Fe limitation decreases transcriptional regulation over the diel cycle in the model diatom Thalassiosira pseudonana

Iron (Fe) is an important growth factor for diatoms and its availability is further restricted by changes in the carbonate chemistry of seawater. We investigated the physiological attributes and transcriptional profiles of the diatom Thalassiosira pseudonana grown on a day: night cycle under different CO2/pH and iron concentrations, that in combination generated available iron (Fe’) concentrations of 1160, 233, 58 and 12 pM. We found the light-dark conditions to be the main driver of transcriptional patterns, followed by Fe’ concentration and CO2 availability, respectively. At the highest Fe’ (1160 pM), 55% of the transcribed genes were differentially expressed between day and night, whereas at the lowest Fe’ (12 pM), only 28% of the transcribed genes displayed comparable patterns. While Fe limitation disrupts the diel expression patterns for genes in most central metabolism pathways, the diel expression of light- signaling molecules and glycolytic genes was relatively robust in response to reduced Fe’. Moreover, we identified a non-canonical splicing of transcripts encoding triose-phosphate isomerase, a key-enzyme of glycolysis, generating transcript isoforms that would encode proteins with and without an active site. Transcripts that encoded an active enzyme maintained a diel expression at low Fe’, while transcripts that encoded the non-active enzyme lost the diel expression. This work illustrates the interplay between nutrient limitation and transcriptional regulation over the diel cycle. Considering that future ocean conditions will reduce the availability of Fe in many parts of the oceans, our work identifies some of the regulatory mechanisms that may shape future ecological communities.铁(Fe)是硅藻重要的生长因子，而海水碳酸盐化学组成的变化会进一步限制其生物可利用性。本研究以昼夜循环培养的假微型海链藻（Thalassiosira pseudonana）为研究对象，设置不同CO₂/pH与铁浓度梯度，组合后得到的可利用态铁(Fe’)浓度分别为1160、233、58与12 pM，并对其生理特性与转录组特征展开分析。研究结果显示，明暗循环是调控转录模式的首要驱动因素，其次依次为可利用态铁浓度与CO₂有效性。在最高可利用态铁浓度（1160 pM）组中，55%的转录基因呈现昼夜差异表达；而在最低可利用态铁浓度（12 pM）组中，仅28%的转录基因表现出相似的昼夜表达模式。尽管铁限制会破坏多数中心代谢通路基因的昼夜表达模式，但光信号分子与糖酵解相关基因的昼夜表达在铁浓度降低的条件下仍保持相对稳定。此外，本研究还发现编码糖酵解关键酶磷酸丙糖异构酶的转录本存在非经典剪接现象，由此产生的转录本异构体可分别编码带有活性位点与不带活性位点的蛋白质。编码活性酶的转录本在低铁条件下仍保持昼夜表达模式，而编码非活性酶的转录本则丧失了昼夜表达节律。本研究阐明了营养限制与昼夜周期中转录调控之间的相互作用。鉴于未来海洋环境将降低全球多数海域的铁生物可利用性，本研究揭示了可能影响未来海洋生态群落结构的部分调控机制。