eDNA) analysis. Stable isotope analysis showed that mangrove plants are the largest contributor to organic carbon pool in the sediments, followed by seston, macroalgae, epiphytes and seagrasses. The combined proportions of sestonic, macroalgal and epiphytic inputs as algal-derived organic matter had contributed a cumulative of 41.4–55.4% of organic matter in the bulk sediment. The eDNA method was used to examine bulk sediments and determine the types of algae present in the seagrass meadow at the genus/family level. Results showed that the main macroalgae present were from member of Coralinalles, Cladophoraceae and Ulvaceae, while diatoms were the predominant microalgae found in the sediment. These findings shed light on the importance of algae in carbon sequestration in the seagrass ecosystem. Understanding the role of algae in carbon storage and their interactions with sediment is crucial in the face of global and local environmental changes.

藻类形成了丰富的沿海栖息地，并贡献了全球最大的二氧化碳 (CO ₂ ) 通量。大多数藻类生物量以颗粒有机物（OM）的形式运输到沿海海洋，其中碳的贡献主要发生在沉积环境中。确定藻类的贡献以及其他主要有机碳 (OC) 来源对于确定蓝碳栖息地的外来和本地贡献至关重要。在这项研究中，我们使用双稳定同位素测量（δ ¹³ C 和 δ ¹⁵N) 和环境 DNA (eDNA) 分析。稳定同位素分析表明，红树林植物是沉积物有机碳库的最大贡献者，其次是水藻、大型藻类、附生植物和海草。作为藻源性有机质的水生、大型藻类和附生输入的组合比例累计贡献了主体沉积物中有机质的 41.4-55.4%。eDNA 方法用于检查大量沉积物并确定海草草甸中存在的属/科水平的藻类类型。结果表明，存在的主要大型藻类来自珊瑚目、刚毛藻科和石莼科。，而硅藻是沉积物中发现的主要微藻。这些发现揭示了藻类在海草生态系统碳封存中的重要性。面对全球和当地的环境变化，了解藻类在碳储存中的作用及其与沉积物的相互作用至关重要。