Cell morphology engineering enhances grazing resistance of Synechococcus elongatus PCC 7942 for non-sterile large-scale cultivation

In the practical scale of cyanobacterial cultivation, the golden algae Poterioochromonas malhamensis is a well-known predator that causes devastating damage to the culture, referred to as pond crash. The establishment and maintenance of monoculture conditions are ideal for large-scale cultures. However, this is a difficult challenge because microbial contamination is unavoidable in practical-scale culture facilities. In the present study, we unexpectedly observed the pond crash phenomenon during the pilot-scale cultivation of Synechococcus elongatus PCC 7942 using a 10 0-L photobioreactor. This was due to the contamination with P. malhamensis, which probably originated from residual fouling. Interestingly, we found that S. elongatus PCC 7942 can alter its morphological structure when subjected to continuous grazing pressure from predators, resulting in cells that were more than 100 times longer than those of the wild-type strain. These hyper-elongated S. elongatus PCC 7942 cells had mutations in the genes encoding FtsZ or Ftn2 which are involved in bacterial cell division. Importantly, the elongated phenotype remained stable during cultivation, enabling S. elongatus PCC 7942 to thrive and resist grazing. The cultivation of the elongated S. elongatus PCC 7942 mutant strain in a 100-L pilot-scale photobioreactor under non-sterile conditions resulted in increased cyanobacterial biomass without encountering pond crash. This study demonstrates an efficient strategy for cyanobacterial cell culture in practical-scale bioreactors without the need for extensive decontamination or sterilization of the growth medium and culture facility, which can contribute to economically viable cultivation and bioprocessing of microalgae. (c) 2024, The Society for Biotechnology, Japan. All rights reserved.