Trophic levels refer to

The trophic level of an organism is the position it occupies in a food web. A food chain is a succession of organisms that eat other organisms and may, in turn, be eaten themselves. ... The word trophic derives from the Greek τροφή (trophē) referring to food or nourishment

Explanation:

this protocol provides a detailed procedure for agarose-based ip to purify and enrich the protein of interest.

i am going to guess that it is co2.