Characterizing the response of coralline algae to ocean acidification

OF THE THESIS Characterizing the Response of Coralline Algae to Ocean Acidification by Brenna Elizabeth Bulach Master of Science in Biology with a Concentration in Ecology San Diego State University, 2012 Future levels of atmospheric carbon dioxide, currently at 380ppm are predicted to reach 1000ppm by the year 2100 and the accompanying increase of dissolved CO2 in the oceans will result in an overall decrease in seawater pH. Most research on ocean acidification has found reduced calcification in animals that create calcium carbonate (CaCO3) structures, but few have focused on photosynthetic coralline algae, such as those prevalent in kelp forest ecosystems. Here, we compare photosynthesis and calcification in three of the dominant species of geniculate coralline algae along the coast of California, Bossiella californica, Calliarthron tuberculosum and Corallina officinalis, with the purpose of identifying how they will respond to elevated pCO2. Specifically we conducted short-term exposure experiments by subjecting algae to two pCO2 levels, 380ppm and 1000ppm, under a range of irradiances for 2.5 hours during bottle incubations. Construction of photosynthesis-irradiance curves indicated the algae suffered reductions in both maximum rates of photosynthesis and saturation irradiances. However, all three species exhibited higher photosynthetic efficiencies under non-saturating irradiances (those below 30 μmol photons·m·s). In contrast, calcification rates under elevated pCO2 varied among the species, declining by 18% in B. californica and 49% in C. officinalis, but increasing by 20% in C. tuberculosum. Likewise, percent CaCO3 in the algal thalli decreased in B. californica and C. officinalis, but increased in C. tuberculosum under elevated pCO2, suggesting that B. californica and C. officinalis may be more strongly adversely impacted by climate change than C. tuberculosum. Assessing only the immediate responses to elevated CO2 neglects the possibility of phenotypic plasticity, which may result in variable abilities to acclimate to changing environmental conditions. To examine this in the three species of geniculate coralline algae, a longer four-week mesocosm experiment was conducted to measure changes in photosynthesis and calcification under elevated pCO2 in all three species. Here, the algae were exposed to one of three pCO2 levels, 500ppm, 1000ppm and 1500ppm, and algal photosynthetic rates, net calcification rates, and CaCO3 thallus contents were measured every seven days. Bleaching was observed in the 1000ppm and 1500ppm mesocosms starting on day 14 and continued to the end of the experiment. Photosynthetic rates when averaged across species decreased with increasing pCO2 by approximately 74% in the 1500ppm mesocosm relative to rates at 380ppm. Calcification rates for all species decreased under elevated pCO2 to an average of -0.896±0.307 μmol C·g·h at 1500ppm, indicating net dissolution of CaCO3. Despite these negative values, net photosynthesis and an eventual stability of calcification rates occurred in all treatments by the end of the experiment, indicating a possible acclimation to elevated CO2.