Hey, I'm not even remotely a biologist (the closest I've ever come to a biology class was invertebrate paleontology) but I thought it a good thing to point out the opinions linked to in my previous message. But I had to throw something else in or be in violation of rule 5 (No bare links etc.).
There doesn't seem to have been the testing required to rule out the notion that this bacterium is merely using mechanisms to survive high arsenic and low phosphorous conditions rather than actually substituting arsenic for phosphorous.
The "bolded" is my "not based of any personal expertise" impression. I must leave it to those who can handle the "gory details" of that second cite to pass a much more qualified judgment.
Now, if you want to get into oddities in preCambrian geology, that's where I have a bit of personal expertise (but just a bit).
Moose