Radiocarbon isotopic evidence for assimilation of atmospheric CO2 by the seagrass Zostera marina.

Submerged aquatic vegetation assimilates dissolved inorganic carbon (DIC) in the water column as a carbon source across its thin cuticle layer. However, it is expected that marine macrophytes also use atmospheric CO₂ when exposed to the air during low tide, although assimilation of atmospheric CO₂ has never been quantitatively evaluated. Using the radiocarbon isotopic signatures (β¹⁴C) of the seagrass Zostera marina and DIC, we show quantitatively that Z. marina takes up and assimilates atmospheric modern CO₂ in a shallow coastal ecosystem. The β¹⁴C values of the seagrass (-36 to -8‰) were significantly higher than those of aquatic DIC (-45 to -18‰), indicating that the seagrass uses a ¹⁴C-rich carbon source (atmospheric CO₂, +17‰). A carbon-source mixing model indicated that the seagrass assimilated ~ 46% (mean: 22%) of its inorganic carbon as atmospheric CO₂. CO₂ exchange between the air and the seagrass may be enhanced by the presence of a very thin water film over the air-exposed leaves during low tide. Our radiocarbon isotope analysis, showing assimilation of atmospheric modern CO₂ as an inorganic carbon source, offers better understanding of the role of seagrass meadows in coastal carbon dynamics. [ABSTRACT FROM AUTHOR]