Long-term effects of UV and visible irradiance on natural populations of a scytonemin-containing cyanobacterium ( Calothrix sp.)

Abstract

A natural warm spring stream with a cyanobacterial biofilm community composed almost entirely of Calothrix sp., a species rich in the UV-absorbing sheath pigment scytonemin, was used for 3 month summer experiments to test the following hypotheses: (1) UV radiation is necessary for the synthesis of high scytonemin content in sheaths. (2) High scytonemin 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 necessary to confirm these conclusions, mainly because fluxes of UV and visible radiation and their ratios are very different under field conditions. Intact mats of Calothrix, siliceous substrate cleared of Calothrix, and artificial foam were treated for 3 months under filters transparent to UV radiation and visible light, and under filters that excluded only UV radiation. Neutral density screens were used in combination with filters to reduce the total irradiance over some sections of the stream. After 3 months, under all treatments, the predominant organism 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 area remained relatively low and showed little change under all treatments. After 3 months, intact Calothrix mat and colonized mat with substantial scytonemin were not inhibited photosynthetically by UV irradiance. Only colonized populations exposed to ∼12% UV and ∼12% visible irradiance, and with very low scytonemin content, showed significant UV-inhibition. Thus, the correlation between UV protection and scytonemin presence was established experimentally for the first time under solar irradiance in a natural, essentially monospecific population of a cyanobacterium.