Using photosystem i as a reporter protein for ¹³C analysis in a coculture containing cyanobacterium and a heterotrophic bacterium

¹³C metabolism analysis of a microbial community is often hindered by the time-consuming and complicated separation procedure for a single species. However, a "reporter protein," produced uniquely by one cell type, retains ¹³C fingerprint information in microbial consortia. This study describes the use of photosystem I (PSI), a multi-subunit protein complex universally found in oxygenic phototrophs, as a reliable reporter protein to probe microalgal metabolism (i.e., cyanobacterium Synechocystis sp. PCC 6803) in a mixed culture with heterotrophic bacteria (i.e., Escherichia coli). We demonstrate that efficient purification of PSI and subsequent ¹³C-based amino acid analyses may decipher photomixotrophic metabolism of Synechocystis 6803 in the coculture. This study also indicates that a supplement of NaHCO₃ at high concentration could significantly improve the robustness of cyanobacterial growth against bacterial contamination.