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

Carbonic anhydrases (CAs) typically possess a Zn2+ cofactor and are involved in a wide range of adaptive metabolisms involving pH and dissolved inorganic carbon homeostasis. In photosynthetic microorganisms, they are involved in CO2 concentrating mechanisms (CCM) that supply CO2 to ribulose-1,5-bisphosphate carboxylase-oxygenase. Here we show that the model diatom Phaeodactylum tricornutum possesses a CA from the newly identified iota class, P. tricornutum ι-carbonic anhydrase (Pt-ιCA), which exhibited catalytic activity without a metal cofactor, in contrast to most CAs. Metal-free Pt-ιCA activity was 2 orders of magnitude lower than that of the Zn2+-containing bovine erythrocyte CA, yet still in the range of expected values for CA. Zn2+ concentrations in 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 atmospheric partial pressure of CO2 (pCO2) to very low pCO2 conditions. The induction of CCM at very low pCO2 and low Zn2+ conditions was concomitant with the induction of light harvesting complexes and remodeling of the pentose phosphate pathways. The total cellular CA activity decreased in the Pt-ιCA deletion mutant (ΔιCA) compared with 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.