>Here's my question:  Does the term "cyanobacteria" refer to the bluish
>color that we see in our aquariums, or does it mean something else?  Many
>of us believe that cyanobacteria are capable of fixing atmospheric
>nitrogen.  I haven't seen experimental evidence that this is true, but it
>sure looks that way from my experiences with my tank.  I'm wondering if
>cyanobacteria may produce internal toxins to make it taste bad or even make
>it poisonous, but if these toxins might be a by-product of nitrogen
>fixation (CN- anions, perhaps).  I'm further wondering if the presence of
>an easier to metabolize nitrogen source such as NH4+ or NO3- might reduce
>the ability to produce such a toxin, if it exists.

They used to be called bluegreen algae, but cyanobacteria is being pushed
as the better name because it draws attention to the fact that they are not
related to other algae, but are large, photosynthetic bacteria.  Some of
them have a bluish pigment in addition to chlorophyll.  There are some
species that can fix nitrogen, but the ones that take over aquaria are
usually varieties of Oscillatoria, which does not have heterocysts, and I
assume it does not fix nitrogen.  The cyanobacteria are well-known
producers of defensive chemicals.  I have found that when the light is cut
way back they become much more palatable to snails and get eaten up fairly
quickly.  I doubt that the toxins are a byproduct of nitrogen fixation.
Those species that do fix N do it only in specialized cells, called
heterocysts that have heavy walls and no chlorophyll.  The enzyme most
directly involved in splitting atmospheric nitrogen apparently can only
function when oxygen levels are very low, and the heavy wall around the
heterocyst may function to keep oxygen out.

Paul Krombholz                  Tougaloo College, Tougaloo, MS  39174
In Jackson, Mississippi, where we had some freezing rain this morning.