Mechanism of Inorganic Carbon Uptake in<i>Chlorella saccharophila</i>: The Lack of Involvement of Carbonic Anhydrase

Abstract

The acid-tolerant green alga &lt;it&gt;Chlorella saccharophila&lt;/it&gt; maintained photosynthesis and accumulated intracellular pools of inorganic carbon over a a range of external pH from 4.0 to 7.5. This accumulation was unaffected by treatment of cells with 10 mol m−3 acetazolamide (AZA). Cells grown at alkaline pH had extracellular carbonic anhydrase (CA), but CA activity was repressed when cells were grown at pH 5.0. Acid-grown cells retained a high affinity for CO&lt;inf&gt;2&lt;/inf&gt;, both at acid and alkaline pH, and the ability to accumulate inorganic carbon. Rates of photosynthesis of acid-grown cells and alkaline-grown AZA-treated cells at pH 8.0 were 2.5-fold higher than the rate of CO&lt;inf&gt;2&lt;/inf&gt; supply from the uncatalysed dehydration of &lt;f&gt;$${\\hbox{ HCO }}_{\\hbox{ 3 }}^{-}$$&lt;/f&gt;, indicating that the cells can take up &lt;f&gt;$${\\hbox{ HCO }}_{\\hbox{ 3 }}^{-}$$&lt;/f&gt; as a source of substrate for photosynthesis. Isotopic disequilibrium experiments with acid-grown cells maintaining steady-state photosynthesis at pH 7.5 demonstrate that 14C from 14CO&lt;inf&gt;2&lt;/inf&gt; was taken up more rapidly than from H14&lt;f&gt;$${\\hbox{ CO }}_{\\hbox{ 3 }}^{-}$$&lt;/f&gt;. This uptake takes place against a concentration gradient. These results demonstrate that C. saccharophila cells have active transport systems for the uptake of both CO&lt;inf&gt;2&lt;/inf&gt; and &lt;f&gt;$${\\hbox{ HCO }}_{\\hbox{ 3 }}^{-}$$&lt;/f&gt; and both operate without the mediation of CA.