Complex sentence: An explanation is that the microbes use sulfate as a catalyst to breathe with ferric iron and metabolize the small amounts of organic matter trapped with them long ago, which has never been observed in a natural environment.
Split into simpler sentences: An explanation is that the microbes use sulfate as a catalyst to breathe with ferric iron and metabolize the small amounts of organic matter trapped with them long ago. While this is theoretically possible, it has never been observed in a natural environment.