Sentence:  [The researchers] noticed many of their pieces of [plastic marine] debris sported surface <WSD>pits</WSD> around two microns across. Such <WSD>pits</WSD> are about the size of a bacterial cell. Closer examination showed that some of these <WSD>pits</WSD> did, indeed, contain bacteria, [...]. 
Meaning of the word marked with <WSD> tags:  A small surface hole or depression, a fossa.