Comments on Coenogonium missouriense Davis, a unique microlichen from a cave in central Missouri.

Abstract: The common occurrence of cyanobacteria colonies attached to the exposed surfaces of the microlichen Coenogonium missouriense and the production of short filaments of cobblestone-shaped fungus cells at the apex of the association are documented. It is possible that the colonies contribute to the survival of the association in the cave environment, and that environmental conditions in the cave maintain the cobblestone shape of the fungus component of the microlichen.

Key Words: cave algae, lichens, microlichens, Physolinum, Trentepohlia

Introduction

While exploring several caverns in central Missouri during 1981-1982, the author discovered a unique microscopic lichen in Onyx Cave. After completing two studies concerned with morphology and ecology of the association (Davis et al. 1989, Davis and Rands 1993), I described Coenogonium missouriense (Davis 1994), a new species of the ascolichens. In its possession of a hyaline, highly characteristic fungal component (the mycobiont) to which cyanobacteria are attached, C. missouriense differs from the microscopic lichens with widely separated hyphae attached to the algal component (Henssen and Jalins 1974), and with the lichens covered by black hyphae (Koch 1982, Skuja and Ore 1933). The present paper reports on a cyanobacterial epiphyte of the microlichen from Onyx cave, and comments on the production of cobblestone-shaped mycobiont cells growing in a series at the apex of the association.

Onyx cave, a dolomite cavern on a cliff above the Gasconade River, is located in Pulaski County in the Ozark region of Missouri. In the 1980's the two mouths of the cave faced northwest and opened into a large entrance room approximately 30 m x 70 m with a ceiling height of 12 m. Photon fluence rates at the rear of the entrance room with the microlichen were 0.01 to 0.05 [mu]E * [m.sup.-2] * [s.sup.-1] at 2:00 PM on a bright day in June (Davis et al. 1989). An opening beyond the back wall of the cave permitted air to flow through the cave. Pastel green patches occurred on the side and back walls where light was able to reach. On the sidewalls the patches contained common species of free-living and lichenized terrestrial algae, but on the moist back wall the colored areas consisted largely of Coenogonium missouriense. The richly branched, pastel green filaments of C. missouriense consisted of a uniseriate algal component (the photobiont) ensheathed by hyaline fungal cells (5 to 8 in a single layer) that adh ered tightly to each other and completely covered the association (Fig. 1). The algal component of the lichen, Physolinum monile (Physolinum = Trentepohlia according to Thompson and Wujek 1997), is a terrestrial member of the filamentous green algae (Chlorophyta); the fungal component is a member of the filamentous ascomycetes. Cells of the mycobiont fit together tightly like cobblestones on a walkway, extended haustoria into the Physolinum cells, and contained concentric bodies. The lichen formed thin mats (1-2 mm) firmly adherent to the moist dolomite substratum (Fig. 2). Organisms among the C. missouriense filaments included Nostoc sp., Melosira roeseana, Synechococcus sp., Hapalosiphon intricatus, Chroococcus turgidis, bacteria, ciliates, and nematodes.

Materials and Methods

During the 1980's the author and co-workers visited the cave and collected Coenogonium missouriense several times during all seasons. Although temperatures and humidifies varied from season to season (Davis et al. 1989), no visible differences in the microlichen were ever observed during the visits. The observations below are from specimens preserved in 3.7% formaldehyde solution or 3% glutaraldehyde-paraformaldehyde, collected during the 1980's. Further methods for study and culturing of the microlichen are detailed in Davis et al. (1989).

Study of a large number of C. missouriense filaments revealed 1) common to abundant occurrence of cyanobacteria colonies attached to the exposed walls of the fungus component, and 2) cobblestone-shaped fungus cells that grew into short filaments beyond the apical cell of the photobiont. The attached colonies have been identified as Synechocystis-like by Davis and Rands (1993). The following comments are offered to supplement information previously published. Because the attached cyanobacteria may be important to the survival of the microscopic lichen in the harsh cave environment and because the mycobiont produces cobblestone-shaped cells in the cave but not in culture.

Results and Discussion

The cyanobacteria colonies of few to numerous cells adhered to the mycobiont at irregular intervals along the association. Cells of the colonies were embedded in a firm transparent matrix which in part contacted the exposed walls of the fungus component. Most colonies adhered to one to three cells of the fungus (Davis et al. 1989), the colonies occasionally covered about half the diameter of the association (Fig. 3). Fungal cells in contact with colonies often were slightly larger than most ensheathing fungal cells of the microlichen.

In specimens preserved directly after collection, fungus cells often extended beyond the apical photobiont of the association (Fig. 4) to form a short series of cells. The shape of these fungal cells was similar to the cobblestone-like cells surrounding the Physolinum algal component. Cyanobacterial colonies also were attached to the cobblestone-like fungal cells that extended beyond the apical photobiont cell. Because of their distance from the photobiont, such fungal cells would seem to obtain part of their organic nutrients from the attached cyanobacterial colonies. The somewhat eroded cell wall of a mycobiont below and attached cyanobacterial colony (Fig. 2, Davis et al. 1989) may indicate a possible pathway of nutrients to the fungus component.

In contrast to the cobblestone shape of the fungus component in the association, the ensheathing cells of Coenogonium missouriense developed cylindrical hyphae after the lichen was cultured with organic nutrients. However, no cylindrical hyphae were ever found in free-living specimens of the lichen.

Lichen genera with green algae as their primary photobiont and cyanobacteria as their secondary one are well known. These cyanobacteria, called cephalodia when contained in gall-like structures, consist of species capable of nitrogen fixation (Friedl and Budel 1996). It seems reasonable to assume that the cyanobacteria attached to Coenogonium missouriense are secondary photobionts and that they contribute to the nitrogen economy of the association. It is also possible that the cyanobacteria on C. missouriense are a primitive expression of cephalodia.

In August 1990, accommodations for commercial tourism in the cave were completed and resulted in higher light levels and decreased air flows through the cave. During several visits in the 1990's to Onyx Cave, I was unable to find C. missouriense. The author hopes the above comments will promote further research on C. missouriense and encourage increased exploration of cave microorganisms in Missouri. Preserved specimens can be obtained by contacting the author.

Literature Cited

Davis, J. S., Rands, D. G., and Lachapelle, M. 1989. Heavily lichenized Physolinum from a dimly lit cave in Missouri. J. Phycology 25:419-428.

Davis, J. S. and Rands, D. G. 1993. Observations on lichenized and free-living Physolinum (Chlorophyta, Trentepohliaceae). J. Phycology 29:819-825.

Davis, Joseph S. 1994. Coenogonium missouriense, a new lichen species from Missouri. The Bryologist 97:186-189.

Friedl, T. and Budel, B. 1996. Photobionts. In: Nash, T. H. (ed.), Lichen Biology. Cambridge University Press, 8-23.

Henssen, A. and Jahns, H. M. 1974. Lichenes. Eine Einfuhrung in die Flechtenkunde mit einem Beitrag von Johan Santesson. Georg Thieme Verlag, Stuttgart, 467 pp.

Koch, W. 1982. Die Gonidie von Racodium rupestre Pers. Fort. Gesamtgeb. Bot. Dtsch. Bot. Ges. N.F. 1:61-64.

Skuja, H. and Ore, M. 1933. Die Flechte Coenogonium nigrum (Huds.) Zahlbr. und ihre Gonidie. Acta Hort. Bot. Univ. Latviens. 7:21-55.

Thompson, R. H. and Wujek, D. E. 1997. Trentepohliales: Cephaleuros, Phycopeltis, and Somatochroon; morphology, taxonomy, and ecology. Science Publishers, Enfield, New Hampshire.