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

Rhodoliths are free-living calcifying red algae that form extensive beds in shallow marine benthic environments (inline-formula<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 inline-formulaδ13C signature of macroalgae can be used to provide an indication of the preferred inorganic carbon source (inline-formulaCO2 vs. inline-formula $M4inlinescrollmathmlchem{\mathrm{normal HCO}}_{normal 3}^{-}$ 33pt16ptsvg-formulamathimg2fbeb9b29341401f243f2ced851d27df bg-15-6139-2018-ie00001.svg33pt16ptbg-15-6139-2018-ie00001.png ). Here we present the total (inline-formulaδ13Cinline-formulaT) and organic (inline-formulaδ13Cinline-formulaorg) inline-formulaδ13C signatures of northeast Atlantic rhodoliths with respect to changing environmental conditions along a latitudinal gradient from the Canary Islands to Spitsbergen. The inline-formulaδ13Cinline-formulaT signatures (inline-formula−11.9 to inline-formula−0.89) of rhodoliths analyzed in this study were generally higher than the inline-formulaδ13Cinline-formulaorg signatures, which ranged from inline-formula−25.7 to inline-formula−2.8. We observed a decreasing trend in inline-formulaδ13Cinline-formulaT signatures with increasing latitude and temperature, while inline-formulaδ13Cinline-formulaorg signatures were only significantly correlated to dissolved inorganic carbon. These data suggest that high-latitude rhodoliths rely more on inline-formulaCO2 as an inorganic carbon source, while low-latitude rhodoliths likely take up inline-formula $M23inlinescrollmathmlchem{\mathrm{normal HCO}}_{normal 3}^{-}$ 33pt16ptsvg-formulamathimgbe55f550a18bd6c852191bf0b8069295 bg-15-6139-2018-ie00002.svg33pt16ptbg-15-6139-2018-ie00002.png directly, but none of our specimens had inline-formula∂13Cinline-formulaorg signatures less than inline-formula−30, suggesting that none of them relied solely on diffusive inline-formulaCO2 uptake. However, depth also has a significant effect on both skeletal and organic inline-formulaδ13C 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 inline-formula $M29inlinescrollmathmlchem{\mathrm{normal HCO}}_{normal 3}^{-}$ 33pt16ptsvg-formulamathimg90b8c208ec8627d5a8bbc1330c52c33c bg-15-6139-2018-ie00003.svg33pt16ptbg-15-6139-2018-ie00003.png use for photosynthesis. This is an important adaptation for marine macroalgae, because inline-formula $M30inlinescrollmathmlchem{\mathrm{normal HCO}}_{normal 3}^{-}$ 33pt16ptsvg-formulamathimgc791e0060a1dd4e6f45ee64831eafa59 bg-15-6139-2018-ie00004.svg33pt16ptbg-15-6139-2018-ie00004.png is available at higher concentrations than inline-formulaCO2 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.

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Hofmann, Laurie C. / Heesch, Svenja: Latitudinal trends in stable isotope signatures and carbon-concentrating mechanisms of northeast Atlantic rhodoliths. 2018. Copernicus Publications.

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