Natural high pCO₂ increases autotrophy in Anemonia viridis (Anthozoa) as revealed from stable isotope (C, N) analysis

Contemporary cnidarian-algae symbioses are challenged by increasing CO₂ concentrations (ocean warming and acidification) affecting organisms' biological performance. We examined the natural variability of carbon and nitrogen isotopes in the symbiotic sea anemone Anemonia viridis to investigate dietary shifts (autotrophy/heterotrophy) along a natural pCO₂ gradient at the island of Vulcano, Italy. δ¹³C values for both algal symbionts (Symbiodinium) and host tissue of A. viridis became significantly lighter with increasing seawater pCO₂. Together with a decrease in the difference between δ¹³C values of both fractions at the higher pCO₂ sites, these results indicate there is a greater net autotrophic input to the A. viridis carbon budget under high pCO₂ conditions. δ¹⁵N values and C/N ratios did not change in Symbiodinium and host tissue along the pCO₂ gradient. Additional physiological parameters revealed anemone protein and Symbiodinium chlorophyll a remained unaltered among sites. Symbiodinium density was similar among sites yet their mitotic index increased in anemones under elevated pCO₂. Overall, our findings show that A. viridis is characterized by a higher autotrophic/heterotrophic ratio as pCO₂ increases. The unique trophic flexibility of this species may give it a competitive advantage and enable its potential acclimation and ecological success in the future under increased ocean acidification.