Algae corona

This photograph was taken by Hans-Jürgen Heyen from Meerbusch. “I discovered the glory effect in August 2012 at the pond of Hugenpoet Castle in Essen-Kettwig. It made me take a closer look at the water because one tends to think that there are mineral oil products on the surface. But there were gigantic carps in the pond and also some big golden fish which looked like koi-carps. And also a beautiful demoiselle flew over the water. And especially this species of dragonflies is very sensitive against environmental pollution. However, what really was there in the water were algae which caused a significant clouding of the water, and this obviously was the reason for the formation of the algae glory.”

These coronae are caused by light diffraction on very small particles. In most cases, they are caused by algae, but there were also such coloured rings observed around pollen which had landed on the water surface. The coronae are caused when a ray of light is split up into partial beams by such a small particle. These partial beams go on into different directions and interfere in the observer´s eye.

Just like pollen coronae, also algae coronae are not always round. When there are unusually shaped algae are involved, also their ring systems on the water surface can be ovally shaped or have bright spots.

Reflected light dewbow on water surface

I and my wife Eliisa Piikki got a hint about a light phenomenon on a lake nearby. Eliisa took some photos and there was a dewbow and a reflected light dewbow in those photos. They were formed on the waterdroplets that were settled down on the Chrysomyxa ledi needle rust (Small-spored spruce-Labrador-tea rust).

That rust can’t be seen in Finland every year, but this year it is very common especially in the Eastern and in the Northern Finland. Because of the rust the young needles of some spruces are brown.

Author: Jari Piiki, Finland

Pollen corona in Deventer, The Netherlands

On April 1st 2009, Peter Paul Hattinga Verschure observed beautiful pollen coronae from his garden in Deventer, The Netherlands. Every time the wind blew through the pine trees, a cloud of pollen was blown away from them. When these clouds of pollen passed exactly in front of the sun, a beautiful pollen corona appeared. So, in one second it was bright and had three rings, in the next moment it disappeared again.

Colourful bacteria film

On September 14, 2007, Reinhard Nitze saw intensely iridescent colours in some puddles on a lane near Barsinghausen/Egestorf (Lower Saxony, Germany). These were probably caused by bacteria of putrefaction coming from decomposing grains of wheat in the water, just as the colours showed up only in those puddles containing larger amounts of grains. The grains originated from the fields along the lane and had probably fallen from a trailer when the harvest was brought in.As some of the grains were already covered with green algae, they must have been lying in the water for some time already. Also remarkable was the rusty colour of those puddles which had already fallen dry.

more pictures

Thin film interference on the water surface

In the city of Lahti there is a landfill hill from which base in some places flows iron rich water. The bottom of these ditches is rusty brown and water has a strong iron smell. In some locations the water surface is completely covered with a film that displays spectacular colors in cloudy weather. This film is caused by iron oxidizing bacteria. The bacteria itself resides in the water, but it produces on the water surface substances from which it hangs down like a chain of sausages. Iron oxide is one of these products and it may be the cause of colors.

Author: Marko Riikonen

Algal optics

The season for algal optics has started, the first display was seen here in Finland already in the beginning of April.

The algal films that display optical phenomena are clearly not as rare as have been thought. In the Baltic sea rocky islands about every 10th freshwater pool came with algal optics last summer. In Bulgaria, the species Chromophyton rosanoffii, which is responsible for the phenomena in the photo, has been described as common.

When it has not been raining for some days, go look for any freshwater ponds, puddles and pools. If the water surface is covered by a thin film, chances are that some sort of optical phenomena is visible in the sun light.

Author: Marko Riikonen

Small coronas from bacterial film on water surface

Most of the cultivation pools in my room are now showing much smaller coronas than earlier. In each pool the corona is of constant size over the whole bacterial film surface, but from pool to pool their size vary.

Even though these coronas are smaller than before, they still are large as compared to pollen coronas. Unfortunately I have no photos for comparison.

In the composite image are two coronas photographed with same lens (not to scale with upper single image, which has been taken with zoom lens). The microphotograph of the bacterial film is from the smaller corona on the left.

The light source was a 50 W halogen spotlight that I made even more concentrated by placing the lamp in a cardboard box, into which a small hole was made for light to come through. When taking photos, the room was otherwise darkened.

[Posted by Marko Riikonen]

Chromulina rosanoffii as a cause of optical phenomena on water surface

Excavating the literature finally gave confirmation that it is the alga Chromulina rosanoffii that causes Quetelet rings and glory on water surface. This unique sort of alga separates itself from the water surface by forming a stalk on top of which it rests. This is seen in the right hand microphoto which is taken parallel to the water surface. Light blue is air and black is water. On the left is a photo taken at right angle to the water surface, thus giving a look at the C. rosanoffii forest from above (transmitted light). If the alga is for some reason submerged in the water, it sheds its stalk and starts immediately swimming. The b&w photos here show C. rosanoffii submerged in water.

Also two photos of the optical phenomena caused by this alga are displayed in the composite. On the left is a fisheye view with Quetelet rings. The glow around the camera shadow is fogbow. Notice the brightening towards nadir in fogbow, indicating possibly non-spherical particles. The photo on right is taken further away from the cultivation tub, showing spectral colored glory rings. Attached to the outmost glory ring is faint, white fogbow.

All said here about the biology of C. rosanoffii was known long time ago. The alga was described in 1880 by Russian botanist Woronin, who encountered it while visiting Finland. One aspect of the studies carried since has been the golden glow that well developed C. rosanoffii surface film displays, as shown beside and here. But I have not yet met in the literature any mention of the spectral colors, which strikes me as a small wonder. However, there is still plenty of reading to be done, so something may come up.

[Posted by Marko Riikonen]

New results on the large coronas on water surface

Some time ago Jari Piikki posted me a couple of drops of the surface film that produced his colourful corona. I mixed it in a bucket full of tap water with also some forest litter added. After a week or so there appeared a bluish film on water surface. Closer inspection with point-like light revealed more colors. It was actually a large corona, with blue inside, and green and red on the outer edge.

Only a segment of the corona could be seen in the small bucket. This is shown in the upper left image. Unlike the peculiar corona described by Piikki, this one has true blue aureole starting right from the reflection image of the lamp. Outside the blue some green and perhaps red maybe distinguished – visually these colors were clear. Upper right image is microphotograph of the film. It consisted of bacteria.

Day after taking the upper photographs the phenomenon in the bucket changed. Now there was similar corona as what Piikki photographed, only less bright. This is shown in lower left. There was no aureole, but instead a dark area surrounded the light source. This also was consistent with Piikki’s observation. Microphotograph of the film on this stage is shown on the lower right.

The most conspicuous difference in the bacterial level between the two stages is the crowding – there are more bacteria in the latter stage. Is this somehow related to the changes in the phenomena? Perhaps Minnaert can give an answer. In the chapter about coronas seen in windows, he describes how these coronas have an aureole replaced by dark area. He speculates the dark area forms because in the two dimensional plane of the window surface the droplets can be evenly spaced – a situation which can not happen when particles are freely floating in the air.

Now, if one looks at the lower right microphotograph, one can imagine that these bacteria indeed are spaced more evenly as compared to the upper photo. In the Piikki’s microphotograph, which looks like a maze, the bacterial film might consist solely on the long strands that are seen in small numbers also in my lower right photo. When packed next to each other, these kind of strands would possibly be very evenly spaced, thus perhaps contributing to the stronger corona. Piikki’s photograph is, however, too much out of focus to be sure of this interpretation.

The bacteria species could be Nevskia ramosa, which is well known neustonic (water surface inhabiting) bacteria. Genomic fingerprinting would be needed for confirmation. For the last note it should be said that the inverted colors between the two microphotographs are due to differences in focus. The upper image is a bit out of focus and thus the bacteria shows as dark spots.

[Posted by Marko Riikonen]

Water colours at Juva & a new optical phenomenon

Jari Piikki have also found some ponds with algal films that show optical phenomena. The ponds were sampled and the alga allowed to breed for some weeks. Extracts were photographed through a microscope. In one pond, Botryococcus alga was abundant nine years ago but then disappeared. Now it is back and quite large Quetelet rings and a corona were visible. Another pond on one island possibly contained Chromulina rosanoffi and microscope images showed it floating on the water surface.

Jari Piikki also found a new optical phenomenon. He took (like Marko) some samples from ponds and cultivated them. In one of them Chromulina disappeared and some other alga appeared. There could be seen a very colourful ring with two zones of spectral colours around the reflection from the surface of the Sun or an artificial light. The ring was oval and became smaller, when the light was brought closer to the water surface. Its diameter in sunlight was about 30 degrees and the inside was dark, so it was not an aureole. also visible when the light shone through the alga from below. Many photographs of the surface were taken with a microscope to show its structure. You can see it in some photos. The alga constantly changes constantly and now shows small colourful blotches.

[Text: Jari Piikki]