Chemical determination of silica in seagrass leaves reveals two operational silica pools in Zostera marina

Abstract Silicon is a major driver of global primary productivity and CO 2 sequestration, and is a beneficial element for the growth and environmental stress mitigation of many terrestrial and aquatic plants. However, only a few studies have examined the occurrence of silicon in seagrasses, and its function within seagrass ecosystems and the role of seagrasses in silicon cycling remain largely unexplored. This study uses for the first time two methods, the wet-alkaline digestion and the hydrofluoric acid digestion, to quantify silicon content in seagrass leaves using the species Zostera marina and elaborates on the potential role of silicon in seagrass biogeochemistry and ecology, as well as the role of seagrass ecosystems as a silicon reservoir. The results revealed that seagrass leaves contained 0.26% silicon:dry-weight, which is accumulated in two forms of silica: a labile form digested with the alkaline method and a resistant form digested only with acid digestion. These findings support chemical digestions for silicon quantification in seagrass leaves and provide new insights into the impact of seagrasses on the marine silicon cycle. Labile silica will be recycled upon leaf degradation, benefiting siliceous organisms, while refractory silica will contribute to the ecosystem’s buried silica stock and coupled carbon sequestration. In the Bay of Brest (France), the seagrass silicon reservoir was estimated at 0.18 ± 0.07 g Si m⁻ 2 , similar to that of benthic diatoms, underscoring the potential role of seagrasses in silicon biogeochemistry in the land–ocean continuum, where they might act as a buffer for silicon transport to the ocean.