Carbon isotopic fractionation in macroalgae from Cádiz Bay (Southern Spain): Comparison with other bio-geographic regions

The ¹³C signature of forty-five macroalgal species from intertidal zones at Cádiz Bay was analysed in order to research the extension of diffusive vs. non-diffusive utilisation of dissolved inorganic carbon (DIC) and to perform a comparison with data published for other bio-geographic regions. The ∂¹³C values ranged from −6.8‰ to −33‰, although the span of variation was different depending on the taxa. Thus, ∂¹³C for Chlorophyta varied from −7‰ (Codium adhaerens) to −29.6‰ (Flabellia petiolata), while all the Phaeophyceae (excepting Padina pavonica with ∂¹³C higher than −10‰) had values between −10‰, and −20‰. The widest variation range was recorded in Rhodophyta, from values above −10‰ (Liagora viscida) to values lower than −30‰ obtained in three species belonging to the subclass Rhodymeniophycidae. Accordingly, the mean ∂¹³C value calculated for red algae (−20.2‰) was significantly lower than that for brown (−15.9‰) and green algae (−15.6‰). Most of the analysed red algae were species inhabiting crevices and the low intertidal fringe which explains that, on average, the shaded-habitat species had a ∂¹³C value lower than those growing fully exposed to sun (i.e. in rockpools or at the upper intertidal zone). The comparison between the capacity for non-diffusive use of DIC (i.e. active or facilitated transport of HCO₃⁻ and/or CO₂) and the ∂¹³C values reveals that values more negative than −30‰ indicate that photosynthesis is dependent on CO₂ diffusive entry, whereas values above this threshold would not indicate necessary the operation of a non-diffusive DIC transport mechanism. Furthermore, external carbonic anhydrase activity (_extCA) and ∂¹³C values were negatively correlated indicating that the higher the dependence of the photosynthesis on the CO₂ supplied from HCO₃⁻ via _extCA, the lower the ∂¹³C in the algal material. The comparison between the ∂¹³C values obtained for the analysed species and those published for species inhabiting other bio-geographic areas (warm-temperate, cold and polar) suggests that globally (at least for the red and brown algae) the non-diffusive entry of DIC is more widely spread among the species from Cádiz Bay than among those of polar regions. If it is assumed that non-diffusive use of DIC implies saturation of photosynthesis at the present-day CO₂ concentration in seawater, our data indicate that the potential impact of the acidification on photosynthesis in the seaweed communities will be different depending on the latitude.