Carbonic anhydrase activity under low zinc in Phaeodactylum tricornutum largely depends on metal-free iota-carbonic anhydrase

Carbonic anhydrases (CA) typically possess a Zn2+-cofactor, and are involved in a wide range of adaptative metabolisms involving pH and dissolved inorganic carbon homeostasis. In photosynthetic microorganisms, they are involved in CO2 concentrating mechanisms (CCM) that supply CO2 to Rubisco. Here we show that the model diatom Phaeodactylum tricornutum possesses a CA from the newly identified iota class (Pt-ιCA) that exhibited catalytic activity without a metal cofactor, in contrast to most CAs. Metal-free Pt-ιCA activity was two orders of magnitude lower than that the Zn2+ containing bovine erythrocyte CA, yet still in the range of expected value for CA. Zn2+ concentrations on the picomolar range induced the expression of Pt-ιCA in P. tricornutum, together with the Zn2+/Co2+ responsive protein A. Zn2+ scarcity did not prevent the induction of Zn2+-containing CAs involved in biophysical CCM when the cells were transferred from high pCO2 to very low pCO2 conditions. The induction of CCM at very low pCO2 and low Zn2+ conditions was concomitant to induction of light harvesting complex and remodeling of the pentose phosphate pathways. The total cellular CA activity decreased in Pt-ιCA deletion mutant (ΔιCA) compared to control cells, indicating that Pt-ιCA is active in P. tricornutum under low Zn2+ conditions. P. tricornutum adapted to the absence of Pt-ιCA by a morphometric change, without affecting photosynthesis or growth rate. The newly identified metal-free Pt-ιCA represents an evolutionary advantage for diatoms that live in metal-limited environments.