Characterization of a CO2-Concentrating Mechanism with Low Sodium Dependency in the Centric Diatom Chaetoceros gracilis

Microalgae induce a CO2-concentrating mechanism (CCM) to overcome CO2-limiting stress in aquatic environments by coordinating inorganic carbon (Ci) transporters and carbonic anhydrases (CAs). Two mechanisms have been suggested to facilitate Ci uptake from aqueous media: Na+-dependent HCO3- uptake by solute carrier (SLC) family transporters and accelerated dehydration of HCO3- to CO2 by external CA in model diatoms. However, studies on ecologically and industrially important diatoms including Chaetoceros gracilis, a common food source in aquacultures, are still limited. Here, we characterized the CCM of C. gracilis using inhibitors and growth dependency on Na+ and CO2. Addition of a membrane-impermeable SLC inhibitor, 4,4 '-diisothiocyano-2,2 '-stilbenedisulfonic acid (DIDS), or the transient removal of Na+ from the culture medium did not impair photosynthetic affinity for Ci in CO2-limiting stress conditions, but addition of a membrane-impermeable CA inhibitor, acetazolamide, decreased Ci affinity to one-third of control cultures. In culture medium containing 0.23 mM Na+C. gracilis grew photoautotrophically by aeration with air containing 5% CO2, but not with the air containing 0.04% CO2. These results suggested that C. gracilis utilizes external CAs in its CCM to elevate photosynthetic affinity for Ci rather than plasma-membrane SLC family transporters. In addition, it is possible that low level of Na+ may support the CCM in processes other than Ci-uptake at the plasma membrane specifically in CO2-limiting conditions. Our findings provide insights into the diversity of CCMs among diatoms as well as basic information to optimize culture conditions for industrial applications.

Automated summary

Microalgae utilize a CO2-concentrating mechanism (CCM) to overcome CO2-limiting stress in aquatic environments. Diverse mechanisms for Ci uptake have been suggested, with studies indicating the use of external carbonic anhydrases in certain diatoms like Chaetoceros gracilis. Low levels of Na+ may also play a role in supporting the CCM in CO2-limiting conditions. These findings provide insight into diatom CCM diversity and offer foundational information for optimizing culture conditions in industrial applications.