Transcriptional response of the extremophile red alga Cyanidioschyzon merolae to changes in CO 2 concentrations.

Cyanidioschyzon merolae (C. merolae) is an acidophilic red alga growing in a naturally low carbon dioxide (CO ₂ ) environment. Although it uses a ribulose 1,5-bisphosphate carboxylase/oxygenase with high affinity for CO ₂ , the survival of C. merolae relies on functional photorespiratory metabolism. In this study, we quantified the transcriptomic response of C. merolae to changes in CO ₂ conditions. We found distinct changes upon shifts between CO ₂ conditions, such as a concerted up-regulation of photorespiratory genes and responses to carbon starvation. We used the transcriptome data set to explore a hypothetical CO ₂ concentrating mechanism in C. merolae, based on the assumption that photorespiratory genes and possible candidate genes involved in a CO ₂ concentrating mechanism are co-expressed. A putative bicarbonate transport protein and two α-carbonic anhydrases were identified, which showed enhanced transcript levels under reduced CO ₂ conditions. Genes encoding enzymes of a PEPCK-type C ₄ pathway were co-regulated with the photorespiratory gene cluster. We propose a model of a hypothetical low CO ₂ compensation mechanism in C. merolae integrating these low CO ₂ -inducible components.
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