FIELD RELATIONSHIPS BETWEEN SCYTONEMIN DENSITY, GROWTH, AND IRRADIANCE IN CYANOBACTERIA OCCURRING IN LOW ILLUMINATION REGIMES

In situ measurements of ultraviolet (UV) irradiance, carbon fixation, and scytonemin pigmentation were made on Scytonema populations from co ntrasting localities in England. Significant negative correlations wer e obtained between the following variate pairs: pigmentation and UV ir radiance; pigmentation and carbon fixation rate. A significant positiv e correlation was found between pigmentation and sheath thickness. The negative correlation between pigmentation and UV irradiance was unexp ected and appeared contrary to the results of previous studies, which indicated a positive correlation between the variates and the recognit ion of scytonemin as a radiation shield. However, by considering how r adiation damage is related to cell division and the water relations of the sites investigated, it was shown that scytonemin is still functio ning as a radiation shield, even in shaded sites. Rivularia colonies p roduced scytonemin only upon their upper, sun-exposed surfaces and wer e positively correlated with UV irradiance. This paper also describes the successful use of some new and inexpensive techniques to measure p igments in cyanobacterium sheaths, and integrated in situ UV-irradianc e.