Nostoc (Anabaena) sp. PCC 7120 is a filamentous cyanobacterial species that fixes N₂
to nitrogenous compounds using specialised heterocyst cells. Changes in
the intracellular ratio of carbon to nitrogen (C/N balance) is known to
trigger major transcriptional reprogramming of the cell, including
initiating the differentiation of vegetative cells to heterocysts.
Substantial transcriptional analysis has been performed on Nostoc
sp. PCC 7120 during N stepdown (low to high C/N), but not during C
stepdown (high to low C/N). In the current study, we shifted the
metabolic balance of Nostoc sp. PCC 7120 cultures grown at 3% CO₂ by introducing them to atmospheric conditions containing 0.04% CO₂
for 1 h, after which the changes in gene expression were measured using
RNAseq transcriptomics. This analysis revealed strong upregulation of
carbon uptake, while nitrogen uptake and metabolism and early stages of
heterocyst development were downregulated in response to the shift to
low CO₂. Furthermore, gene expression changes revealed a
decrease in photosynthetic electron transport and increased
photoprotection and reactive oxygen metabolism, as well a decrease in
iron uptake and metabolism. Differential gene expression was largely
attributed to change in the abundances of the metabolites
2-phosphoglycolate and 2-oxoglutarate, which signal a rapid shift from
fluent photoassimilation to glycolytic metabolism of carbon after
transition to low CO₂. This work shows that the C/N balance in Nostoc
sp. PCC 7120 rapidly adjusts the metabolic strategy through
transcriptional reprogramming, enabling survival in the fluctuating
environment.