Abstract
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.
Highlights
Natural high pCO2 increases autotrophy in Anemonia viridis (Anthozoa) as revealed from stable isotope (C, N)
We examined the natural variability of carbon and nitrogen isotopes in the symbiotic sea anemone Anemonia viridis to investigate dietary shifts along a natural pCO2 gradient at the island of Vulcano, Italy. d13C 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 d13C 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. d15N values and carbon to nitrogen ratios (C/N) ratios did not change in Symbiodinium and host tissue along the pCO2 gradient
Summary
Natural high pCO2 increases autotrophy in Anemonia viridis (Anthozoa) as revealed from stable isotope (C, N). Natural CO2 vents at sub-tropical coastal areas[3,4,5] and tropical reefs[6] serve as natural laboratory locations to study long-term effects of elevated pCO2 (pH) across many biological and spatial scales. Such a location has been reported in the Levante Bay of Vulcano Island (Italy) in the Mediterranean Sea where many studies have examined physiological adaptations of biota to OA, including seagrass[7], benthic micro- and macroalgaes[8,9], sea urchins[10], and sea anemones[11,12]. Other reports on the response of A. viridis near CO2 vents discovered changes in their associated microbial communities[19], reduced dimethylsulfoniopropionate (DMSP) production[12] and enhanced productivity[3,11]
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