Global transcriptome analysis of salt acclimated Prochlorococcus AS9601

Molecular processes leading to salt stress acclimation in the model cyanobacterium Prochlorococcus are not known. To address this, we used RNA sequencing (RNAseq) to compare the global transcriptome of two exponential-phase populations of Prochlorococcus AS9601 cells - acclimated to high salt (5%, w/v) and normal seawater salt (3.8%, w/v). Experiments showed that salt acclimated cells exhibit slower growth rates with a doubling time almost twice as controls. Approximately 1/3 of the genome was found to be differentially expressed (p-value <0.05), but a considerably large number of these genes are "hypothetical proteins" with unknown function. Transcript abundance were higher for genes involved in respiratory electron flow, carbon fixation, osmolyte/compatible solute biosynthesis and inorganic ion transport. Many of the highly expressed genes are 'high light inducible proteins' believed to be part of the general Prochlorococcus stress response. Transcript abundance were lower for genes involved in photosynthetic electron transport and cell division. The relative reduction in transcript abundance for genes encoding proteins containing heme groups and iron transporters suggests cellular iron requirements in salt acclimated cells maybe lower. The results presented here provide the first glimpse into global gene expression changes in Prochlorococcus cells due to salt stress.