The CO ₂-concentrating mechanism of Synechococcus WH5701 is composed of native and horizontally-acquired components

The cyanobacterial CO ₂-concentrating mechanism (CCM) is an effective adaptation that increases the carbon dioxide (CO ₂) concentration around the primary photosynthetic enzyme Ribulose-1,5-bisphosphate Carboxylase/ Oxygenase (RuBisCO). α-Cyanobacteria (those containing Form1-A RuBisCO within cso-type α-carboxysomes) have a limited CCM composed of a small number of Ci-transporters whereas β-cyanobacteria (those species containing Form-1B RuBisCO within ccm-type β-carboxysomes) exhibit a more diverse CCM with a greater variety in Ci-transporter complement and regulation. In the coastal species Synechococcus sp. WH5701 (a-cyanobacteria), the minimal α-cyanobacterial CCM has been supplemented with β-cyanobacterial C _itransporters through the process of horizontal gene transfer (HGT). These transporters are transcriptionally regulated in response to external Ci-depletion however this change in transcript abundance is not correlated with a physiological induction. WH5701 exhibits identical physiological responses grown at 4% CO ₂ (K1/2 ≈ 31 μM Ci) and after induction with 0.04% CO ₂ (K1/2 ≈ 29 μM Ci). Insensitivity to external C _iconcentration is an unusual characteristic of the WH5701 CCM which is a result of evolution by HGT. Our bioinformatic and physiological data support the hypothesis that WH5701 represents a clade of α-cyanobacterial species in transition from the marine/oligotrophic environment to a coastal/freshwater environment.