Prolonged exposure to elevated CO2 promotes growth of the algal symbiont Symbiodinium muscatinei in the intertidal sea anemone Anthopleura elegantissima

摘要

Some photosynthetic organisms benefit from elevated levels of carbon dioxide, but studies on the effects of elevated P CO2 on the algal symbionts of animals are very few. This study investigated the impact of hypercapnia on a photosynthetic symbiosis between the anemone Anthopleura elegantissima and its zooxanthella Symbiodinium muscatinei . Anemones were maintained in the laboratory for 1 week at 37?Pa P CO2 and pH?8.1. Clonal pairs were then divided into two groups and maintained for 6 weeks under conditions naturally experienced in their intertidal environment, 45?Pa P CO2, pH?8.1 and 231?Pa P CO2, pH?7.3. Respiration and photosynthesis were measured after the 1-week acclimation period and after 6 weeks in experimental conditions. Density of zooxanthellal cells, zooxanthellal cell size, mitotic index and chlorophyll content were compared between non-clonemate anemones after the 1-week acclimation period and clonal anemones at the end of the experiment. Anemones thrived in hypercapnia. After 6 weeks, A. elegantissima exhibited higher rates of photosynthesis at 45?Pa (4.2?μmol O2 g?1 h?1) and 231?Pa (3.30?μmol O2 g?1 h?1) than at the initial 37?Pa (1.53?μmol O2 g?1 h?1). Likewise, anemones at 231?Pa received more of their respiratory carbon from zooxanthellae (CZAR ?=?78.2%) than those at 37?Pa (CZAR ?=?66.6%) but less than anemones at 45?Pa (CZAR ?=?137.3%). The mitotic index of zooxanthellae was significantly greater in the hypercapnic anemones than in anemones at lower P CO2. Excess zooxanthellae were expelled by their hosts, and cell densities, cell diameters and chlorophyll contents were not significantly different between the groups. The response of A. elegantissima to hypercapnic acidification reveals the potential adaptation of an intertidal, photosynthetic symbiosis for high P CO2.