Usage of the word "steward"

It's right there in the definition you quote, entry (2).

"Someone who protects or is responsible for ... etc." - what they are responsible for is 'the reputation and honor of science'

See also stewardship

the responsible overseeing and protection of something considered worth caring for and preserving.

It's common to see steward used in that context: A steward of the land; a steward of health care; a steward of the profession. It's used in the way that "servant" would be used in the selfsame contexts. If you google "a steward of" or "a servant of" you'll find numerous examples.