Carbon stable isotope discrimination during respiration in three seaweed species

Very little is known about seaweed stable carbon isotope discrimination during respiration (Delta(r), defined here as the difference between respired CO2 delta C-13 and algal tissue delta C-13). However, Delta(r) can give information on carbon metabolic pathways in seaweeds, and can also be helpful to better understand their role in carbon cycling. Here we measured the Delta(r) of Ulva sp. (Chlorophyta), Pterocladia capillacea Bornet (Rhodophyta) and Sargassum sp. (Ochrophyta) under 3 different experimental conditions: at 15 and 25 degrees C after strong illumination for 30 to 60 min, and at 25 degrees C after at least 20 h in the dark. We observed a correlation between Delta(r) and the respiratory quotient (RQ, defined as the ratio between CO2 release and O-2 consumption), suggesting changes in the organic substrate used for respiration in the different treatments. Delta(r) was positive in most cases, suggesting that carbohydrates were the likely substrate for respiration; however, Delta(r) (and the RQ) decreased with an increase in temperature, suggesting increased use of lipids as substrates. Although Delta(r) was not large (averaging 3 parts per thousand), the influence of seaweed respiration on the delta C-13 of dissolved inorganic carbon may need to be taken into account in ecosystems where seaweeds dominate primary production (i.e. eutrophic lagoons). Delta(r) may also be important when interpreting seaweed delta C-13 values to determine the form of inorganic carbon used in photosynthesis.