The blemishes are called “foxing spots” and have probably plagued the drawing for centuries (Sorry, Leo). Recently, a team from the University of Natural Resources and Life Sciences in Vienna extracted DNA from the spots and found evidence of fungi from the phylum Ascomycota.
They found one uncharacterized species from the genus Acremonium. The cephalosporins, a type of beta-lactam antibiotic, are derived from this genus. The group from Vienna, led by Guadalupe Pinar, also used electron microscopy to characterize fungal spores found on the drawing. They found a wide variety of spores - smooth spheres covered in filaments and flat disks with cross-hatched surfaces.
Pinar suggests that the foxing spots are formed through a multi-step chemical process involving fungi. First, iron particles come to rest on the paper and damage its cellulose structure. Second, fungal spores burrow into the damaged cellulose. And third, the fungi produce oxalic acid that then produces calcium oxalate crystals.
Now that art conservators know the (likely) cause of the damage, they can work to curb further damage to da Vinci’s self portrait.
Source: Discover Magazine, Wikipedia