Physiological Response Of A Coralline Alga To Changes In CO₂ Concentration And Solar UV Radiation

Impacts of global warming and ocean acidification associated with elevated atmospheric CO₂ concentration due to industrial activities on marine calcifying organisms are of general concerns. At the same time, increased UVB (280–315 nm) irradiance caused by decreased ozone layer has also been receiving attention. Calcareous algae play an important role in reef building and marine carbon cycle. However, little is known on the effects of ocean acidification and UVR (280–400) on macroalgal calcification to date. In this paper, the coralline alga Corallina sessilis was studied in order to see its physiological responses to ocean acidification (High CO₂ / low pH) and UVR, as well as their combined effects. The main results are as follows:Under the conditions of increased CO₂ concentration (1000 ppmv) and changes of seawater carbonate system (pCO₂ increased, pH decreased), the calcification of thalli decreased significantly, and the growth was also inhibited. The effect of UVR on photosynthesis and calcification under high CO₂ concentration (1000 ppmv) increased significantly comparing with that under the control (380 ppmv). UVR inhibited the growth, photosynthesis and calcification of thalli, and reduced the contents of photosynthetic pigments (Chl.a and PE). Such negative impacts were enlarged significantly under the conditions related to ocean acidification. The results indicated that ocean acidification exacerbated the UVR-related harms. Further investigations showed that contents of UV-absorbing compounds and carotenoid increased under the conditions of ocean acidification.On the other hand, the ratios of PIC to POC of thalli affected the physiological response to UVR. At higher levels of PIC/POC, rates of photosynthesis and calcification were higher, and UVR-induced inhibition on photosynthesis was smaller, indicating the protective role of PIC. When the value of PIC/POC was lower, greater UVR-induced inhibition on the growth, photosynthesis and calcification was observed. At the same time, higher levels of UV-absorbing compounds were found, indicating a defensive strategy against UVR at the cost of growth.On the basis of the above results, we verified the ecological effects of progressive acidification by increasing the CO₂ concentration step by step (150, 380, 800 and 1000 ppmv). The photosynthetic oxygen evolution and calcification rates were higher at current level of CO₂ (380 ppmv) than at 150 ppmv CO₂, but were lower at elevated CO₂ concentrations..In conclusion, ocean acidification reduced the calcification, photosynthesis and growth of the coralline alga, enhanced UVR-induced damages. Althouth the alga operated UVR-defensive strategy via increasing UV-absorbing compounds, it was insufficient to alleviate the negative effects of UVR under the ocean acidification condition.