Macromolecular material from the slime layer of the cyanobacterium Microcystis flos-aquae C3-40 was defined as material that adhered to cells during centrifugation in growth medium but was dislodged by washing with deionized water and retained within dialysis tubing with a molecular-weight cutoff of 3,500. At each step of this isolation procedure, the slime was observed microscopically. Cells in the centrifugal pellet were surrounded by large amounts of slime that excluded negative stain, whereas cells that had been washed with water lacked visible slime. Two independently isolated lots of slime contained no detectable protein (<1%, wt/wt) and consisted predominantly of anthrone-reacting polysaccharide. Sugars in a hydrolysate of slime polysaccharide were derivatized with trimethylsilylimidazole and examined by gas chromatography-mass spectrometry. The composition of the slime polysaccharide was 1.5% (wt/wt) galactose, 2.0% glucose, 3.0% xylose, 5.0% mannose, 5.5% rhamnose, and 83% galacturonic acid. This composition resembles that of the plant polysaccharide pectin, which was treated in parallel as a control. Consistent with earlier indications that M. flos-aquae slime preferentially binds certain cations, the ratio of Fe to Na in the dialyzed slime was 104 times that in the growth medium. The composition of the slime is discussed with respect to possible mechanisms of cation binding in comparison with other cyanobacterial exopolysaccharides and pectin.

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The properties of purified capsular polysaccharide from the cyanobacterium Microcystis flos-aquae C3-40 were examined by capillary viscometry. Capsule suspensions exhibited similar viscosities between pH 6 and 10 but were more viscous at pH <4 than at pH 6 to 11. At pH 7, a biphasic effect of metal ion concentration on capsule viscosity was observed: (i) capsule viscosity increased with increasing metal ion concentration until a maximal viscosity occurred at a specific concentration that was a reproducible characteristic of each metal ion, and (ii) the viscosity decreased with further addition of that ion. Because the latter part of the biphasic curve was complicated by additional factors (especially the precipitation or gelation of capsule by divalent metal ions), the effects of various metal chlorides were compared for the former phase in which capsule viscosity increased in the presence of metal ions. Equivalent increases in capsule viscosity were observed with micromolar concentrations of divalent metal ions but only with 10 to 20 times greater concentrations of Na ؉. The relative abilities of various metal salts to increase capsule viscosity were as follows: CdCl 2 , Pb(NO 3) 2 , FeCl 2 > MnCl 2 > CuCl 2 , CaCl 2 ӷ NaCl. This pattern of metal efficacy resembles known cation influences on the structural integrity of capsule in naturally occurring and cultured M. flos-aquae colonies. The data are the first direct demonstration of an interaction between metal ions and purified M. flos-aquae capsule, which has previously been proposed to play a role in the environmental cycling of certain multivalent metals, especially manganese. The M. flos-aquae capsule and the plant polysaccharide pectin have similar sugar compositions but differ in their relative responses to various metals, suggesting that capsular polysaccharide could be a preferable alternative to pectin for certain biotechnological applications.

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