Cell size influences inorganic carbon acquisition in artificially selected phytoplankton

Cell size influences the rate at which phytoplankton assimilate dissolved inorganic carbon (DIC), but it is unclear whether volume-specific carbon uptake should be greater in smaller or larger cells. On the one hand, Fick’s Law predicts smaller cells to have a superior diffusive CO₂ supply. On the other, larger cells may have greater scope to invest metabolic energy to upregulate active transport per unit area through CO₂-concentrating mechanisms (CCMs). Previous studies have focused on among-species comparisons, which complicates disentangling the role of cell size from other covarying traits. In this study, we investigated the DIC assimilation of the green alga Dunaliella tertiolecta after using artificial selection to evolve a 9.3-fold difference in cell volume. We compared CO₂ affinity, external carbonic anhydrase (CA_ext), isotopic signatures (δ¹³C) and growth among size-selected lineages. Evolving cells to larger sizes led to an upregulation of CCMs that improved the DIC uptake of this species, with higher CO₂ affinity, higher CA_ext and higher δ¹³C. Larger cells also achieved faster growth and higher maximum biovolume densities. We showed that evolutionary shifts in cell size can alter the efficiency of DIC uptake systems to influence the fitness of a phytoplankton species.