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Pond corona formed by algae imaged by Marko Riikonen (site) on Tvärminne Longskär Island, Finland. ©Marko Riikonen, shown with permission.

Coronae, more familiar as coloured rings around the moon, are made when light is scattered, diffracted, by small droplets or particles. The scatterers need not be transparent and airborne pollen makes excellent coronae.

Here, algae of the genus Nautococcus on the pond surface are scattering sunlight. The cell shape protruding from the water is hemispherical and this produces oval coronae. The apparent brightenings are most likely parts of bands produced by another interference phenomenon, Quételet scattering.

More on algal optics by Marko Riikonen