EXOSYMBIONT. “Like other eukaryotes, diatom cells contain mitochondria evolved from a single primary endosymbiotic event involving an α-proteobacterium. No trace of mitochondria remain in the secondary endosymbiont, and so it is believed that diatom mitochondria are derived from the exosymbiont.”
http://jxb.oxfordjournals.org/content/early/2012/01/20/jxb.err441.full