Marine macroalgae are an overlooked sink of Si in coastal systems.

Across the marine landscape - from estuaries to the open ocean, biota take up silicon (Si) as monosilicic acid (Si(OH4 )) and deposit it into their tissues as biogenic silica (BSi, SiO2 ). Along the coast, vegetated ecosystems such as salt marshes and mangroves sequester a significant amount of Si in their tissues and likely help regulate the availability of Si in surrounding waters (Carey & Fulweiler, 2014; Elizondo et al., 2021).