Sentence:  Oxygen levels on Earth skyrocketed 2.4 billion years ago, when cyanobacteria evolved photosynthesis: the ability to convert water and carbon dioxide into carbohydrates and waste oxygen using solar energy. 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 manganese <WSD>atoms</WSD> and a calcium <WSD>atom</WSD>. 
Meaning of the word marked with <WSD> tags:  The smallest possible amount of matter which still retains its identity as a chemical element, now known to consist of a nucleus surrounded by electrons. (from 16th c.)