Anyone knows why this is happening?

[pwnell]

It is a piece of algae photographed under polarized lighting. The first image is feflected polarization, the second is transmitted polarization. What is the white/purple band in the middle of the filament in the reflected polarized shot and why is it not in the transmitted polarized photo?


Tropical Aquarium algae, 40x, EPI-POL


Tropical Aquarium algae, 40x, Transmitted POL

[Cactusdave]

Do you have a coverslip over the algae?

[pwnell]

Yes I did.

[Jacek]

interesting .... ?

[Ecki]

I think this is the top of the filament.

[Cactusdave]

I'm guessing that in the epi-polarising picture there is reflection from the upper surface of the algal filament, which is behaving like a shiny cylinder. The blue colour is presumably due to some retardation effects on the polarised light. I'm a little surprised that the effect should be so strong with a coverslip in place which would tend to flatten the upper surface of the cylindrical algal filament. It would be interesting to see if the effect changes or disappears as the preparation dries out and the coverslip presses more strongly on the filament. Some algae deposit considerable amounts of calcium salts in their cell wall making them incompressable and this could also be a factor in the effect you are seeing. An image of a larger portion of the alga wold be helpful in trying to identify it.

[pwnell]

Here is a DIC and fluorescent closeup (40x), and a dark field @ 10x.

[Cactusdave]

This is very similar to an alga I see in the UK. Unfortunately I don't know it's name. We are blessed with lots of subject experts on the forum, but I thing we need to recruit an algologist.