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

The C-13 signature of forty-five macroalgal species from intertidal zones at Cadiz 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 partial derivative C-13 values ranged from -6.8 parts per thousand to -33 parts per thousand, although the span of variation was different depending on the taxa. Thus, PC for Chlorophyta varied from -7 parts per thousand (Codium adhaerens) to -29.6 parts per thousand (Flabellia petiolata), while all the Phaeophyceae (excepting Padina pavonica with partial derivative C-13 higher than -10 parts per thousand.) had values between -10 parts per thousand, and -20 parts per thousand, The widest variation range was recorded in Rhodophyta, from values above -10 parts per thousand (Liagora viscida) to values lower than -30 parts per thousand obtained in three species belonging to the subclass Rhodymeniophycidae. Accordingly, the mean partial derivative C-13 value calculated for red algae (-20.2 parts per thousand.) was significantly lower than that for brown (-15.9 parts per thousand) and green algae (-15.6 parts per thousand). 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 partial derivative C-13 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 HCO(3) over bar and/or CO2) and the partial derivative C-13 values reveals that values more negative than -30 parts per thousand 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 ((ext)CA) and partial derivative C-13 values were negatively correlated indicating that the higher the dependence of the photosynthesis on the CO2 supplied from HCOT via extCA, the lower the partial derivative C-13 in the algal material. The comparison between the C-partial derivative 13 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 Cadiz 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. (C) 2009 Elsevier Ltd. All rights reserved.