Retention of a SulP-family bicarbonate transporter in a periplasmic N2-fixing cyanobacterial endosymbiont of an open ocean diatom

Symbioses between diatoms and the N2-fixing, heterocyst-forming cyanobacterium Richelia spp. are widespread and contribute to primary production. Unique to these symbioses is a variation in the symbiont location: one lives in the host cytoplasm (endobiont) vs. residing between the host frustule and plasmalemma (periplasmic endobiont). Both partners are photosynthetic, yet how the partners acquire, share, or compete for bicarbonate necessary for their photosynthesis is unknown. The genomes of both endobionts (ReuHH01 and RintRC01, respectively) contain genes encoding SulP-family proteins, which are oxyanion transporters. To study the possible involvement of these transporters in bicarbonate uptake, we used complementation in a Synechocystis sp. PCC 6803 mutant that is unable to grow at air levels of CO2 because all five of its inorganic carbon uptake systems have been inactivated. Of the five genes tested, only one (RintRC_3892) from the periplasmic endobiont complemented the mutant to grow with air levels of CO2 or at low bicarbonate concentrations. The complemented strain showed strong sodium-dependent and low-affinity bicarbonate uptake that was consistent with bicarbonate concentrations expected in the diatom periplasm. Additionally, all the amino acids involved in the bicarbonate binding site of BicA from Synechocystis sp. PCC 6803 are conserved in RintRC_3892. Finally, the importance of the RintRC_3892 protein was confirmed by the consistent detection of its transcripts in wild Richelia populations from three different oceans. Combined our results showed no evidence for a bicarbonate transporter in the cytoplasmic endobiont, whereas the periplasmic endobiont has retained a SulP-type bicarbonate transporter for its own photosynthesis.