Sentence:  Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis:.... The evolutionary precursor of photosynthesis is still under debate, and a new study sheds light. The critical component of the photosynthetic system is the "water-oxidizing complex", made up of <WSD>manganese</WSD> atoms and a calcium atom. 
Meaning of the word marked with <WSD> tags:  (Q731)  A metallic chemical element ("symbol" Mn) with an atomic number of 25, not a free element in nature but often found in minerals in combination with iron, and useful in industrial alloy production.