Effects of nitrogen source on the synthesis of the UV‐screening compound, scytonemin, in the cyanobacterium Nostoc punctiforme PCC 73102

The effects of nitrogen source (N 2 , NO 3 − and NH 4 + ) on scytonemin synthesis were investigated in the heterocystous cyanobacterium Nostoc punctiforme PCC 73102. With the required UVA radiation included, Nostoc synthesized three to seven times more scytonemin while fixing nitrogen than when utilizing nitrate or ammonium. A similar increase in scytonemin synthesis occurred when nitrate or ammonium became depleted by growth and Nostoc switched to diazotrophic metabolism with the differentiation of heterocysts. In addition, UVA‐exposed cultures grown in medium with both NO 3 − and NH 4 + synthesized some scytonemin but synthesis increased when NH 4 + was depleted and growth had become dependent on NO 3 − reduction. Although the mechanism is unclear, these results suggest that the greater the restriction in nitrogen accessibility, the greater the production of scytonemin. Perhaps the entire response may be an interaction between this restriction and a resultant sensitivity to UV radiation that acts as a cue for determining the level of scytonemin synthesis. Scytonemin is a stable UVR screening compound and appears to be synthesized by cyanobacteria as a long‐term solution for reducing UVR exposure and damage, but mainly or solely, when metabolic activity is absent. It is likely that during metabolic resurgence, the presence of a dense scytonemin sheath would facilitate the recovery process without the need for active defenses against UV radiation.