Very nice!
Paranassula is a marine genus, I believe (and not a nassulid, oddly! It is currently considered a peniculid, like
Paramecium).
Nassula aurea is now in the genus
Obertrumia. That genus is differentiated from other nassulids by a small difference in a structure called the "hypostomial frange," which in
Nassula is continuous and in
Obertrumia is in two parts. Overall,
O. aurea has a very different shape, without the anterior constriction we see here (it is typically a rather plump ciliate, and broader in the anterior than the posterior). I'm quite sure this isn't
O. aurea.
As far as I know,
N. gracilis has not been redescribed since Kahl's description in 1931. Your ciliate doesn't have a distinct pigment spot, as reported for
N. gracilis, and it lacks the slight "beak" mentioned & depicted by Kahl. The cell shape does not strike me as similar.
I think this is likely a species of
Nassulopsis (which, I'm sorry to say, is not a nassulid but a synhymeniid, like
Chilodontopsis and
Zosterodasys 
). That genus is characterized by an oblique hypostomial frange which goes all the way around the cell. We don't really see the frange, here, but we the overall appearance of the cell is consistent with that genus. However, we don't see the number and distribution of contractile vacuoles clearly, and pigment granules don't seem to be concentrated in the anterior (forming a distinct blue or purple spot). So, I'm not completely sure! If you see them again, try to get a look at the frange...if it goes all the way across the cell,
Nassulopsis is likely.
(Another possibility is
Zosterodasys henarensis, a synhymeniid with a distinctly constricted anterior region, originally found in the Henares river, in Spain. However, according to Vdacny & Tirjakova, "The generic home of this species is doubtful because of the
Nassulopsis-like body shape.")
