There is no way that a statement like that can be believed. It is a theory, as with the people who believe that the earth turned into a snowball and ended most life or was that another time.
As the article says: "Finding evidence of green sulfur bacteria would provide evidence for hydrogen sulfide as the cause of the mass extinctions."
It's research in progress, not intended to be believed, but rather examined and weighed.
Finding evidence of green bacteria would prove nothing. We are in the middle of an ice age. We are in the warming cycle of that ice age. Thank goodness it is warming or the ice would be puching at our door.
http://www.museum.state.il.us/exhibits/ice_ages/when_ice_ages.html