The end result appears to be bacteria that have incorporated arsenate into their metabolism. They still have phosphate in them, but not enough to keep everything running on a phosphate basis. Some parts have switched over to arsenate, without gumming up the works completely. That surprises me quite a bit - I really wouldn't have thought that things could be pushed that far. After all, in higher organisms, it's that arsenate-for-phosphate switch that's responsible for arsenic's reputation as a poison. Eventually, some key enzyme systems can't handle the switch and cease to function.
But not in these bacteria. They look different and grow more slowly than their phosphate-saturated brethren, and they'd clearly like ditch the arsenic at the first opportunity (add phosphate and they start growing more vigorously). But they're getting by, presumably with just enough phosphate to hold things together. (Have they hit the wall, one wonders?) A number of physical methods all point in the same direction, to arsenate being incorporated into their biomolecules. We still don't know where most of it goes, or how the various phosphate-manipulating enzymes manage to still work, but working out those details will keep a lot of people busy for quite a while. Personally, I'd love to see some X-ray structures of aresenate-containing proteins or nucleic acids, and I'm sure that the people who reported this are trying to get some.
So what does this mean? Well, you can apparently bend the most basic chemistry of life as we know it quite a bit before it breaks. As I said, I really would not have thought that this could be possible - we're all going to have to keep rather more open minds about what biochemical systems can handle. This makes the arsenic-from-the-ground-up idea look a lot more plausible, too, and you can be sure that the search for such organisms (using arsenate naturally, without having to be forced in the lab) will intensify.
