LONG-TERM EFFECTS OF UV AND VISIBLE IRRADIANCE ON NATURAL-POPULATIONSOF A SCYTONEMIN-CONTAINING CYANOBACTERIUM (CALOTHRIX SP.)

A natural warm spring stream with a cyanobacterial biofilm community c omposed almost entirely of Calothrix sp., a species rich in the UV-abs orbing sheath pigment scytonemin, was used for 3 month summer experime nts to test the following hypotheses: (1) UV radiation is necessary fo r the synthesis of high scytonemin content in sheaths. (2) High scyton emin content is required for uninhibited photosynthesis under high UV flux. Both of these hypotheses were answered affirmatively in earlier experiments with laboratory cultures of other cyanobacterial species. However, the tests of these hypotheses under natural conditions were n ecessary to confirm these conclusions, mainly because fluxes of UV and visible radiation and their ratios are very different under field con ditions. Intact mats of Calothrix, siliceous substrate cleared of Calo thrix, and artificial foam were treated for 3 months under filters tra nsparent to UV radiation and visible light, and under filters that exc luded only UV radiation. Neutral density screens were used in combinat ion with fillers to reduce the total irradiance over some sections of the stream. After 3 months, under all treatments, the predominant orga nism was still the same morphotype of Calothrix. Intact Calothrix mats produced high levels of scytonemin over the summer, except under low UV and low visible irradiance, while chlorophyll-a values per unit are a remained relatively low and sh owed little change under all treatmen ts. After 3 months, intact Calothrix mat and colonized mat with substa ntial scytonemin were not inhibited photosynthetically by UV irradianc e. Only colonized populations exposed to similar to 12% UV and similar to 12% visible irradiance, and with very low scytonemin content, show ed significant UV-inhibition. Thus, the correlation between UV protect ion and scytonemin presence was established experimentally for the fir st time under solar irradiance in a natural, essentially monospecific population of a cyanobacterium.