Buellia pullata 1

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Identification Of Two Black Coatings On
Boulders In The Palomar College Arboretum

1. Black Coating On Boulders

As of 6 Mar. 2022, I now conclude that this is an unknown fungus that grows on boulders especially in the shade of trees. It is probably in the fungus division Ascomycota

For many decades I have pondered over the identity of two black, crustose "lichens" covering monzogranite rocks and boulders in the Palomar College Arboretum. Many years ago, I attempted to key these "lichens" down to species with two very old, classic references. I checked the names with the University of California Checklist of California Lichens; however, I never felt comfortable with the identity of the following black coating on boulders.

  1. Fink, B. 1935. The Lichen Flora of the United States. University of Michigan Press. 424 p.

  2. Hasse, H.E. 1913. "The Lichen Flora of California." Contributions From the United States National Herbarium 17 (1): 1-132.

  3. University of California Checklist Of California Lichens by Shirley C. Tucker & Bruce D. Ryan

Dark coating on rocks & boulders in the Palomar College Arboretum. I once thought it was the lichen Buellia pullata; however, I now think this may be a non-lichenized rock fungus. My conclusion explains the lack of characteristic lichen structures, such as cup-shaped apothecia of ascolichens. Under microscopic examination (400x) with compound microscope it lacks green, photosynthetic algal cells (photobionts), hymenial layer and paraphyses. It does have brown, 2-celled spores mixed with numerous colorless fungal cells. See following images taken 7 Mar. 2022.

   See Rock Inhabiting Fungus In The Superstition Mtns Of Arizona  

2. Verrucaria cf. nigrescens Pers.

Verrucaria Family (Verrucariaceae)

Another dark brown (black), crustose rock lichen in the Palomar College Arboretum. The thallus is areolate (cracked into sections called areoles) like dried, cracked mud. It is thicker than previous black lichen and the ascocarps are perithecia rather than apothecia. I am reasonably confident that it is a species of Verrucaria. The orange lichen is Caloplaca bolacina. The thallus is composed of small scalelike, lobed (lobulate) squamules. The squamules are similar to areoles except they lift slightly from the rock surface along the margins. The apothecia have an orange disk and paler thalline rim (margin). Since the rim is the same color as the thallus and contains photobiont cells, it called lecanorine.

Another crustose rock lichen with a dark brown or black thallus grows on rocks and boulders in the Palomar College Arboretum. This is a pyrenocarpous lichen with perithecia rather than apothecia. The perithecia are embedded in the thallus and are very small. Younger perithecia are almost completely embedded in the thallus. In older perithecia, the emergent portion of the perithecium is conical-spherical, 0.2-0.4 mm in diameter. The spores are colorless and simple (non-septate), about 20 µm long and 8 per ascus. The paraphyses break down or gelatinize (deliquesce) and are not visible when the spores are mature. The thallus is areolate, cracked into irregular sections (areoles) like dried, cracked mud. Another species in San Diego County, V. viridula, is very similar except the cracks between the areoles are wider. In the latter species the areoles are more distinctly separated from each other. In addition, the thallus of V. viridula appears greenish-brown when wet. V. nigrescens has an enormous worldwide distribution, including North America, Europe, Asia and northern Africa. Aquatic species of Verrucaria grow on boulders along streams and waterfalls, and the marine V. maura grows on rocks of the wave battered intertidal zone.

Magnified view of Verrucaria nigrescens. Younger perithecia are almost completely embedded in the thallus. The cracks in thallus are thinner than V. viridula. In addition, V. viridula has more widely separated and distinct areoles. When wet, the thallus of V. nigrescens does not appear greenish as in V. viridula. 20x under dissecting microscope.

Magnified view of an ascus of Verrucaria nigrescens. The colorless, simple (nonseptate) spores are about 16-22 µm in length. 1000x under compound microscope.

 See The Marine Lichen Verrucaria maura 
Ascus & Spores Of Verrucaria viridula