diatom Actinocyclus octonarius

René · Post by **René** » Mon Dec 20, 2010 12:22 pm

Hi guys, came across this beauty today in a fresh marine sample:

It's pretty common in these waters, but at 80um dia it is a big one. The valve pattern is pretty pronounced, with the typical actinocyclus 'eye' at around 5 o'clock. I've cropped it here out of a full frame at 25x25um:

60/1.4 oil, DIC/inverted

Best wishes, Rene

Franz Neidl · Post by **Franz Neidl** » Tue Dec 21, 2010 2:27 am

Hello Renè,

An interesting alga!
What is the function of the "eye" ?
The red-brownish colour is coming from the fixation (with Iodine?) ?

Greetings
Franz

Charles Krebs · Post by **Charles Krebs** » Tue Dec 21, 2010 12:35 pm

Great looking diatom and pictures. The three focus "layers" is a good way of showing these.

I too am curious about the reddish coloration. Is that about the color you see when fresh, or is it a DIC "thing" or something else?

René · Post by **René** » Wed Dec 22, 2010 10:48 am

Yes, no, the sample is fresh, but the reddish tone is a white-balance artefact. Possibly related to the led illumination (Cree 6000K), which otherwise works fine (in brightfield), but the DIC system introduces a lot of blue. Have to find a way round that. How's that working with the BH2 system, Charlie?

Thanks, René

Oh yes, Franz, no idea of the function of the 'eye'. Might be a remnant of the processes of other centrics. Apparently it is closed internally, or at least not perforated (Round, p194). Still a lot remains to be discovered...

Mitch640 · Post by **Mitch640** » Fri Dec 24, 2010 6:27 am

I have looked at this diatom over several days and just now saw that nucleus, or at least I think it is, in the center of #3? And the other objects are chloroplasts?

Very interesting diatom. I have not seen a round one yet. Is he from fresh water?

René · Post by **René** » Fri Dec 24, 2010 9:22 am

Hi Mitch, it's marine. But centrics are also common in freshwater, but most often in chains. Yes, the blob in the middle is the nucleus, the large greenish blobs the chloroplast, and small globules of strorage material, might be oily, not sure about that in this sp.

I found an old image set in my collection (actually one of my first stereopairs), if you can bend your eyes enough to make use of that 3D effect. The nucleus is better visible here, in the middle of the cell (actually in a column spanning the 2 valves). Chloroplasts are always on the outside, against the silica frustule. Most of the diatom appears empty, but is really a large water-filled vacuole like most plant cells have.

Best wishes, Rene

Mitch640 · Post by **Mitch640** » Fri Dec 24, 2010 9:26 am

Ah yes, I have never had a problem with cross eyed stereo. The chloroplasts and food vacuoles just jump right out. This is the best way to really look at micrographs.