Chapter 25. Resident from another order. Cyanobacteria identification 8.

Change in my aquarium comes slowly (if at all), so I was interested when a fluffy brown organism began growing on my bogwood.

W312. The brown fluffy organism attached to the bogwood is probably cyanobacteria from the genus Scytonema.

Previous experience suggested that it was some form of cyanobacteria (almost every sample I have ever examined from the tank has turned out to be some form of cyanobacteria) but it had some unusual characteristics that made me wonder. Most noticeably, during the weekly water change when I remove the bogwood from the tank and scrub it with a toothbrush, this organism remained attached. This was not true for any of the other species I have described, although the brown unicellular species can take a lot of effort to remove (Chapter 18).

Scytonema filaments sampled from an aquarium ornament (x100 magnification).

At x100 magnification a network of what looked very much like cyanobacteria filaments was revealed. But from which genus? Returning to "Freshwater algae of North America: ecology and classification" (Wehr and Sheath Eds) and specifically chapter 4 on filamentous cyanobacteria, I made the following identification. Q1-Heterocytes and/or akinetes develop commonly or occasionally in trichomes?

Scytonema filament with a heterocyte. Filament width 14.5 to 16.5 micrometers (x1000 magnification).

From reading descriptions of heterocytes (cells specialising in fixing elemental Nitrogen N2) I would say the cell in the centre of the above image is a heterocyte (heterocyst). It looks very different from the other cells in the filament as it lacks the brownish pigment and cytoplasmic granules. Notice the pores connecting it to adjacent cells are clearly visible. The presence of heterocytes is significant for two reasons: Firstly because it means that this species is from a different order of filamentous cyanobacteria to those previously identified which were all Oscillatoriales; Secondly, because it suggests to me that N2 levels in the aquarium must be very low indeed. I say that because according to "Freshwater algae of North America" the frequency of heterocytes along the filament declines with greater levels of NH4+ or NO3- and I see lots of heterocytes. So I can say that a species of cyanobacteria that is not only capable of N2 fixation, but that is actively fixing N2, has out competed the cyanobacteria/algae normally found on the bogwood (see Chapter 22) . Q2-Trichomes never branched or only with false branching? The branches below look like diagrams and pictures of false branching to me, so I say yes and that means this species if from the order Nostocales!

False branching Scytonema filament (x400 magnification)

Q3-Filaments with lateral false branches? Yes. Q4-Filaments isopolar, solitary, entangled in clusters, or forming woolly mats with common lateral branches in pairs (V shape), or rarely with single (Y shape) branch? I don't see lateral false branches in pairs, but I do see the rarer Y shape branches so Yes=family Scytonemataceae. Q5-Trichomes cylindrical up to the ends, apical trichome ends not attenuated, end cells more or less rounded? Yes=genus Scytonema.

Once again my aquarium has proven itself an ideal habitat for cyanobacteria, which seem to be increasing in diversity. Also, Scytonema provided a good first test of my new microscope and camera which have captured some beautiful detail I think. It's a shame that cyanobacteria look so manky from a distance.