An in situ enclosure experiment to test the solar UVB impact on plankton in a high-altitude mountain lake. II. Effects on the microbial food web

We studied the impact of ambient levels of solar ultraviolet B (UVB) radiat ion on the planktonic microbial food web (viruses, heterotrophic bacteria, heterotrophic nanoflagellates and ciliates) of a high-mountain lake (2417 m above sea level) under in situ conditions for 16 days. Enclosures of 1 m(3 ) receiving either the full sunlight spectrum or sunlight without UVB radia tion were suspended at the lake surface. We found that the abundance of het erotrophic nanoflagellates was always lower in the +UVB treatment than in t he -UVB one. In addition, bacterial consumption, measured by the disappeara nce of fluorescently labelled bacteria, was significantly (P < 0.05) reduce d in the +UVB treatment. The abundance of non-filamentous bacteria (<10 mu m long) was also lower in the +UVB treatment, suggesting a direct effect of UVB on their growth. This was supported by the significantly (P < 0.05) lo wer cell-specific activity ([H-3]thymidine incorporation) found on the fift h day of the experiment. In contrast, WE radiation had no effect on filamen tous bacteria (>10 mu m long), which represented only a small fraction of t he total abundance (<4%), but up to similar to 70% of the total bacterial b iovolume. Ciliates, mainly Urotricha pelagica and Urolricha furcata, were l ess impacted by UVB radiation, although the net growth rate during the firs t week of the experiment was lower in the +UVB treatment than in the -UVB o ne (0.22 and 0.39 day(-1), respectively). The abundance of virus-like parti cles during the first week of the experiment was higher in the -UVB treatme nt. After reaching the maximum value for the interaction viruses x bacteria , viral abundance decreased dramatically (by similar to 85%) in both treatm ents with a decay rate of similar to 0.017 h(-1). This study illustrates th e complexity in assessing the impact of UVB radiation when more than one tr ophic level is considered and indicates the existence of different sensitiv ity to UVB radiation among components of the microbial food web.