Similar(?) nitrogen fixing organisms are found in soils, and my impression is that a little more is known about them.

Interesting!
It's a while since I was formally into all this stuff but my recollection is that all nitrogen-fixing. whether in root nodules of legumes or by free living organisms, involves cyanobacterial activity at some point. Most of the free-living organisms are species of cyanobacteria ("blue-green algae" in the older texts) but it would be interesting to see whether the current DNA analysis is of something independent and completely new or freed fragments of something that previously had an integral (if unknown) existence but has subsequently become disintegrated. It is reasonable to infer from the article that the source of the DNA is "single-celled" but I'm not yet sure they've eliminated "all the usual suspects"; so little is known at this level.

Cheers, Rowan