Recently, the genome for an exotic microbe known as syntrophus aciditrophicus was sequenced. The link goes to the PDF of the news article. I'd originally read it as an HTML story, but I'm not sure if that's open access. You may need access to Science to read these things anymore.
Anyhow, the microbe is already interesting because it's a syntroph. This means that it can't exist on its own but relies on a symbiotic relationship with another microbe, typically a methanogen. S. aciditrophicus creates H2 in the course of its metabolism. However, build-up of the gas shuts down its metabolic processes, making them energetically unfavorable. Thus it partners with another microbe which can utilize the H2 and thus survives.
This diagram take from the article linked aboveThere's quite a few syntrophs out there, and they have similar metabolic situations. What makes the genomic sequence of S. aciditrophicus so interesting is that it's missing all kinds of genes that they would expect to be there for the metabolism it seems to perform. Many of the genes found have only limited homology to known genes with appropriate functions, but many are just simply not there.
This leads to all kinds of questions about how the thing actually survives. Does it utilize its partner for these missing steps? Does it do the steps with some protein which simply hasn't been identified? Or does it use some novel pathway for accomplishing these processes?
It's an exciting mystery as far as I'm concerned. Your mileage may vary.