Penny for your thoughts.
Healthy cells of bioluminescent bacteria produce light, those that are damaged are dimmer, and those that are dead are dark. Because of this, they are commonly used in laboratories as sensitive and effective monitors of pollution, for testing environmental samples and drinking water for example. Here coins have been placed onto a confluent layer of bioluminescent bacteria. These have given rise to obvious zones of darkness around the coins which means that chemical toxins have diffused from these into the media and killed the bacteria. Most likely this toxic effect is due to the presence of metals like copper and sliver in the coins. There is also an obvious metaphor here.