Very little is known about seaweed stable carbon isotope discrimination during respiration (Δ r , defined here as the difference between respired CO 2 δ 13 C and algal tissue δ 13 C). 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 CO 2 release and O 2 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 δ 13 C 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 δ 13 C values to determine the form of inorganic carbon used in photosynthesis.
KEY WORDS:13 C · Fractionation · Macroalgae · Respiration · Respiratory quotient · Short-term incubation
Resale or republication not permitted without written consent of the publisherMar Ecol Prog Ser 437: [41][42][43][44][45][46][47][48][49] 2011 the bulk algal tissue) influence on the overall organism δ 13 C can be important. Aquatic respiration is typically dominated by microbial respiration (Del Giorgio & Williams 2005), including respiration of seaweed detritus. However, respiration by seaweeds comprises the bulk of the respiration in some coastal communities (Middelburg et al. 2005), which means that CO 2 respired by seaweeds may influence the δ 13 C of dissolved inorganic carbon in nearby waters, especially in rocky tidal pools during low tide.In terrestrial plants, there has been a growing recognition that not only Δ p , but also Δ r , can shape plant δ 13 C (Ghashghaie et al. 2003, Pataki 2005, Tcherkez et al. 2010, although this has largely been ignored in aquatic plants. It is un known if the patterns found for Δ r in land plants are also observed in algae; thus, studies addressing Δ r could enhance understanding of the variation of δ 13 C in algae. Investigations in land plants have shown that temperature and light exposure affect Δ r (Tcherkez et al. 2003, Gessler et al. 2009, Werner et al. 2009). An increase in temperature can cause an increased rate of lipid consumption, which is reflected in Δ r (Tcherkez et al. 2003). Light exposure inhibits part of the Krebs cycle, leading to the accumulation of metabolites which are preferentially consumed in the first minutes of darkness,...