Effects of Elevated CO 2 Concentrations on 13 C Fractionation during Photosynthesis, Post-Photosynthesis and Night Respiration in Mangrove Saplings Avicennia marina and Rhizophora stylosa

Carbon fractionation (Δ 13 C) is well documented for various plants functional types. Yet, specific studies on Δ 13 C on mangroves are particularly rare although they have a key role in coastal carbon (C) cycling. In this study, we investigated the 13 C exchanges between leaves and the atmosphere and between the main plant’s organs in two common mangroves species, Avicennia marina and Rhizophora stylosa subjected to two different CO 2 concentrations. Two-years-old saplings were grown in mesocosms during one year under 400 ppm and 800 ppm of CO 2 . At the end of the experiment, the isotopic value of the night-respired CO 2 was measured on six individuals for each species and CO 2 treatment. Then, 60 saplings were harvested to measure the organs δ 13 C values, and, finally, carbon fractionation (Δ 13 C) during photosynthesis, post-photosynthesis and apparent Δ 13 C during night respiration were calculated. Results indicated that elevated CO 2 reduced Δ 13 C during photosynthesis by 13% and during night respiration by 20%. Alongside, within-plant Δ 13 C was twice higher in the saplings grown under elevated CO 2 concentrations. These results showed that ongoing and future increases in atmospheric CO 2 concentrations have the potential to modify the δ 13 C values of mangrove trees. These results could have important implications in Blue Carbon sciences, and particularly in the comprehension of future carbon cycling in coastal wetlands, mangroves being an essential link in terrestrial and marine food webs along tropical and subtropical coastlines.