https://phys.org/news/2018-06-pores-scientists-gold.html

Gold-bearing nanoparticles—tiny gold particles fused with another element such as oxygen or copper—were found in both types of plant. In E. Canadensis these particles were around 20-50 nm in diameter and had an irregular form. The gold-bearing particles in B. nivea were circular, elliptical or bone-rod shaped with smooth edges and were 5-15 nm.

"The abundance of gold in the crust is very low and there was no metal deposit in the sampling area so we speculate that the source of these gold nanoparticles is a nearby electroplating plant that uses gold in its operations, " explains Jianjin Cao who is a co-author of the study.