Sentence:  [The researchers] noticed many of their pieces of [plastic marine] debris sported surface pits around two microns across....Closer examination showed that some of these pits did, indeed, contain bacteria, and that in several cases these bacteria were dividing and thus, by the perverse arithmetic of biological terminology, <WSD>multiplying</WSD>. 
Meaning of the word marked with <WSD> tags:  To breed or propagate.