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.
Effects of sodium bicarbonate as an inorganic carbon source on the growth of scenedesmus dimorphus
