Presence of toxin-antitoxin systems in picocyanobacteria and their ecological implications

Picocyanobacteria (mainlySynechococcusandProchlorococcus) contribute significantly to ocean's primary production. Toxin-Antitoxin (TA) systems present in bacteria and archaea are known to regulate cell growth in response to environmental stresses. However, little is known about the presence of TA systems in picocyanobacteria. This study investigated complete genomes ofSynechococcusandProchlorococcusto understand the prevalence of TA systems in picocyanobacteria. Using the TAfinder software, Type II TA systems were predicted in 27 of 33 (81%)Synechococcusstrains, but none of 38Prochlorococcusstrains contain TA genes.Synechococcusstrains with larger genomes tend to contain more putative type II TA systems. The number of TA pairs varies from 0 to 42 inSynechococcusstrains isolated from various environments. A linear correlation between the genome size and the number of putative TA systems in both coastal and freshwaterSynechococcuswas established. In general, open oceanSynechococcuscontain no or few TA systems, while coastal and freshwaterSynechococcuscontain more TA systems. The type II TA systems inhibit microbial translationviaribonucleases and allow cells to enter the dormant stage in adverse environments. Inheritance of TA genes in freshwater and coastalSynechococcuscould confer a recoverable persister mechanism important to survive in variable environments.