The carbonic anhydrase CAH1 is an essential component of the carbon-concentrating mechanism in .

Aquatic photosynthetic organisms cope with low environmental CO concentrations through the action of carbon-concentrating mechanisms (CCMs). Known eukaryotic CCMs consist of inorganic carbon transporters and carbonic anhydrases (and other supporting components) that culminate in elevated [CO] inside a chloroplastic Rubisco-containing structure called a pyrenoid. We set out to determine the molecular mechanisms underlying the CCM in the emerging model photosynthetic stramenopile, , a unicellular picoplanktonic alga that lacks a pyrenoid. We characterized () as an essential component of the CCM in CCMP1779. We generated insertions in this gene by directed homologous recombination and found that the mutant has severe defects in growth and photosynthesis at ambient CO We identified CAH1 as an α-type carbonic anhydrase, providing a biochemical role in CCM function. CAH1 was found to localize to the lumen of the epiplastid endoplasmic reticulum, with its expression regulated by the external inorganic carbon concentration at both the transcript and protein levels. Taken together, these findings show that is an indispensable component of what may be a simple but effective and dynamic CCM in .