It is known that the cell walls of algae are birefringent as well as the starch grains and crystals within the plasm. The cell wall of the cyanobacteria is birefringent too and between crossed pol filters the outline of the solitary cells or colonies becomes visible. But the effect is weak. In my favored pond Simmelried I found a “bright” exception 15 years ago. At this time the ID of the species was out of my skills but I could fix the genus: Oscillatoria. The species is very common in the Simmelried and I see the species almost every day. Nine years later, when I started to screen my archive for the PM, Vol. 3 I was able to identify the species as Oscillatoria chlorina. For the Vol. 3 I tried to get some better images of it. My microscope is equipped with a rotatable stage and I tried to fit the allignment of the filaments optimal to the image frame by rotation of the stage. During this rotation I could observe that the filament changed in DIC the color from bright green to reddish/pink or sometimes to orange. In the years before I wasn’t aware for this effect. I repeated the experiment between crossed pol filters and could observe the light up and extinction of the filaments in 90 degree steps. Later, I cultivated O. chlorina in a microaquarium and after some days I found spirally curled specimens. This specimens showed a perfect Maltese cross-shaped interference pattern under pol oberservation as well as in DIC.
To my knowledge this effect wasn’t described before and no other cyanobacteria are known to be birefringent. The reason for the birefringence of the filaments is unknown to me. But the very fine transverse striation of the cells can give a hint. My opinion is, that this structures are responsible for the effect. To investigate the nature of the striation a TEM preparation would be necessary.
How to identify Osciallotria chlorina? The following features are important:
- shape of the terminal cell
- diameter and length of filaments
- color
- are there any inclusions or vesicles
- show the cell surface any structures
- shape of the cross walls
- is there a mucous sheath
Oscillatoria chlorina can be identified by a hyaline cap at the terminal cells. No mucous sheath. The cell surface is smooth but each cell shows a very fine and regular striation. Cells broader than long. A unconspicuous constriction at the cross walls. The color is yellowish green (in BF). The most important feature is the stration. Each cell shows 4 – 8 lines. You can find O. chlorina for example in Streble/Krauter: Das Leben im Wassertropfen, p. 126 and in PM, Vol. 3, p. 22.
Below 3 images and a short video with a demonstration of the color change.
Martin
The link to the video:
http://www.einzell.de/Oscillatoriachlorina.wmv
The “true” color of O. chlorina is yellowish-green only visible in brightfield.
HC = hyaline cap
ST = striation
CW = cross walls
In DIC the color of the spirally curled filaments changes from green to reddish, almost pink:
The following image shows very impressive the birefringent character of O. chlorina. The Maltese cross-shaped interference pattern becomes clearly visible between crossed pol filters.
