In the sentence extracted from biomedical literature, identify all the chemical entities. The required answer format is the same sentence with HTML <span> tags to mark up specific entities. 
### Entity Markup Guides:
Use <span class="chemical"> to denote a chemical.

### Input Text: The in vivo protection was associated to the prevention of oxidative stress and preservation of mitochondrial oxygen consumption and activity of respiratory complex I , and the in vitro protection was associated to the prevention of ROS production .
### Output Text:
The in vivo protection was associated to the prevention of oxidative stress and preservation of mitochondrial <span class="chemical">oxygen</span> consumption and activity of respiratory complex I , and the in vitro protection was associated to the prevention of ROS production . 