About 20 years ago the shingle manufacturers started using Limestone as a filler in their asphalt shingles. This worked great as a filler but their was an unforeseen drawback. Gloeocapsa Magma (an algae) absolutely loves to eat the stuff! That is why the Ugly Dark Streaks that are so common today weren't around a long time ago.
Now the Major Shingle manufacturers such as Certainteed, GAF and Owens Corning are coming out with shingles (AR) that are algae resisitant. Certainteed actually fills some of the granules with copper and wraps the copper in ceramic in order to maintain the great appearance. The only issue being the word resistant that is used. They are not Algae Proof!
The algae, lichen and moss can and will become detrimental to the health of your roof. If you see those dark streaks on your roof you should have it cleaned by your local Certified Roof Cleaning Company. Once the streaks spread enough the algae also prevents the ceramic coating on the granules from doing one of it's jobs, which is reflecting the sun's rays. Adding to your air conditioning expense.
The only approved method of cleaning by the Asphalt Roofing Manufacturers Association (ARMA) is a solution consisting of Bleach, Water and TSP. This solution should be applied with no more pressure than a garden hose puts out. If high pressure is used you WILL lose granules (we already discussed their importance).
Do not let a power washer or a pressure washer near your roof, all it will do is remove granules from your roof and spread the algae to your siding and your neighbors homes.
Many homes are so affected by algae that I get calls all the time for a full roof replacement. It is at those times when I do my best to decide if its best to clean or replace, usually it is about 50/50.