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 contrasting localities in England. Significant negative correlations were obtained between the following variate pairs: pigmentation and UV irradiance; pigmentation and carbon fixation rate. A significant positive correlation was found between pigmentation and sheath thickness.The negative correlation between pigmentation and UV irradiance was unexpected and appeared contrary to the results of previous studies, which indicated a positive correlation between the variates and the recognition of scytonemin as a radiation shield. However, by considering how radiation damage is related to cell division and the water relations of the sites investigated, it was shown that scytonemin is still functioning as a radiation shield, even in shaded sites. Rivularia colonies produced scytonemin only upon their upper, sun-exposed surfaces and were positively correlated with UV irradiance. This paper also describes the successful use of some new and inexpensive techniques to measure pigments in cyanobacterium sheaths, and integrated in situ UV-irradiance.