These organisms are not completely different in their chemical makeup from what we already know.

From what I can tell, the microbes prefer to live "normally" but may insert arsenic as a substitute for phosphorus when conditions demand it -- arsenic can play the same role that phosphorus would play under normal circumstances. This is a great novelty. Arsenic is bigger and heavier than phosphorus, and its compounds are less stable. These organisms would not have done this unless they didn't have any other choice. Just like Dr. Gerald Joyce, who was quoted in the New York Times today, I feel sorry for these creatures.