Latitudinal trends in stable isotope signatures and carbon-concentrating mechanisms of northeast Atlantic rhodoliths

Abstract

Abstract. 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 carbon-concentrating mechanisms (CCMs) of rhodoliths along a latitudinal gradient in the northeast Atlantic using natural stable isotope signatures. The δ13C signature of macroalgae can be used to provide an indication of the preferred inorganic carbon source (CO2 vs. HCO3-). Here we present the total (δ13CT) and organic (δ13Corg) δ13C signatures of northeast Atlantic rhodoliths with respect to changing environmental conditions along a latitudinal gradient from the Canary Islands to Spitsbergen. The δ13CT signatures (−11.9 to −0.89) of rhodoliths analyzed in this study were generally higher than the δ13Corg signatures, which ranged from −25.7 to −2.8. We observed a decreasing trend in δ13CT signatures with increasing latitude and temperature, while δ13Corg signatures were only significantly correlated to dissolved inorganic carbon. These data suggest that high-latitude rhodoliths rely more on CO2 as an inorganic carbon source, while low-latitude rhodoliths likely take up HCO3- directly, but none of our specimens had ∂13Corg signatures less than −30, suggesting that none of them relied solely on diffusive CO2 uptake. However, depth also has a significant effect on both skeletal and organic δ13C 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 CCMs 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.