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

Submerged aquatic vegetation takes up water-column dissolved inorganic carbon (DIC) as a carbon source across its thin cuticle layer. It is expected that marine macro-phytes also use atmospheric CO2 when exposed to air during low tide, although assimilation of atmospheric CO2 has never been quantitatively evaluated. Using the radiocarbon isotopic signatures (Delta C-14) of the seagrass Zostera marina, DIC and particulate organic carbon (POC), we show quantitatively that Z. marina takes up and assimilates atmospheric modern CO2 in a shallow coastal ecosystem. The Delta C-14 values of the seagrass (-40 to -10 parts per thousand) were significantly higher than those of aquatic DIC (-6 to -18 parts per thousand), indicating that the seagrass uses a C-14-rich carbon source (atmospheric CO2, + 17 parts per thousand). A carbon-source mixing model indicated that the seagrass assimilated 0-40% (mean, 17%) of its inorganic carbon as atmospheric CO2. CO2 exchange between the air and the seagrass might be enhanced by the presence of a very thin film of water over the air-exposed leaves during low tide. Our radiocarbon isotope analysis, showing assimilation of atmospheric modern CO2 as an inorganic carbon source, improves our understanding of the role of seagrass meadows in coastal carbon dynamics.