Reduction of Spermidine Content Resulting from Inactivation of Two Arginine Decarboxylases Increases Biofilm Formation in Synechocystis sp. Strain PCC 6803

The phototropic bacteriumSynechocystis sp. strain PCC 6803 is able to adapt its morphology in order to survive in a wide range of harsh environments. Under conditions of high salinity, planktonic cells formed cell aggregates in culture. Further observations using crystal violet staining, confocal laser scanning microscopy, and field emission-scanning electron microscopy confirmed that these aggregates wereSynechocystis biofilms. Polyamines have been implicated in playing a role in biofilm formation, and during salt stress the content of spermidine, the major polyamine inSynechocystis , was reduced. Two putative arginine decarboxylases, Adc1 and Adc2, inSynechocystis were heterologously expressed inEscherichia coli and purified. Adc2 had high arginine decarboxylase activity, whereas Adc1 was much less active. Disruption of theadc genes inSynechocystis resulted in decreased spermidine content and formation of biofilms even under nonstress conditions. Based on the characterization of theadc mutants, Adc2 was the major arginine decarboxylase whose activity led to inhibition of biofilm formation, and Adc1 contributed only minimally to the process of polyamine synthesis. Taken together, inSynechocystis the shift from planktonic lifestyle to biofilm formation was correlated with a decrease in intracellular polyamine content, which is the inverse relationship of what was previously reported in heterotroph bacteria.IMPORTANCE There are many reports concerning biofilm formation in heterotrophic bacteria. In contrast, studies on biofilm formation in cyanobacteria are scarce. Here, we report on the induction of biofilm formation by salt stress in the model phototrophic bacteriumSynechocystis sp. strain PCC 6803. Two arginine decarboxylases (Adc1 and Adc2) possess function in the polyamine synthesis pathway. Inactivation of theadc1 andadc2 genes leads to biofilm formation even in the absence of salt. The shift from planktonic culture to biofilm formation is regulated by a decrease in spermidine content inSynechocystis . This negative correlation between biofilm formation and polyamine content, which is the opposite of the relationship reported in other bacteria, is important not only in autotrophic but also in heterotrophic bacteria.