背景回顾：Although most of the tens of thousands of diatom species are photoautotrophs, a small number of heterotrophic species no longer photosynthesize. 

主要研究：We sequenced the genome of a nonphotosynthetic diatom, Nitzschia Nitz4, to determine how carbon metabolism was altered in the wake of this trophic shift.

结果1：Nitzschia Nitz4 has retained its plastid and plastid genome, but changes associated with the transition to heterotrophy were cellular-wide and included losses of photosynthesis-related genes from the nuclear and plastid genomes, elimination of isoprenoid biosynthesis in the plastid, and remodeling of mitochondrial glycolysis to maximize ATP yield. 

结果2：The genome contains a β-ketoadipate pathway that may allow Nitzschia Nitz4 to metabolize lignin-derived compounds. 

结果3：Diatom plastids lack an oxidative pentose phosphate pathway (oPPP), leaving photosynthesis as the primary source of NADPH to support essential biosynthetic pathways in the plastid and, by extension, limiting available sources of NADPH in nonphotosynthetic plastids. 

结论：The genome revealed similarities between nonphotosynthetic diatoms and apicomplexan parasites for provisioning NADPH in their plastids and highlighted the ancestral absence of a plastid oPPP as a potentially important constraint on loss of photosynthesis, a hypothesis supported by the higher frequency of transitions to parasitism or heterotrophy in lineages that have a plastid oPPP.

 摘 要 

尽管在数万种硅藻类植物中，绝大多数物种都是光自养生物，但还存在少数的异养物种不再进行光合作用。本文中，作者对非光合类硅藻Nitzschia Nitz4进行了全基因组测序，研究碳代谢在这种营养方式转变后是如何改变的。Nitzschia Nitz4保留了质体和质体基因组，但与异养转化相关的变化是整个细胞范围的，包括核和质体基因组中与光合作用相关基因的丢失，并且质体中不再合成异戊二烯，线粒体中糖酵解途径发生了重塑，从而最大限度地提高了ATP产量。Nitzschia Nitz4基因组包含一条β-己二酸酮途径，该途径可能使得Nitzschia Nitz4能够代谢木质素衍生的化合物。硅藻质体缺少一个氧化戊糖磷酸途径（oPPP），使得光合作用成为NADPH的主要来源，以支持质体中基本的生物合成途径，进而限制非光合质体中的NADPH来源。Nitzschia Nitz4基因组揭示了非光合硅藻和顶复门原虫之间在质体中供应NADPH方面的相似性，并突出了质体oPPP的古老缺失是光合作用丧失的潜在重要制约因素，而在具有质体oPPP的谱系中，向寄生或异养转变的频率更高。