COS 127-1
Impacts of geography, taxonomy and functional group on carbon uptake and isotope fractionation patterns in marine macrophytes: A global meta-analysis of 532 species

Thursday, August 13, 2015: 1:30 PM
339, Baltimore Convention Center
Courtney C. Stepien, Committee on Evolutionary Biology, University of Chicago, Chicago, IL
Catherine A. Pfister, Department of Ecology & Evolution, University of Chicago, Chicago, IL

While carbon uptake in terrestrial plants consists primarily of the uptake of atmospheric CO2, aquatic macrophytes operate in a more complex system, where Dissolved Inorganic Carbon (DIC) occurs in multiple forms. Because the majority of DIC in seawater exists in the form of 13C-heavier bicarbonate (HCO3-) rather than relatively 13C-lighter CO2, many marine macrophytes not only passively access CO2 for photosynthesis, but also actively concentrate both CO2 and HCO3- using carbon concentration mechanisms (CCMs), processes that lead to changes in macrophyte carbon isotope fractionation signatures. I previously found that macrophytes with CCMs can also raise seawater pH as high as 10.4 in enclosed mesocosm pH drift assays, in which the maximum pH value reached is termed pH*. I assembled a global dataset of 1753 marine macrophyte δ13C and pH drift assay values from literature and experimental data to assess (i) how macrophyte δ13C varies with the abiotic parameters sea surface temperature, latitude, and habitat, and the organismal traits species taxonomy, calcification status and functional group, and (ii) how tightly species δ13C is related to CCM presence or absence, as determined by a pH drift assay.


Analysis of 532 macrophyte species found that collection site distance from the equator was significantly negatively related to species δ13C, indicating that macrophytes rely more on CO2 compared to bicarbonate further from the equator. Macrophyte δ13C differed among oceanic basins, was higher at sites from the western side of basins, and was higher in intertidal compared to subtidal habitats. Species from phylum Rhodophyta had lower δ13C than all other phyla, and though functional group was related to δ13C, this was largely due to significantly higher δ13C in calcifying species. Multinomial single factor logistic analyses confirmed that as species δ13C increases, so does the probability of species pH* > 9.0, the threshold value for CCM presence, indicating that the comparatively more rapid sampling of macrophyte δ13C may be a more efficient method of assessing CCM presence than a pH drift assay. Macrophyte communities may respond differently to ocean acidification-induced changes in the relative availability of carbon and to changes in sea surface temperature. Because macrophytes vary in carbon uptake traits based on phylum membership, calcifying status and habitat, macrophyte identity is crucial to understanding species interactions with the seawater carbon cycle.