• Although most of the tens of thousands of diatom species are photoautotrophs, a small number of heterotrophic species no longer photosynthesize. We sequenced the genome of a nonphotosynthetic diatom, Nitzschia Nitz4, to determine how carbon metabolism was altered in the wake of this trophic shift.
• 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 adenosine triphosphte (ATP) yield. The genome contains a β-ketoadipate pathway that may allow Nitzschia Nitz4 to metabolize lignin-derived compounds.
• 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.
• 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.

中文翻译：

---

非光合作用硅藻的基因组提供了对异养代谢转变和光合作用损失限制的见解

• 尽管数以万计的硅藻物种中的大多数是光合自养生物，但少数异养物种不再进行光合作用。我们对非光合硅藻Nitzschia Nitz4 的基因组进行了测序，以确定在这种营养转变之后碳代谢是如何改变的。
• Nitzschia Nitz4 保留了其质体和质体基因组，但与向异养性转变相关的变化是整个细胞范围的，包括核和质体基因组中光合作用相关基因的丢失、质体中类异戊二烯生物合成的消除以及线粒体糖酵解的重塑最大限度地提高三磷酸腺苷 (ATP) 的产量。基因组包含一个 β-酮己二酸途径，可能允许Nitzschia Nitz4 代谢木质素衍生化合物。
• 硅藻质体缺乏氧化戊糖磷酸途径 (oPPP)，光合作用成为 NADPH 的主要来源，以支持质体中的基本生物合成途径，进而限制非光合质体中 NADPH 的可用来源。
• 基因组揭示了非光合硅藻和顶复门寄生虫之间在质体中提供 NADPH 的相似性，并强调了质体 oPPP 的祖先缺失是光合作用丧失的潜在重要限制，这一假设得到了谱系中向寄生或异养的更高频率转变的支持具有质体 oPPP。