Sentence:  [The researchers] noticed 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, and that in several cases these bacteria were dividing and <WSD>thus</WSD>, by the perverse arithmetic of biological terminology, multiplying. 
Meaning of the word marked with <WSD> tags:  As a result.