Sentence:  Plastics are energy-rich substances, which is why many of them burn so readily. Any organism that could unlock and use that energy would do well in the Anthropocene. Terrestrial bacteria and fungi which can manage this trick are <WSD>already</WSD> familiar to experts in the field. 
Meaning of the word marked with <WSD> tags:  Prior to some specified time, either past, present, or future; by this time; previously.