Microbial mats are laminated microbial communities encased within organic matrices composed of excreted polymers which are ubiquitous fixtures of a variety of marine and freshwater environments. The microbial mats utilized in this research are hypersaline cyanobacterial mats from Solar Lake, Sinai, Egypt (Fig. 1) (Cohen et al., 1977). Hypersaline microbial mats are often dominated by the filamentous cyanobacteria Microcoleus chthonoplastes which are often found growing in groups or bundles surrounded by a common sheath (D'Amelio et al., 1989; Stal et al., 1985). The large number of sheaths, and filaments within them, provide significant structure to the mat and may serve an important role in transport of both chemical species and migrating microbial populations. In proximity to the cyanobacteria are a wide variety of heterotrophic and chemolithotrophic bacteria (e.g., sulfide-oxidizing bacteria), eager to utilize excreted organic matter and oxygen (Stolz, 1984).

Figure 2 is a scanning electron micrograph image of a transverse section of the surface (0 to 1 mm) of a microbial mat from Solar Lake, Sinai, Egypt. Visible are the filaments (cross section) of M. chthonoplastes (M) and the surrounding sheath material, which is devoid of any other bacteria.  Just outside the sheath are a host of unidentified microorganisms.

Figure 3 is a high-magnification cross section of the second image, showing more clearly the internal structures of M. chthonoplastes. Marked are the radial thylakoids, flattened double-membrane disks surrounding an intermembrane space, which contain all the components required for photosynthesis (Brock et al., 1994). Also visible is a carboxysome which consists of a polymer of ribulose 1,5-bisphosphate carboxylase, the key enzyme of the Calvin cycle.

These samples were prepared for microscopy by being frozen in a high-pressure freezer (Balzers HPM 010) which allows cryofixation of the sample without formation of ice crystals.  The frozen hydrated samples were then freeze fractured and sputter coated with platinum before viewing.

References

1. Brock, T. D., M. T. Madigan, J. M. Martinko, and J. Parker. 1994. Biology of Microorganisms, 7th ed. Prentice-Hall, Englewood Cliffs, N.J.
2. Cohen, Y., W. E. Krumbein, M. Goldberg, and M. Shilo. 1977. Solar Lake (Sinai). 1. Physical and chemical limnology. Limnology and Oceanography 22:597-608.
3. D'Amelio, E. D., Y. Cohen, and D. J. Des Marais. 1989. Comparative functional ultrastructure of two hypersaline submerged cyanobacterial mats: Guerrero Negro, Baja California Sur, Mexico, and Solar Lake, Sinai, Egypt, p. 97-113. In Y. Cohen and E. Rosenberg (ed.), Microbial Mats: Physiological Ecology of Benthic Microbial Communities. American Society for Microbiology, Washington, D.C.
4. Stal, L. J., H. van Gemerden, and W. K. Krumbein. 1985. Structure and development of a benthic marine microbial mat. FEMS Microbial Ecology 31:111-125.
5. Stolz, J. F. 1984. Fine structure of the stratified microbial community at Laguna Figueroa, Baja California, Mexico. II. Transmission electron microscopy as a diagnostic tool in studying microbial communities in situ, p. 23-28. In Y. Cohen, R. W. Castenholz, and H. O. Halvorson (ed.). Microbial Mats: Stromatolites. Alan R. Liss Inc., New York, N.Y.