Rhodoliths are free-living calcifying red algae that form extensive beds in shallow marine benthic environments (< 250 m), which provide important habitats and nurseries for marine organisms and contribute to carbonate sediment accumulation. There is growing concern that these organisms are sensitive to global climate change, yet little is known about their physiology. Considering their broad distribution along most continental coastlines, their potential sensitivity to global change could have important consequences for the productivity and diversity of benthic coastal environments. The goal of this study was to determine the plasticity of dissolved inorganic carbon (DIC) uptake mechanisms of rhodoliths along a latitudinal gradient in the Northeast (NE) Atlantic using natural stable isotope signatures. The d13C signature of macroalgae can be used to provide an indication of the preferred inorganic carbon source (CO2 vs. HCO3-). Here we present the total (d13CT) and organic (d13Corg) d13C signatures of NE Atlantic rhodoliths with respect to changing environmental conditions along a latitudinal gradient from the Canary Islands to Spitsbergen. The d13CT signatures (-11.9 to -0.89) of rhodoliths analysed in this study were generally higher than the d13Corg signatures, which ranged from -25.7 to -2.8. We observed a decreasing trend in d13CT signatures with increasing latitude and temperature, while d13Corg signatures were only significantly correlated to DIC. These data suggest that high latitude rhodoliths rely solely on CO2 as an inorganic carbon source, while low latitudes rhodoliths likely take up HCO3- directly. However, depth also has a significant effect on both skeletal and organic d13C signatures, suggesting that both local and latitudinal trends influence the plasticity of rhodolith inorganic carbon acquisition and assimilation. Our results show that many species, particularly those at lower latitudes, have carbon concentrating mechanisms that facilitate HCO3- use for photosynthesis. This is an important adaptation for marine macroalgae, because HCO3- is available at higher concentrations than CO2 in seawater, and this becomes even more extreme with increasing temperature. The flexibility of CCMs in northeast Atlantic rhodoliths observed in our study may provide a key physiological mechanism for potential adaptation of rhodoliths to future global climate change.
