Heterocyte production, gene expression, and phylogeography in Raphidiopsis (=Cylindrospermopsis) raciborskii

The response of the cyanobacterium Raphidiopsis raciborskii to nitrogen limitation by following heterocyte production and its genetic background, and new understandings of its phylogeography. Raphidiopsis (=Cylindrospermopsis) raciborskii was described as a subtropical-tropical cyanobacterium, later reported expanding into temperate regions. Heterocyte presence used to distinguish Cylindrospermopsis from the very similar Raphidiopsis, but recently the two genera were recognized as one and unified. This study aimed to investigate how heterocyte production is related to nitrogen (N) limitation in heterocytous and non-heterocytous strains of R.raciborskii. High N-concentrations did not inhibit heterocyte development in some strains, while prolonged N-starvation periods never stimulated production in others. RT-qPCR was used to examine the genetic background, through the expression patterns of nifH, ntcA and hetR. While gene expression increased under N-restriction, N-sufficiency did not suppress nifH transcripts as previously observed in other diazotrophyc cyanobacteria, suggesting that heterocyte production in R. raciborskii is not regulated by N-availability. Heterocytous and non-heterocytous strains were genotypically characterized to assess their phylogenetic relationships. In the phylogenetic tree, clusters were intermixed and confirmed Raphidiopsis and Cylindrospermopsis as the same genus. The tree supported previous findings of earlier splitting of American strains, while contesting the African origin hypothesis. The existence of two lines of Chinese strains, with distinct evolutionary patterns, is a significant addition that could lead to new hypotheses of the species biogeography.

Automated summary

This study investigated the response of cyanobacteria Raphidiopsis raciborskii to nitrogen limitation and its genetic background, shedding new light on heterocyte production and phylogeography. The findings suggest that heterocyte development in R. raciborskii is not regulated by nitrogen availability. Genetic analysis revealed differences between strains. The study also confirmed Raphidiopsis and Cylindrospermopsis as the same genus and proposed new hypotheses on the biogeography of the species.