Complex sentence: An unexpected finding in "Chromera" was that a large (1μm diameter) ever-present organelle bounded by two membranes, and originally thought to be the mitochondrion, is not a mitochondrion but an extrusosome called the "chromerosome".
Split into simpler sentences: An unexpected finding in "Chromera" was a large (1 μm diameter) ever-present organelle bounded by two membranes, originally thought to be the mitochondrion. This organelle may not be a mitochondrion, but an extrusosome called the "chromerosome".