Life at elevated CO<sub>2</sub> modifies the cell composition of <i>Chromera velia</i> (Chromerida)

<p>We investigated the response to high CO<sub>2</sub> of <i>Chromera velia</i>, a photosynthetic relative of apicomplexan parasites that is possibly involved in symbiotic associations with scleractinian corals. The inorganic C content in the proximity of the symbiotic algal cells within the tissues of scleractinians is disputed. According to some authors, it is very high. A higher C content in the endodermal tissues of scleractinians than in the external environment may have favoured the constitution of symbiosis with organisms such as <i>Symbiodinium</i> and <i>Chromera</i> that have a type II Rubisco, which is intrinsically ill suited to low CO<sub>2</sub> environments. We thus cultured <i>C. velia</i> at the very high inorganic C estimated by some authors and assessed its growth and photosynthetic performance. We also evaluated whether these conditions affected C allocation and elemental stoichiometry in <i>C. velia</i> cells by state-of-the-art Fourier transform infrared spectroscopy and total reflection X-ray fluorescence spectrometry in combination with more traditional biochemical and physiological techniques. Our results demonstrated that <i>C. velia</i> was capable of coping with very high CO<sub>2</sub>, which even stimulated biomass production and increased N, P, Mn, Fe and Zn use efficiency. Growth at elevated CO<sub>2</sub> changed the stoichiometric relationships among elements in <i>C. velia</i> cells, but had no effect on the relative abundance of the main organic pools. The high CO<sub>2</sub> in the animal tissue surrounding the photosynthetic cells may therefore facilitate <i>C. velia</i> life in symbiosis.</p>