Ocean acidification (OA) continues to be a threat to many species of marine life and is projected to become more severe over the next 50 years. Areas that experience upwelling of low pH water, such as the Salish Sea and Puget Sound, are particularly susceptible to even more intense OA conditions. While much of OA literature has focused on the negative impacts to calcifying organisms, economically important shellfish, and zooplankton, not all species may be threatened by OA. Photosynthesizing organisms, including eelgrasses and macroalgae, have shown some benefits from the increased CO2 in their environment. Few studies have analyzed how this alteration of carbonate chemistry may affect animals that form symbiotic relationships with photosynthetic algae. This study focuses on one of the most abundant intertidal organisms in the Pacific Northwest, the aggregating anemone (Anthopleura elgantissima), and the two photosynthetic symbionts they host- Symbiodinium muscatinei and Elliptochloris marina. Four states of anemones were analyzed for this study: those hosting each symbiont individually, those hosting both simultaneously, and those lacking any symbionts. These anemones were subjected to three different levels of OA over the course of 10 weeks. Photosynthetic efficiency, symbiont cell density, reactive oxygen species (ROS), and CZAR score (the percent of organic carbon the anemone received from the symbiont) were measured during the experiment for all four symbiont states. The objectives of this study were to determine if anemones benefitted from increased OA conditions and if the effect differed depending on symbiotic state.
Results/Conclusions
Reactive oxygen species increased with OA level and was greatest in S. muscatinei anemones. Anemones with no symbionts had a lower concentration of ROS, indicating symbiotic cells are a source of ROS to the host anemone. Photosynthetic efficiency and CZAR score increased in intermediate levels of OA for both symbiont types but declined at the highest levels, suggesting symbiotic anemones may benefit under slightly increased OA until a threshold is reached that damages photosystem mechanisms. Lastly, anemones that started with 50% of each symbiont type shifted toward a higher percentage of S. muscatinei with increased OA. This study suggests anemones hosting S. muscatinei may have a competitive advantage over anemones hosting E. marina under future conditions and even benefit from increased OA until a certain threshold is reached. This symbiont shifting has future implications for the diversity and range of anemones within intertidal systems of the Salish Sea and other areas subject to more severe OA conditions.