What the streaks actually are
It is algae eating your roof's mineral filler, not mold.
The dark streaks are a blue-green algae with a tongue-twisting name: Gleocapsa magma. It is not mold, it is not mildew, and it is not the same thing that grows on a damp bathroom wall. It is an airborne algae that lands on your roof, takes hold, and feeds. What it feeds on is the surprising part. Asphalt shingles contain a limestone filler mixed into the granule layer, and that limestone is a food source for this particular algae. Once it has a colony established, it keeps growing, and the dark protective sheath the algae forms over itself is what you see as the black streak.
Florida is close to a perfect home for it. Gleocapsa magma loves heat and humidity, and the Gulf Coast hands it both nearly year-round. Warm nights, heavy moisture in the air, frequent rain, and a long growing season mean an algae colony that might take years to show up in a dry climate can streak a Florida roof in a season or two. That is also why it spreads the way it does. The algae reproduces by releasing spores that ride the wind, which is exactly why your neighbor's roof has the same streaks and why a whole street can look matched after a few humid summers. One roof seeds the next.
Here is the part that keeps homeowners from overreacting: in the early years this is a cosmetic problem, not a structural one. A roof with light streaking is usually still doing its job of keeping water out. The honest concern is what happens over the long haul. The algae mat holds moisture against the shingle surface instead of letting it dry, and it darkens the roof so it absorbs more heat. Held moisture plus extra heat, repeated across many Florida summers, can age the shingle faster and shave time off the roof's life. So streaking is not an emergency, but it is also not nothing. It is a wear factor worth managing before it compounds.
