Abstract
Chromera velia (Alveolata) is a close relative to apicomplexan parasites with a functional photosynthetic plastid. Even though C. velia has a primitive complement of pigments (lacks chlorophyll c) and uses an ancient type II form of RuBISCO, we found that its photosynthesis is very efficient with the ability to acclimate to a wide range of irradiances. C. velia maintain similar maximal photosynthetic rates when grown under continual light-limited (low light) or light-saturated (high light) conditions. This flexible acclimation to continuous light is provided by an increase of the chlorophyll content and photosystem II connectivity under light limited conditions and by an increase in the content of protective carotenoids together with stimulation of effective non-photochemical quenching under high light. C. velia is able to significantly increase photosynthetic rates when grown under a light-dark cycle with sinusoidal changes in light intensity. Photosynthetic activities were nonlinearly related to light intensity, with maximum performance measured at mid-morning. C. velia efficiently acclimates to changing irradiance by stimulation of photorespiration and non-photochemical quenching, thus avoiding any measurable photoinhibition. We suggest that the very high CO2 assimilation rates under sinusoidal light regime are allowed by activation of the oxygen consuming process (possibly chlororespiration) that maintains high efficiency of RuBISCO (type II). Despite the overall simplicity of the C. velia photosynthetic system, it operates with great efficiency.
Highlights
Most of all the diverse assemblage of eukaryotic oxygenic photosynthetic autotrophs present today belong to either the green or red plastid lineages [1,2,3]
C. velia is effectively a mixture of different organisms: heme synthesis as observed in Apicomplexans, simple pigmentation as in Eustigmatophyceae, primitive type II RuBisCO as in Dinoflagellates, and antenna organized as observed in Bacillariophyceae
To C. velia, we propose that members of this new family of Chromeraceae use photorespiration, together with the thermal energy dissipation via non-photochemical fluorescence quenching (NPQ), as a mechanism in their photoacclimation strategy
Summary
Most of all the diverse assemblage of eukaryotic oxygenic photosynthetic autotrophs present today belong to either the green (chlorophyll b-containing) or red (chlorophyll c-containing) plastid lineages [1,2,3]. The apicomplexans, which are non-photosynthetic sporozoan parasites (e.g., the malaria organism, Plasmodium falciparum), have a relic unpigmented plastid (apicoplast) indicating that the ancestors of these organisms were once photosynthetic, and that part of the plastid metabolic machinery is indispensable to the present organism. This may include the fatty acid synthesis enzymes [4] and isoprenoid biosynthesis [5]. While apicomplexans are not currently placed in the polyphyletic group ‘algae’ by taxonomists, their algal roots have been long acknowledged [7,8,9]
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