Abstract
Ocean acidification (OA) has important implications for the persistence of coral reef ecosystems, due to potentially negative effects on biomineralization. Many coral reefs are dynamic with respect to carbonate chemistry, and experience fluctuations in pCO2 that exceed OA projections for the near future. To understand the influence of dynamic pCO2 on an important reef calcifier, we tested the response of the crustose coralline alga Porolithon onkodes to oscillating pCO2. Individuals were exposed to ambient (400 µatm), high (660 µatm), or variable pCO2 (oscillating between 400/660 µatm) treatments for 14 days. To explore the potential for coralline acclimatization, we collected individuals from low and high pCO2 variability sites (upstream and downstream respectively) on a back reef characterized by unidirectional water flow in Moorea, French Polynesia. We quantified the effects of treatment on algal calcification by measuring the change in buoyant weight, and on algal metabolism by conducting sealed incubations to measure rates of photosynthesis and respiration. Net photosynthesis was higher in the ambient treatment than the variable treatment, regardless of habitat origin, and there was no effect on respiration or gross photosynthesis. Exposure to high pCO2 decreased P. onkodes calcification by >70%, regardless of the original habitat. In the variable treatment, corallines from the high variability habitat calcified 42% more than corallines from the low variability habitat. The significance of the original habitat for the coralline calcification response to variable, high pCO2 indicates that individuals existing in dynamic pCO2 habitats may be acclimatized to OA within the scope of in situ variability. These results highlight the importance of accounting for natural pCO2 variability in OA manipulations, and provide insight into the potential for plasticity in habitat and species-specific responses to changing ocean chemistry.
Highlights
Anthropogenic CO2 emissions have been rising steadily since the Industrial Revolution with the increase in fossil fuel consumption, agriculture, and land development [1], resulting in greater concentrations of CO2 entering the surface ocean [2]
Our results demonstrate that the response of a coralline alga to variability in pCO2 depends upon the conditions in the original habitat
These findings support our hypothesis that exposure to oscillating, high pCO2 within the natural range of variability, had a modulating effect on coralline calcification compared to the static high pCO2 treatment
Summary
Anthropogenic CO2 emissions have been rising steadily since the Industrial Revolution with the increase in fossil fuel consumption, agriculture, and land development [1], resulting in greater concentrations of CO2 entering the surface ocean [2]. Ocean acidification (OA) refers to the equilibration of atmospheric CO2 with the surface ocean, and the subsequent changes to seawater carbonate chemistry. These reactions are manifest as a decrease in mean ocean pH, carbonate ion concentration (CO322) and CaCO3 saturation state (V), an increase in pCO2 and bicarbonate ion concentration (HCO32), and no change in total alkalinity (AT). Saturation state is an important factor that influences rates of biomineralization, where CaCO3 precipitation is favored when V is .1 [3]. Changes in V associated with ocean acidification have important implications for biogenic calcifiers and for ecosystems that are structurally dependent on carbonate platforms, such as coral reefs [3]. The accelerated rate of OA may outpace the ability of marine organisms to adapt to the changing environment [5]
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