EFFECT OF UV-RADIATION ON THE BACTERIVORY OF A HETEROTROPHIC NANOFLAGELLATE

The effects of UV-B radiation on the heterotrophic nanoflagellate Bodo saltans (Kinetoplastida) were examined under controlled conditions wi th artificial UV sources and also under natural solar radiation in an oligotrophic lake. In both types of experiments, the characteristic el ongated cell morphology of this flagellate changed into a spherical on e. This effect was due to UV-B but also to W-A radiation, and after 1 h of exposure at 0.5 m of depth, 99% (UV-B plus UV-A plus photosynthet ically active radiation) and 69% of the cells (W-A plus photosynthetic ally active radiation) were spherical, At 6 m of depth where only 10% of the UV-B (305 nm) at the surface was measured, no significant effec t was observed. The spherical cells were nonmotile, but before the mor phological change took place, the swimming speed was ca. 3.5 times low er in the plus-UV-B treatment. The negative relation between the abund ance of spherical cells and the average ingestion of fluorescently lab eled bacteria per cell indicates that these cells are not able to feed upon bacteria, In bacterivory experiments lasting for 6 h, the total number of grazed bacteria was up to 70% lower in the plus-UV-B treatme nt than in the control without UV-B, This resulted in a positive feedb ack between UV-B and bacterial growth, The high sensitivity of B, salt ans to solar W-B and UV-A radiation strongly reduces its ability to li ve near the surface at times of high UV radiation.