Eliminating the Carbon Dixoide Concentrating Mechanism: Towards Open Pond Cultivation of Genetically Modified Cyanobacteria

Recent approaches to chemical production through cultivation of photosynthetic microorganisms have relied on photobioreactor designs of increasing complexity. In order to improve the economic efficacy of such a process, it is desirable to reduce capital costs by moving to an open pond cultivation scheme. One concern when considering an open pond scheme is environmental contamination by genetically modified organisms. In order to address this concern, it is necessary to develop a photosynthetic host that can thrive in the production environment but fails to propagate once introduced to the surroundings. In this work, a strategy for containment has been developed wherein a cyanobacterium (Synechococcus sp. strain PCC 7002) has been genetically modified in such a manner that it no longer has a functioning carbon dioxide-concentrating mechanism. This organism exhibits carbon fixation rates equivalent to that of the wild type when cultivated in liquid media in equilibrium with a high concentration of CO₂ (~10% CO₂ in air) but fails to propagate in ambient CO₂ conditions (~400 ppm CO₂ in air). This organism could be used in an open pond cultivation scheme supplemented with waste CO₂ from flue gas and the risk of environmental contamination would be mitigated without sacrificing efficiency. In fact, because this strategy involves the elimination of large native proteins, a metabolic benefit may be harnessed for increased carbon fixation efficiency.