Sentence:  [The researchers] <WSD>noticed</WSD> many of their pieces of [plastic marine] debris sported surface pits around two microns across. Such pits are about the size of a bacterial cell. Closer examination showed that some of these pits did, indeed, contain bacteria, [...]. 
Meaning of the word marked with <WSD> tags:  To become aware of; to observe. (from 17th c.)