Life at elevated CO₂ modifies the cell composition of Chromera velia (Chromerida)

We investigated the response to high CO₂ of Chromera velia, 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 Symbiodinium and Chromera that have a type II Rubisco, which is intrinsically ill suited to low CO₂ environments. We thus cultured C. velia 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 C. velia 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 C. velia was capable of coping with very high CO₂, which even stimulated biomass production and increased N, P, Mn, Fe and Zn use efficiency. Growth at elevated CO₂ changed the stoichiometric relationships among elements in C. velia cells, but had no effect on the relative abundance of the main organic pools. The high CO₂ in the animal tissue surrounding the photosynthetic cells may therefore facilitate C. velia life in symbiosis.