Hello, colleagues;
Ordinarily I pursue identifications on my own or with the use of keys and papers that are available, but when it comes to filamentous cyanobacteria I have yet to find a useful resource in terms of dichotomous keys, cell/filament/heterocyte measurements, etc. For this reason I hope to reach out to the expertise of the community to help me identify this specimen.
It was collected at a terrestrial site of moderate humidity (shaded, dark, sheltered, and with reasonably high snowmelt and precipitation, though about 600 m from the nearest open water). The site is highly calcareous, and characterized by exposed limestone rock faces and fissures. The colony was found on a limestone ledge that was covered with soil and moss clumps, with the colony on top.
The colony was a striking turquoise color when it was collected, which has faded somewhat after collection to a pale/whitish turquoise. I have attached photos of the colony itself, closeup of the filaments with a dissecting scope, as well as high magnification photos of the filaments, cells, heterocytes, and the branching pattern.
A few notes about the specimen:
1. The average cell sizes are 8.1 x 6.5 μm (average of 9 cells, from different filaments). The cells range in shape from rectangular in one sense through roughly square to rectangular in the other sense, and seem to be thinner toward the filament tips. The apical cells are rounded, and some seem to be slightly swollen at the tips (not yet pictured).
2. Average filament width is 6.5 μm, not including the sheath.
3. Average heterocyte size is 8.3 x 7.3 μm (average of five).
4. The filament sheath has either a mucilagenous/gel sheath, or perhaps lime dust/calcium dust etc., on the outside (see photos).
5. False branching is present, but it is quite rare and so far seems to be mostly single branching. I have not yet seen any double branching. Branching does seem to sometimes occur at heterocytes which I understand is characteristic of Tolypothrix, but the specimen is very fragile and it is very challenging to photograph branching as it seems to fall into tiny pieces incredibly easily. The other possibility that comes to mind is Scytonema, of course.
Thank you all! Hopefully I can find some cyanobacterial nerds (I mean this as the most high praise) who might be interested. Please pass on accordingly if you know of anyone.
I am working on taking more photos of the branching patterns and am happy to take more photos if it will help identify this.
Michael.
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