re: Two lifeforms merge in once-in-a-billion-years evolutionary event

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Yeah, they're called legumes. Although, I think they use bacteria inhabiting their root systems to fix nitrogen.

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If I recall from my college microbiology days these bacteria are provided an environment by the plant to survive and the bacteria fix atmospheric nitrogen for the plants nutrients. This is an endosymbiotic relationship as opposed to an ectosymbiotic relationship. A very common example of ectosymbiosis are mistletoe growing in trees or remora fish hanging out with sharks.

What is unique about this ectosymbiotic relationship is that when the host algae divides via reproduction so does the internal bacterium acting as an eukaryotic organelle. At least that is my understanding of the key finding from the article.