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

Background/Question/Methods

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.

Results/Conclusions

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.