Abstract
The effects of nitrogen source (N2, NO3 and NH4
1) 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 NO3 and NH4
1 synthesized some scytonemin but synthesis
increased when NH4
1 was depleted and growth had become dependent on NO3
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 longterm
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.