EFFECT OF UV-B RADIATION ON CILIATED PROTOZOA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 232

Effect of UV-B radiation on photomotile responses
Photomotile responses of both B. japonicum and F. salina are impaired by UV-B
irradiation 18,19 .
B. japonicum reacts to light stimuli exhibiting step-up photophobic responses:
upon a sudden increase in light fluence rate the cells stop, turn and start swimming
again in a new direction. Action spectra indicated that these responses are mediated by
the endogenous pigment blepharismin 27-29 . UV-B irradiation caused a specific inhibition
of step-up photophobic responses (see Fig. 8): after 30 min of UV-B irradiation, as
described in the previous section, 50% of still motile cells were unable to respond to
photic stimuli and after 60 min irradiation the photoresponsiveness was totally
suppressed. This inhibition was specifically determined by UV-B radiation; in fact,
irradiation with visible light only did not affect in any way the photophobic response 18 .
Fig. 8. Fraction of motile cells of B. japonicum showing step-up photophobic response as a function
of irradiation time in the visible and in the UV-B range (redrawn from ref. 18).
F. salina shows two kinds of photomotile responses: positive phototaxis (cell
displacement towards the light source) and step-down photophobic reaction (upon a
sudden decrease in light fluence rate the cells stop, turn and start swimming again in a
new direction) 30,31 . As previously mentioned it is not clear at present if these responses
are mediated by a rhodopsin like pigment and/or by a hypericin-like pigment 32 .
Preliminary, qualitative, observations indicate a UV-induced damage to the photophobic
step-down reaction (unpublished data).
F. salina phototactic reaction is strongly inhibited by UV-B irradiation 19 .
Phototaxis decreased with UV-B irradiation time (at the above-described experimental
conditions) and completely disappeared after one hour (Fig. 9). Control samples, as
defined in the previous section, show a slight decrease in phototaxis after two hour
irradiation, but this decrease is significantly smaller than that of UV-B treated samples.
The inhibition of phototactic responsiveness, except for the shortest exposure time, was
irreversible (see Fig. 10).