Hi Priit. Long time ago.  For my part I am not sure that it is an algae. Could it be a hyroid, e.g. Cordylophora caspia ? But you probably know that introduced species? They form a bit of a net and are quite stiff but you may see the thecas now and then if they are happy and alive. just look at pictures on the net. Cheers Lena

Did you pick this algae in a place with Mytilus? Because it seem like byssus.

HI It is not byssus from Mytilus. These threads have cellwalls and are hollow which is not the case in byssus.

It is true. I did not check properly.

Hi Lena! When I saw the object, my first imression was also hydrozoa but now I am quite sure that this is not Cordylophora or any kind of hydrozoa. This is about mophology, because C. caspia has much longer and thicker stem (dimensions is over 10x smaller in that case). I also took some mesures of the object:  width of the axis: 15 µm; filaments club-shape apical parts: 28 µm; it has wider nodes middle of the filaments (28 µm). The filaments does not have ring shaped nodes, there are not alternate branches but opposite. I attached another picture of nodes between filaments, I hope there are some information about taxonomy of the algae.

I hope you identify that algae soon, 

glad to see a lot of helpful experts!

I certainly agree with Lena - this is not an algae.

I glad to agree with Priit

My guess is that it is a tube forming diatom.  There is one here on the Oregon coast that looks just like a monosiphonious Brown until you put it under the scope.....then you see this strand of diatoms held together by a tube. See University of Guam, C.S. Lobban. 

Just to react on the last comment for a 'tube forming diatom'. It is not! There should be diatom cells in the tubes, at least dead ones, or just some cells. There are no diatom cells on your photo!

This coeonocytic wavy filaments remind me xanthophyte from genus Vaucheria. Is this an options in that case?

Could it be a marine fungus?

It doesn't seem to be an algae for me too.

I agree with others. This thing doesn't look as algae. It can be fungus.

One more for a fungus...

Microscope photos look a lot like hypae. 

Please take a SEM photo as it will allow for clearer viewing of the topographical features and assist in identification. 

One remote possibility is a colony of cyanobacteria such as Trichodesmium or the like, but the pictures do not show clear filament structure so I cannot tell for sure. 

Lots of good suggestions.  I missed the sizes given from start so it might still be an algae  i.e. a brown algae and an epiphytic/endophytic group such as Myriothrichia Streblonema. There are some drawings in Waern 1952, where the Myriothrichia repens has some similarities but then your speciemens are too large. My only suggestion nwo would be to send the pictures to Inger Wallentinus and ask her  she has looked at these small stuff in the Baltic Sea.

If it has siphonaceous filaments (as mentioned), occasionally branched and entangled filaments, then appears to me to be correspond  to Vaucheria despite its brownish colouration. Examination under higher power should readily confirm its identity.   

It does not look like Vaucheria. I also think about a fungus.

I think it look like Fungus with some algal filaments, I suggest to do a simple test with Indian ink to be sure there are algal filaments or not.

Regards

Thank you all for practical suggestions. Currently I don't have a chance to try SEM or Indian ink. Therfore I attached some additional micrographs (incl. darkfield and phase contrast)  of this challenging object. I hope that on that pictures will be more identification clues about potential hypha or siphonous alga. Pictures are taken under 10x, 100x and 400x magnification.

It looks like a lichen. good luck!

Fascinating and some great photos. I hope you are having as much fun with this as I am.   Well we need to analyze and list what we know.  First it appears that it is not segmented.  So that lets out a host of algae possibilities.  Since it is brown and not green, I am not competent to comment further.  Guessing, as some are doing, does not help but adds confusion and false trails.  So, start with what you do know about it.  Brackish water, Temperature range, etc. and you may begin to back into the identification. 

The University of Tartu is a Medical university.  I you telling me the university does not have a SEM?  That would be odd.  There must be one on campus somewhere. 

Dear Cloud Jon, I was not claiming that University of Tartu hasn't SEM facility at all. My point is that at the moment I don't have an opportunity to test this object under SEM. Therefore I try optical microscopy at first.

Thank you for the clarification.  Good luck. 

Dear Pritt, I think it appears the rizhoidal attachement system of some phaeophyceae, even Fucus. 

Hi Priit!

Since you identified that algae is? Your algae, the algae seems Chrysophyta Division, which you mention, according to your description. Your photos do not help me much, to tell you more. Maybe if you separate a filament, and you take pictures at higher magnification 40x, 60x or 100x, I could help you more. You can do a cross-section to see the parenchyma, you say? You fix the thallus immediately after collecting? o is an alga with some decomposition?

Javier, thank You for very useful remark about rhizoids since the object is found from Fucus vesiculosus sample.

Agree with Javier

Can you try extracting chlorophyll or other pigments to determine if it is an algae? Interesting idea about impossibly being a rhizoid. Look at the image at balticseaweeds.com.

Not impossibly, possibly!! Spell check got me again. The image on the Baltic site looks lot like yours. (Fucus rhizoids).

Now I had tested if it has chlorophyll a or not. Chl concentration  value is 0.

 For closer view of this specimen I took some DIC illumination pictures (attached) under 200x and 600x magnification. There can be see some cytoplasma in the filaments/hypae and a few brown granules (vacuole or nucleus). Since I couldn't detect chlorophyll at all in the speciemen I can rather conclude that it probably not rhizoids of Fucus or any algal tissue. I found some evidence of marine fungi that similar filaments growing on the Fucus (link below). As I understand that marine fungi can be identify to species/genus is possible only by molecular methods with fresh material.

http://www.aphotomarine.com/fungi_marine_saw_wrack_03-09-13.html

Great photos. Without chlorophyll I agree with the decision to go with fungus. 

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