EFFECT OF UV-B RADIATION ON CILIATED PROTOZOA - Environmental UV Radiation: Impact on Ecosystems and Human Health and Predictive Models - page 223

Among these systems, ciliates deserve a particular attention, for their importance
in many ecosystems. The Ciliata, or Ciliophora, include about 7000 known species of
some of the most complex single-cell organisms ever. Ciliates include organisms such
as Tetrahymena , Paramecium , Stentor , Euplotes , Fabrea , Blepharisma and
Ophryoglena . They derive their name from the Latin word cilium (eyelash), which gives
an immediate idea of the aspect of many ciliates. In fact, some or all of the surface of a
ciliate is covered with relatively short, dense and fine hair-like structures, the cilia, that
beat rhythmically and in an orderly fashion to propel the ciliate through the water and/or
to draw in food particles. Ciliates can be found in almost every environment with liquid
water: ocean waters, marine sediments, lakes, ponds and rivers, and even soils. This
have a great ecological importance. For instance, ciliates are especially important
trophic links in microbial food webs, because they are the major consumers of bacteria,
pico- and nano-plankton, diatoms, dinoflagellates, and amoebae. In their turn, they are
eaten by higher organisms, such as crustacea in the zooplankton and larval fish.
They also play a role in cleaning water in sewage treatment plants, where some
species eat sewage and others feed upon the bacteria that grow on the waste. Because of
their role in microbial food network and their sensitivity to pollutants, which may vary
greatly from a species to another, they may be used as reliable indicators of the state of
the microbial environment and can give quick information on the degree of its pollution.
Even though ciliates are unicellular organisms, they do possess a complex
morphology and physiology and are, moreover, extremely sensitive to environmental
signals; they can perceive a wide variety of stimuli, from mechanical to thermal, from
chemical to optical and gravitational. In many cases, their reaction to these stimuli is
shown as an alteration of their motile behavior. These perception functions are all
integrated in a single cell and all participate in controlling the motile behavior; in fact,
the final step of a stimulus-induced behavior consists in an alteration of the membrane
potential and a modulation of the activity of the motile apparatus. The presence of all
these control circuits in the same ciliate cell has led some author to speak of ciliates as
“walking neurons”.
It has been known for a long time that exposure to artificial UV irradiation at
high doses can kill these cells; at lower doses, it can destroy (or seriously damage) many
of the control circuits and of the ultrastructural elements of the cell itself.
This suggests that also the increase of environmental UV can alter the ciliate
physiology, leading to a decrease of their survival 2-4 . Since ciliates are decisive
components of the food chain, it is very important, from an ecological point of view, to
understand how they are affected by UV irradiation, and UV-B (280-320 nm) in
particular, and if they possess repair mechanisms.
Many authors have worked on this subject, exploring a wide range of possible
effects of UV exposure (from cell elongation to alterations in the reproduction cycle)
often using different investigation methods; the first part of this chapter will give the
reader a schematic summary of these works.
3. Early studies and general considerations
The first studies were mainly aimed at determining the relationship between UV
exposure and cell survival, trying to establish the minimum dose sufficient to severely