Macroalgal Performance and Competition under Elevated CO2

Abstract

Since the industrial revolution, atmospheric carbon dioxide (CO2) concentrations have been increasing, and the surface waters of the global oceans have absorbed 30% of the anthropogenic CO2 released into the atmosphere. A higher CO2 concentration in surface ocean waters shifts the carbon chemistry, resulting in higher concentrations of bicarbonate ions (HCO3-) and protons (H ) and lower concentrations of carbonate ions (CO32-). Such a shift in ocean carbon chemistry decreases the pH and the saturation state of the seawater with respect to CO32- thereby making the precipitation of CaCO3 less kinetically favorable. These changes in ocean chemistry termed ocean acidification) are expected to have negative impacts on marine calcifying organisms, which deposit CaCO3 in the form of aragonite, calcite and high-magnesium calcite into their shells and skeletons. Because calcifying marine primary producers are very important to the carbon cycle and for rocky shore habitat structure and stability, investigating how they will respond to future oceanic CO2 levels is a relevant and important topic of research. Therefore, two calcifying marine macroalgae were chosen as the central organisms for investigation in this thesis. I investigated the physiological responses of the temperate calcifying coralline rhodophyte alga Corallina officinalis (L.) and the tropical calcifying chlorophyte alga Halimeda opuntia (L.) J. V. Lamouroux to elevated CO2 concentrations which are expected to occur by the end of this century. Furthermore, the effect of elevated CO2 on the competitive interactions between these two calcifiers and their noncalcifying counterparts was investigated in order to predict how macroalgal communities will respond to future surface ocean CO2 levels in both temperate and tropical environments. Because CO2 concentrations are increasing in surface ocean in parallel with other abiotic stressors, I also chose to investigate the response of H. opuntia to the combined effect of elevated CO2 and inorganic nutrients, which replicates a likely scenario for the condition of some eutrophied tropical coral reefs at the end of this century. The studies carried out during this thesis revealed that there are differences in the physiological responses of calcifying macroalgae to elevated CO2, but similar patterns of competitive interactions between calcifiers and noncalcifiers occur under elevated CO2 regardless of species and latitude. I found that the temperate coralline alga C. officinalis was highly sensitive to elevated CO2, as shown by lower growth and 3 photosynthetic rates and less calcified cell walls than under normal conditions. On the other hand, the tropical calcifying chlorophyte alga H. opuntia was only moderately sensitive to elevated CO2 concentrations, as this species had lower growth rates but maintained normal calcification rates and increased electron transport rates. Enzyme activity (external carbonic anhydrase and in situ nitrate reductase) in both species was affected by CO2 indicating that external carbonic anhydrase plays an important role in calcification by regulating the speciation of inorganic carbon, and that nitrogen assimilation in these species is affected by elevated CO2. The effect of CO2 on energy balance in these two species is also discussed. The different calcification mechanisms utilized by these two species is likely to account for some of the observed differences in physiological responses, and is discussed in detail below. While these two species showed different susceptibilities to elevated CO2 in isolation, they both showed similar sensitivity to overgrowth and outcompetition by noncalcifying algae when grown with their natural communities under elevated CO2 conditions. This trend was amplified under conditions of inorganic nutrients. The results of this thesis indicate that calcifying macroalgae show differences in their susceptibility to ocean acidification, but regardless of their sensitivity in isolation, both temperate and tropical species are likely to be outcompeted by noncalcifying macroalgae under elevated CO2 conditions. Tropical systems are especially susceptible to a shift in community composition (from calcifier- to noncalcifier-dominated) when eutrophication and ocean acidification occur simultaneously.