Global Transcriptional Responses of the Toxic Cyanobacterium, Microcystis aeruginosa, to Nitrogen Stress, Phosphorus Stress, and Growth on Organic Matter

Whole transcriptome shotgun sequencing (RNA-seq) was used to assess the transcriptomic response of the toxic cyanobacterium Microcystis aeruginosa during growth with low levels of dissolved inorganic nitrogen (low N), low levels of dissolved inorganic phosphorus (low P), and in the presence of high levels of high molecular weight dissolved organic matter (HMWDOM). Under low N, one third of the genome was differentially expressed, with significant increases in transcripts observed among genes within the nir operon, urea transport genes (urtBCDE), and amino acid transporters while significant decreases in transcripts were observed in genes related to photosynthesis. There was also a significant decrease in the transcription of the microcystin synthetase gene set under low N and a significant decrease in microcystin content per Microcystis cell demonstrating that N supply influences cellular toxicity. Under low P, 27% of the genome was differentially expressed. The Pho regulon was induced leading to large increases in transcript levels of the alkaline phosphatase phoX, the Pst transport system (pstABC), and the sphX gene, and transcripts of multiple sulfate transporter were also significantly more abundant. While the transcriptional response to growth on HMWDOM was smaller (5–22% of genes differentially expressed), transcripts of multiple genes specifically associated with the transport and degradation of organic compounds were significantly more abundant within HMWDOM treatments and thus may be recruited by Microcystis to utilize these substrates. Collectively, these findings provide a comprehensive understanding of the nutritional physiology of this toxic, bloom-forming cyanobacterium and the role of N in controlling microcystin synthesis.

本研究采用全转录组鸟枪法测序（RNA-seq），旨在评估产毒蓝细菌铜绿微囊藻（Microcystis aeruginosa）在三种培养条件下生长时的转录组响应：低浓度溶解态无机氮（低氮，low N）、低浓度溶解态无机磷（低磷，low P），以及高浓度高分子量溶解态有机物（HMWDOM）环境。在低氮条件下，该菌约三分之一的基因组基因发生差异表达，其中nir操纵子、尿素转运基因（urtBCDE）与氨基酸转运蛋白相关基因的转录本水平显著上调，而光合作用相关基因的转录本水平显著下调；同时，微囊藻毒素合成基因簇的转录水平显著降低，且单株铜绿微囊藻的微囊藻毒素含量显著下降，表明氮供给水平可影响细胞毒性。在低磷条件下，27%的基因组基因发生差异表达。Pho调控子被激活，导致碱性磷酸酶phoX、Pst转运系统（pstABC）及sphX基因的转录本水平大幅上调，同时多种硫酸盐转运蛋白的转录本丰度也显著升高。尽管在高分子量溶解态有机物培养条件下的转录响应幅度较小（差异表达基因占比为5%~22%），但该处理组中多种与有机物转运及降解特异性相关的基因转录本丰度显著提升，提示铜绿微囊藻可通过调控此类基因的表达来利用这些底物。综上，本研究结果全面解析了这一产毒水华蓝细菌的营养生理特性，以及氮元素在调控微囊藻毒素合成过程中的作用。