Future levels of atmospheric carbon dioxide, currently at 380ppm are predicted to reach 1000ppm by the year 2100 and the accompanying increase of dissolved CO_ 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 (CaCO_) 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 pCO_. Specifically we conducted short-term exposure experiments by subjecting algae to two pCO_ 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 _mol photons×m-2×s-1). In contrast, calcification rates under elevated pCO_ varied among the species, declining by 18% in B. californica and 49% in C. officinalis, but increasing by 20% in C. tuberculosum. Likewise, percent CaCO_ in the algal thalli decreased in B. californica and C. officinalis, but increased in C. tuberculosum under elevated pCO_, 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 CO_ 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 pCO_ in all three species. Here, the algae were exposed to one of three pCO_ levels, 500ppm, 1000ppm and 1500ppm, and algal photosynthetic rates, net calcification rates, and CaCO_ 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 pCO_ by approximately 74% in the 1500ppm mesocosm relative to rates at 380ppm. Calcification rates for all species decreased under elevated pCO_ to an average of -0.896±0.307 _mol C×g-1×h-1 at 1500ppm, indicating net dissolution of CaCO_. 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 CO_. The in situ effects of ocean acidification are challenging to study due to methodological constraints and difficulties in maintaining experiments in aquatic environments. The third component of this research assessed the impacts of elevated pCO_ on photosynthesis and calcification of all three species of geniculate coralline algae in field incubations. Here, the coralline algae were incubated in translucent bags filled with seawater at two pCO_ levels, 500ppm and 1000ppm. The bags were placed within the kelp forest at 10 meters depth for a 2.5-hour incubation period, during which algae experienced natural light and wave energy (hereafter 'immediate incubation'). A second collection of algae was brought back to the laboratory and placed in mesocosms (500ppm and 1000ppm) for two weeks. Following this acclimation period, the algae were then incubated for 2.5 hours in the field as described above (hereafter 'acclimated incubation'). Two immediate incubations and two acclimated incubations were conducted. The three species averaged 45% higher photosynthesis rates under 1000ppm pCO_ than under 500ppm pCO_ during the immediate incubations, but only 30% higher during the acclimated incubations. This suggests carbonfertilization, yet also suggests the algae can acclimate to elevated pCO_. Further, calcification rates decreased in all species at 1000ppm during both immediate and acclimated incubations; immediate incubations resulted in a 44% average decrease in calcification under elevated CO_, whereas acclimated incubations resulted in an 81% average decrease in calcification. This indicates that photosynthesis in these algae has the potential to acclimate to elevated CO_, whereas calcification does not but rather declines rapidly. The response of calcifying autotrophs will be dependent on the ability of calcification rates to maintain CaCO_ structures and less on the potential fertilization effects of CO_ on photosynthesis. Together, the results from all three experiments suggest negative impacts of future levels of CO_ on temperate coralline algae. The variable responses among species were not substantial enough to give one species a physiological advantage over another and this study stresses the importance of understanding the impacts of ocean acidification on a group of species that fulfill similar ecological roles.