Carbon stable isotope discrimination during respiration in three seaweed species

Very little is known about seaweed stable carbon isotope discrimination during respiration (Δ_r, defined here as the difference between respired CO₂ δ¹³C and algal tissue δ¹³C). However, Δ_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 Δ_r of Ulva sp. (Chlorophyta), Pterocladia capillacea Bornet (Rhodophyta) and Sargassum sp. (Ochrophyta) under 3 different experimental conditions: at 15 and 25°C after strong illumination for 30 to 60 min, and at 25°C after at least 20 h in the dark. We observed a correlation between Δ_r and the respiratory quotient (RQ, defined as the ratio between CO₂ release and O₂ consumption), suggesting changes in the organic substrate used for respiration in the different treatments. Δ_r was positive in most cases, suggesting that carbohydrates were the likely substrate for respiration; however, Δ_r (and the RQ) decreased with an increase in temperature, suggesting increased use of lipids as substrates. Although Δ_r was not large (averaging 3‰), the influence of seaweed respiration on the δ¹³C of dissolved inorganic carbon may need to be taken into account in ecosystems where seaweeds dominate primary production (i.e. eutrophic lagoons). Δ_r may also be important when interpreting seaweed δ¹³C values to determine the form of inorganic carbon used in photosynthesis.