基于玛氏骨条藻(Skeletonema marinoi)转录组的碳固定代谢途径分析

Description of carbon fixation pathway based on Skeletonema marinoi transcriptome

以玛氏骨条藻(Skeletonema marinoi)转录组信息为基础,分析了其碳固定代谢途径,共发现18种酶对应的34个编码基因,构建了玛氏骨条藻进行碳固定代谢途径的通路图.这些编码基因的序列比对结果表明,其与假微型海链藻(Thalassiosira pseudonana)的同源基因具有较高的一致性.对这些样品进行数字基因表达谱的差异基因分析,获得了不同生长时期碳固定代谢途径酶编码基因的差异表达情况.通过分析发现,C3和C4代谢途径中存在表达差异的基因分别有7和3个,其中果糖-1,6-二磷酸酶和丙酮酸磷酸双激酶的编码基因表达在指数生长期之后出现显著上调.这有助于对玛氏骨条藻碳固定代谢途径中关键编码基因调控过程的解析,为进一步研究硅藻的固碳机制奠定了基础,也为深入了解碳的生物地球化循环提供了新的研究方向.

Diatoms can fix approximately 1016 g CO2 into organic carbon every year, equivalent to roughly 20% of global primary production. They are the basis for important food webs, having considerable impacts on carbon cycle and biological carbon pump that draws down CO2 from the atmosphere to the ocean interior. Nevertheless, the Km of their ribulose- 1,5-bisphosphate carboxylase/oxygenase （Rubisco） is 40-60 μmol L-1, higher than the apparent photosynthetic Km （CO2） for ambient CO2. Growth and kinetic studies have shown that diatoms can avoid CO2 limitation through CO2 concentrating mechanisms. Recently the carbon fixation metabolism pathway in diatoms is further elucidated by the presence of a complete set of genes essential for this metabolic route in Thalassiosira pseudonana and Phaeodactylum tricornutum because of their entire availablegenome information. In this study, we used RNA-Seq technology to confirm the existence of necessary genes for plant-like C4 activity in Skeletonema marinoi, which is a complex biological trait that enables S. marinoi to either has high water-use efficiency or accumulate biomass at a fast rate. The carbon fixation metabolism pathway of S. marinoi involves 20 enzymes and 34 coding genes, including the complete key enzymes especially for C4 pathway, which are phosphoenolpyruvate carboxylase （PEPC）, phosphoenolpyruvate carboxykinase （PEPCK） and pyruvate orthophosphate dikinase （PPDK）. Comparing with the homologous genes of carbon fixation metabolism pathway in T. pseudonana and P. tricornutum, these genes showed little divergence. Besides, the gene differential expression of S. marinoi in carbon fixation metabolism pathway was identified at growth stages using digital gene expression profiling. The results indicated the number of differential expression genes was 7 and 3 in C3 and C4 pathway, respectively. As for the differential expression of coding genes in growth stages, there were 8 and 10 coding genes in the stationary and decline phase, respectively versus to those in the exponential phase. The gene expression of fructose-l,6-bisphosphatase （FBP）, fructose-bisphosphate aldolase （ALDO） and PPDK was changed significantly after the exponential phase. The FBP and ALDO are not only responsible for the carbon fixation metabolism pathway, but also as the key enzymes in the glycolytic metabolism pathway, which links closely with the flow direction of carbon and the synthesis of carbohydrates. The PPDK, a cardinal enzyme of the C4 pathway, catalyzing the regeneration of phosphoenol pyruvate（PEP）, evolved as an adaptation to high light intensity, high temperature, low concentration of CO2 and dryness. Therefore, the formation of PEP through PPDK is considered to be very important in the C4 pathway. This study will contribute to analyze the gene regulation of key enzymes involved in the carbon metabolism of S. marinoi, explain the reason for huge amounts of primary production by diatom, and enhance our knowledge on the carbon fixation mechanism, which provides a new insight into understanding carbon biogeochemical cycle.