Algal contribution to organic carbon sequestration and its signatures in a tropical seagrass meadow

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.