Sodium alginate is a natural, water-soluble polysaccharide polymer which is not known so far to crystallize spontaneously. Here we show that the controlled drying of wetting films of the aqueous solution of this biopolymer can lead to the formation of large-scale self-assembly and crystallization structures. We show that the structures which grow exclusively within the final drying spot arise from the ordered assembly of the concentrated polysaccharide chains mediated by the condensation of trace cations present in the system. Moreover, we show that these trace cations, which are dragged with the polysaccharide chains and concentrated by the drying process inside the residual spot, come mainly from the ultrapure water used for the experiment. This study does not bring only new insights on the behavior and structural properties of polysaccharide polymers. It also shows that the use of water, even of high purity (conductivity <= 1 mu S.cm(-1)), can lead to singular and unexpected effects when dealing with sensitive materials and experiments (polysaccharide/DNA, drying drops).