Data_Sheet_1_Acclimation and stress response of Prochlorococcus to low salinity.docx

Prochlorococcus is an obligate marine microorganism and the dominant autotroph in tropical and subtropical open ocean. However, the salinity range for growing and response to low salinity exposure of Prochlorococcus are still unknown. In this study, we found that low-light adapted Prochlorococcus stain NATL1A and high-light adapted strain MED4 could be acclimated in the lowest salinity of 25 and 28 psu, respectively. Analysis of the effective quantum yield of PSII photochemistry (Fv/Fm) indicated that both strains were stressed when growing in salinity lower than 34 psu. We then compared the global transcriptome of low salinity (28 psu) acclimated cells and cells growing in normal seawater salinity (34 psu). The transcriptomic responses of NATL1A and MED4 were approximately different, with more differentially expressed genes in NATL1A (525 genes) than in MED4 (277 genes). To cope with low salinity, NATL1A down-regulated the transcript of genes involved in translation, ribosomal structure and biogenesis and ATP-production, and up-regulated photosynthesis-related genes, while MED4 regulated these genes in an opposite way. In addition, both strains up-regulated an iron ABC transporter gene, idiA, suggesting low salinity acclimated cells could be iron limited. This study demonstrated the growing salinity range of Prochlorococcus cells and their global gene expression changes due to low salinity stress.

原绿球藻（Prochlorococcus）是专性海洋微生物，亦是热带及亚热带开阔大洋中的优势自养生物。然而，目前学界对于原绿球藻的适宜生长盐度范围及其对低盐暴露的响应机制仍不明晰。本研究发现，低光照适应型菌株NATL1A与高光照适应型菌株MED4可分别在盐度低至25与28 psu（实用盐度单位，practical salinity unit）的环境中完成驯化。通过对光系统II（Photosystem II, PSII）光化学有效量子产率（Fv/Fm）的分析结果显示，当盐度低于34 psu时，两株菌株均会受到胁迫。随后，我们对比了低盐（28 psu）驯化细胞与正常海水盐度（34 psu）下培养细胞的全转录组表达谱。NATL1A与MED4的转录组响应模式存在显著差异：NATL1A的差异表达基因（differentially expressed genes, DEGs）数量达525个，多于MED4的277个。为应对低盐胁迫，NATL1A下调了与翻译、核糖体结构与生物发生及ATP生成相关的基因转录本，并上调了光合作用相关基因；而MED4的基因表达调控趋势恰好相反。此外，两株菌株均上调了铁ABC转运蛋白基因idiA，提示低盐驯化的原绿球藻细胞可能面临铁限制。本研究明确了原绿球藻的可生长盐度范围，并揭示了其在低盐胁迫下的全基因组表达变化特征。