The combined effects of ocean acidification, warming and light on Caulerpa spp.: The role of inorganic carbon physiology and species’ ranges

<p><strong>Caulerpa is a widely distributed genus of chlorophytes (green macroalgae) that is important for its dietary, social, and coastal ecosystem value. Ocean acidification (OA) and ocean warming (OW) both threaten to change the distribution of macroalgae in New Zealand and globally. Two of the most common Caulerpa species in Te Whanganui a Tara Wellington, Aotearoa New Zealand, Caulerpa brownii and C. geminata, could have vastly different responses to both OA and OW because of their divergent dissolved inorganic carbon (DIC) uptake strategies and distance from range edge. However, responses to both stressors of OA and OW could be heavily mediated by light. To address these concerns, I conducted four manipulative laboratory experiments to test the effects of OA, OW and marine heatwaves (MHWs), and light and interactive effects of two or more stressors on Caulerpa species.</strong></p><p>Responses in physiology and photo-physiology of both Caulerpa species to OA suggest that those macroalgae without a CO2 concentrating mechanism (CCM) and relying solely on diffusive CO2 for DIC will benefit more from OA due to the benefits of alleviation of current DIC limitation than those species which possess a CCM that are not currently DIC limited. Physiological and photo-physiological responses to OA were significantly impacted by irradiance, demonstrating the role of irradiance in mediating DIC physiology in Caulerpa species tested. I found that any benefits received from elevated CO2 under OA may be dependent on energy limitations which in turn affects photo-physiology differentially depending on the DIC uptake strategy operating. Growth and photosynthesis responses of Caulerpa spp., demonstrated that the species with a warm edge distribution in Wellington was more negatively impacted by OW than the species with a cold edge in Wellington, which was more tolerant to OW and MHWs. Exposure to multiple stressors (OA, OW, reduced irradiance) of a future ocean exhibited less severe responses to a combined stressors effect than the singular stressor effects. Overall, my research contributed new knowledge to add to literature about the effects of ocean change on macroalgae with various DIC uptake mechanism and distribution edge positions. The findings here provide evidence that for chlorophytes, especially Caulerpa spp., some species will likely survive under OA in future OW and reduced irradiance conditions in Wellington unless irradiance is further limited. These results suggest a shift in distribution range of Caulerpa spp. in New Zealand can be expected that could potentially change the community structure in these locations.</p>