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B.W.,

No, Gloeocapsa isn't actually still classified as an algae, although its still called "blue-green algae" my many. Its actually considered to be bacteria, of the cyanbacteria type. Cyanbacteria can "Feed" from the air, light and moisture, photosynthesizes and perform nitrogen fixation. Protected by encapsulation method in dry periods, and bound together and to host roof by a bioslime.

Gloeocapsa is a unicellular cynobacteria. The gelatinous material (bioslime)surrounding each cell causes the cells to stick together (and to other things, such as the roofing surface). Like plants, cynobacteria can have the photosynthetic pigment chlorophyll A and they use water as an electron donor during photosynthesis. When water molecules are split, oxygen is liberated.

Catching Fungi spores, which can feed off it, and other "nutrient rich goodies" caught in/on the bioslime layer, and prematurely break down the asphalt.

Moisture rich environ of the slime - the discoloration nulling the granules' ability to deflect UV, sometimes daily near "boiling off" temperatures the moisture at the edge of the slime layer, holding moisture within the shingle environment, and the asphalt further becomes exposed to air, UV, and greater temperatures than otherwise, the moisture if freezing can pop off granules, and the premature weathering of the asphalt - the granuals are lost prematurely, thus a premature rapid aging of the roof. Hosting Lichen (bacteria plus fungi), enjoying the moisture rich, now nutrient rich environment further deteriorating shingle surface, etc.

Seems to me I recently read about a similar, same family (OS20?) from Antartic? colony hosted on a piece of cliff rock from U.K. taken to the International Space Station and placed outside the station in orbit. IIRC it was exposed to UV, the boil-off of space vacuum for many months (at least five I belive) and later collected and returned to earth for examination. Apparently it survived quite well. (I think I was reading about this sometime last August or so, an extended exposure experiment on the International Space Station, after a few years earlier shorter exposure on the line of several days in near space, this time many months), thus producing some discussions regarding terraforming with cynobacteria. Anyway that's the way I remember it.

Anyway, regarding weathering of roofing materials, I think you might find the following article informative. It will start to get interesting on the subject around part six, but suffer through if you can from the beginning. The references, several of which I think you'll also find very enlightening, should be available via your local library reference desk, if not freely available on the www.

Here is the title, authors, publication date, a (clickable) link to view same, and the abstract:

From: Weathering of Roofing Materials - An Overview
Paul Berdahl, Hashem Akbari, and Ronnen Levinson
Lawrence Berkeley National Laboratory Berkeley, CA 94720
and
William A. Miller
Oak Ridge Naitonal Laboratory
Oak Ridge, TN 37831.
Published June 11, 2008
See (clickable linkeScholarship: Weathering of Roofing Materials-An Overview
Abstract:
"An overview of several aspects of the weathering of roofing materials is presented. Degradation of materials initiated by ultraviolet radiation is discussed for plastics used in roofing, as well as wood and asphalt. Elevated temperatures accelerate many deleterious chemical reactions and hasten diffusion of material components. Effects of moisture include decay of wood, acceleration of corrosion of metals, staining of vlay, and freeze-thaw damage. Soiling of roofing materials causes objectional stains and reduces the solar reflectance of reflective materials. (Soiling of non-reflective materials can also increase solar reflectance.) Soiling can be attributed to biological growth (e.g. cyanbacteria, fungi, algae), deposits of organic and mineral particles, and to the accumulation of flyash, hydrocarbons and soot from combustion."

A few selected paragraphs below:

"As far as biological growth on roofing surfaces is concerned, it is clear that a large variety of species is expected to be present. For example, some of the species of fungi that occur in soil may be expected to be present. However, the ultraviolet radiation incident on roofing can kill many organisms. We have noted earlier that the cyanobacteria Gloeocapsa infests mineral roofing granuels, leading to black colored stains. In a study of biomass accumulation on single ply roofing membranes exposed in Oak Ridge, Tennessee [44], it was found by phospholipid fatty acid analysis that the biological growth was in this case primarily fungal in nature.

C.C. and P.M. Gaylarde [45] recently published a study of 230 biofilms found on the exterior of buildings in Europe and in Latin America. They classified the biofilms according to the predominant microscopic organisms found, of which the most important were various cynobacteria, algae, and fungi. The substrate was important with, for example, fungi rarely found on mineral substrates and often on paint. Climate was believed to be important as well, with algae more prevalent at cool damp European sites and dark-colored cynobacteria frequently found in tropical locations at elevations above 1000 m."

"In the roofing industry it is often not known exactly what species make up the biomass deposits on roofing. In fact, the terms algae and fungi are somtimes used interchangeably. However, it is well known that in humid climates, such as the southeastern United States, the growth of biomass on roofing leads to visible stains. In some cases, it has been noted that staining does not occur near copper and galvanized (zinc) flashings. In a patent by Narayan et al., [49] reviewing prior art, it is noted that algae growth on asphalt shingles can be inhibited by metal An particles, ZnO, ZnS, and cuprous oxide Cu2O). Thus it appears that zinc and copper ions can inhibit biological growth. A more recent patent [50] likewise discloses that Cu2O, either alone or with zinc compounds, is effective in roof granules, to prevent growth of cyanobacteria (such as Gloeocapsa) and also fungi."