EFFECT OF UV-B RADIATION ON CILIATED PROTOZOA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 225

Table 2. Relative resistance of different protozoa to UV-irradiation. The dose for 50% immobilization
of P. multimicronucleatum was set at 100 and the other values are relative to this (from ref. 5).

50%
rotation

50%
immobilization

Protozoan Size (µm)

Tetrahymena glaucoformia 25 x 50 146 ± 4.4

Colpidium colpoda 55 x 88 132 ± 6.8

Stylonychia curvata 48 x 98 34 ± 6.4 80 ± 4.7

Paramecium bursaria 43 x 105 87 ± 6.3 103 ± 37.0

Paramecium Aurelia 49 x 120 92 ± 0.3 119 ± 5.7

Paramecium multimicronucleatum

N. 1 51 x 175 76 ± 6.9 100 ± 4.2

N. 2 64 x 123 73 ± 22.0 116 ± 19

Paramecium caudatum 70 x 173 58 ± 4.4 101 ± 3.4

Blepharisma undulans 52 x 175 43 ± 3.2 59 ± 6.1

Spirostomum ambiguum 36 x 369 32 ± 3.3

Bursaria truncatella 200 x 337 112 ± 6.3 147 ± 14.1

Fabrea salina 169 x 225 555 ± 196 896 ± 126

the pigment deprivation produces an increase in UV-vulnerability 7 . However,
UV-resistance is not always correlated with pigmentation, as shown by our results on an
apparently colorless strain of Fabrea salina , which is more resistant to the same
radiation doses than the pigmented protozoa B. japonicum (see below). Even if these
findings are derived from measures of different parameters and are difficult to compare
to those of Giese 5-8 , they indicate that there is no direct relationship between pigment
content and UV-resistance.
Structural changes in the cell after UV-exposure have been described: cells can
elongate, vacuolate or the cell wall can break causing a cytolysis. Particular attention
has been given to the so called retardation effect: it has been noticed, in fact, that cells
exposed to UV (and UV-B in particular) strongly reduce the frequency of division 9 . The
delay in reproduction has been measured as a function of the UV-dose received and of
some other environmental factors such as composition of the culture medium, starving
period, etc., finding also in this case an interplay of all these variables.
In order to try to identify the molecular targets of UV radiation in ciliates, the
action spectrum for the retardation response has been determined in some species; the
results are, in certain cases, difficult to interpret: the action spectrum at low doses
resembles the absorption spectrum of cytoplasmic proteins, whereas the action spectrum
at larger doses is complex, with a contribution surely due to the DNA 8 (Figure 1).
The relationship between UV-damage and nucleic acids has been pointed out by
different authors; in general it is suggested that the molecular phenomena which take
place after UV exposure are complex, and involve both proteins and nucleic acids 8 .
Moreover, there are evidences demonstrating that after UV-exposure there may be some
dark reactions, which, in certain cases (for example in Didinium nasutum ), may amplify
the damages produced by UV itself 10 . In fact, a pulsed UV stimulation, at the same dose
of a continuous one, is much more efficient in inducing damages. These dark reactions,
being independent of light, are of a termochemical nature and, therefore, strongly
dependent on temperature 8-10 .