Multidisciplinary Evidences that <em>Synechocystis</em> PCC6803 Exopolysaccharides Operate in Cell Sedimentation and Protection against Salt and Metal Stresses

<div><p>Little is known about the production of exopolysaccharides (EPS) in cyanobacteria, and there are no genetic and physiological evidences that EPS are involved in cell protection against the frequently encountered environmental stresses caused by salt and metals. We studied four presumptive EPS production genes, <em>sll0923, sll1581, slr1875</em> and <em>sll5052</em>, in the model cyanobacterium <em>Synechocystis</em> PCC6803, which produces copious amounts of EPS attached to cells (CPS) and released in the culture medium (RPS) as shown here. We show that <em>sll0923, sll1581, slr1875</em> and <em>sll5052</em> are all dispensable to the growth of all corresponding single and double deletion mutants in absence of stress. Furthermore, we report that <em>sll0923</em>, <em>sll1581</em> and <em>slr1875</em> unambiguously operate in the production of both CPS and RPS. Both <em>sll1581</em> and <em>slr1875</em> are more important than <em>sll0923</em> for CPS production, whereas the contrary is true for RPS production. We show that the most EPS-depleted mutant, doubly deleted for <em>sll1581</em> and <em>slr1875</em>, lacks the EPS mantle that surrounds WT cells and sorbs iron in their vicinity. Using this mutant, we demonstrate for the first time that cyanobacterial EPS directly operate in cell protection against NaCl, CoCl₂, CdSO₄ and Fe-starvation. We believe that our EPS-depleted mutants will be useful tools to investigate the role of EPS in cell-to-cell aggregation, biofilm formation, biomineralization and tolerance to environmental stresses. We also suggest using the fast sedimenting mutants as biotechnological cell factories to facilitate the otherwise expensive harvest of the producer cell biomass and/or its separation from products excreted in the growth media.</p>