Do you happen to know why one of them lives inside Ficus carica? It
supposedly fixes extra nitrogen, & in the process makes the tree smell like you
forgot to clean the cat's litter pan.

I've not heard of this. Are you sure?

That is what I was told by a botany professor.

Where inside the plant are the blue-greens supposed to live?

In the leaves. I used to have a bonsai F. carica. When it was in the house, the
smell got strong whenever the sun shone directly on the leaves.

Cyanobacteria do live symbiotically in various plants [e.g., in some water
ferns (_Azolla_ spp.), in cycad "coralloid roots", inside the stems of
_Gunnera_] and do fix nitrogen, but I've never heard of any such role in Ficus
spp.. A web search and a try with Biological Abstracts both just turned up
nothing.

There are more things in heaven and earth, Horatio, than are met with on the
Web.

_Ficus carica_ foliage is often noticeably aromatic, either fresh or dried,
but I've never noticed the smell to be at all offensive the way you describe.

it is definitely an ammonia type smell. I got the same smell from fig trees in
Israel, and I also heard it occurs in Florida. When the tree is growing in the
ground outdoors, it is not that noticeable unless you walk right up to it.

Perhaps some cats have been doing their business around the fig tree?

On the contrary. At the time I had the fig tree I didn't own a cat. A friend of
mine compained that her neighbor's cat was "spraying" her fig tree, & had to
laugh when I told her it was the bacteria in the tree.
I gather that actually cyanobacteria are very busy in the higher plants.
This arrangement is not so strange when you stop to consider that F. carica is
the only really cold hardy member of the genus, and apparently the only one
with this symbiotic arrangement. Probably at some time during the Ice Age, F.
carica hit on this solution to the problem of a shorter growing season and less
light than its fellow species. Any genus that can come up with an inside-out
inflorescence should have no difficulty capturing a bacterium to further its
own ends.

Iris,
Central NY, Zone 5a, Sunset Zone 40
"If we see light at the end of the tunnel, It's the light of the oncoming
train."
Robert Lowell (1917-1977)