In vitro anti-cyanobacterial activity of copper roof granules towards Gloeocapsa alpicola

Gloeocapsa spp. are a type of airborne cyanobacteria that are known for causing black streaks on asphalt roofs. Embedding copper containing granules into asphalt shingles was developed to prevent cyanobacterial growth on roof shingles. There are currently no established methods for testing the anti-cyanobacterial activity of copper roofing granules. First, the minimum inhibitory concentration (MIC) of Cu2+ ions in solution was evaluated by monitoring growth in a 96 well plate over 14 days. Using a Gompertz function the MIC of Cu2+ was determined to be 0.592 ppm for G. alpicola. To test copper granules used in shingles, the granules were either autoclaved in BG11 media or sterilized using anhydrous ethanol, dried, and added to sterile BG11. Once inoculated with G. alpicola, growth was monitored using optical density (OD) and fluorescence measurements. Debris from the granules interfered with OD measurements but fluorescence clearly showed granules formulated with copper prevented cyanobacterial growth and those without did not. This work is of practical use for companies looking to formulate anti-cyanobacterial products like roof granules as it reports a specific MIC for this airborne species of cyanobacteria often found on roofs and also validates a methodology for testing roof granules.