Contrasting modes of inorganic carbon acquisition amongst <i>Symbiodinium</i> (Dinophyceae) phylotypes

<jats:title>Summary</jats:title><jats:p> <jats:list list-type="bullet"> <jats:list-item><jats:p>Growing concerns over ocean acidification have highlighted the need to critically understand inorganic carbon acquisition and utilization in marine microalgae. Here, we contrast these characteristics for the first time between two genetically distinct dinoflagellate species of the genus <jats:italic>Symbiodinium</jats:italic> (phylotypes A13 and A20) that live in symbiosis with reef‐forming corals.</jats:p></jats:list-item> <jats:list-item><jats:p>Both phylotypes were grown in continuous cultures under identical environmental conditions. Rubisco was measured using quantitative Western blots, and radioisotopic <jats:sup>14</jats:sup>C uptake was used to characterize light‐ and total carbon dioxide (TCO<jats:sub>2</jats:sub>)‐dependent carbon fixation, as well as inorganic carbon species preference and external carbonic anhydrase activity.</jats:p></jats:list-item> <jats:list-item><jats:p>A13 and A20 exhibited similar rates of carbon fixation despite cellular concentrations of Rubisco being approximately four‐fold greater in A13. The uptake of CO<jats:sub>2</jats:sub> over <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/nph12379-math-0001.gif" xlink:title="urn:x-wiley:0028646X:media:nph12379:nph12379-math-0001" /> was found to support the majority of carbon fixation in both phylotypes. However, A20 was also able to indirectly utilize <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="graphic/nph12379-math-0002.gif" xlink:title="urn:x-wiley:0028646X:media:nph12379:nph12379-math-0002" /> by first converting it to CO<jats:sub>2</jats:sub> via external carbonic anhydrase.</jats:p></jats:list-item> <jats:list-item><jats:p>These results show that adaptive differences in inorganic carbon acquisition have evolved within the <jats:italic>Symbiodinium</jats:italic> genus, which thus carries fundamental implications as to how this functionally key genus will respond to ocean acidification, but could also represent a key trait factor that influences their productivity when <jats:italic>in hospite</jats:italic> of their coral hosts.</jats:p></jats:list-item> </jats:list> </jats:p>