Chrysolykos planctonicus is a planktonic chrysophyte flagellate in the genus Chrysolykos, a small genus of loricate or semi-loricate golden algae in the class Chrysophyceae. Like other Chrysolykos species, it inhabits the open water column of freshwater lakes and ponds, where its planktonic lifestyle is reflected in the specific epithet planctonicus. The cell is typically enclosed in a lorica or cellulose housing that may be less rigid than those of Chrysococcus species, with a flagellum extending through an apical pore. Chrysolykos species are photosynthetic and potentially mixotrophic, using chlorophylls a and c and fucoxanthin-type carotenoids for light energy capture. The genus is documented from Scandinavian freshwaters and other northern temperate lakes, where chrysophytes are often the dominant flagellated algae in spring and early summer. C. planctonicus contributes to primary production in oligotrophic and mesotrophic lake systems. The species has not been evaluated for conservation status by the IUCN and is listed as Not Evaluated. Chrysolykos is a less-studied genus than Chrysococcus or Mallomonas, and its ecology and phylogeny remain incompletely characterized.