From isolation to insights: mitochondrial complex I in the diatom <i>Phaeodactylum tricornutum</i>

Diatoms are among the most ecologically successful microalgae, contributing significantly to marine primary production and global carbon cycling. Their distinctive metabolic architecture, shaped by a complex evolutionary history involving secondary endosymbiosis, includes a highly compartmentalized cell organization and unique metabolic pathways. In Phaeodactylum tricornutum, a model pennate diatom, chloroplasts with four membranes and mitochondria of likely exosymbiotic origin exhibit intricate physical and metabolic interactions that support integrated carbon and nitrogen metabolism. The mitochondrial electron transport chain, essential for ATP synthesis, shows clade-specific structural and compositional adaptations. Despite its importance, detailed proteomic characterization has remained limited. Here, we report a method for the isolation of mitochondrial complex I from P. tricornutum and present a comprehensive proteomic analysis. Our results confirm the presence of carbonic anhydrase and bridge modules, both previously proposed as ancestral features of mitochondrial complex I, and identify at least one novel, clade-specific subunit that resembles NAD(P)H-dependent trans-2-enoyl-CoA/ACP reductases (TER) from other species. The subunit is similar to proteins involved in mitochondrial fatty acid biosynthesis. Our findings provide new insights into the composition, evolutionary conservation, and potential biotechnological relevance of this essential respiratory protein complex in diatoms.