About the Cover
Cover
The background image on the cover of this issue shows an area (1-2 m2) of intertidal microbial mats, 0.5 to 2 cm thick, adjacent to the largest salt works in North America-Exportadora de Sal-located in Guerrero Negro, Baja California Sur, Mexico. Hypersaline ponds belonging to the salt works harbor extensive cyanobacterial mats, which are extraordinarily diverse, complex, and highly organized ecosystems. On p. 160 of this issue, David J. Des Marais reviews his own and others' comprehensive studies of these mats. He asks how such systems respond in a coordinated fashion to cyclical or transient environmental changes and how they influence sedimentation and produce gases.
Microbial mats are laminated, and the component microorganisms in the community are localized to layers at specific depths. Layering is visible in the inset on the cover-a Nomarski image of a section through the upper 2 mm of a mat. The productivity of the mat is dominated by Microcoleus chthonoplastes, a filamentous cyanobacterium (the yellow-green region near the top of the section). During the day, photosynthesis by Microcoleus and many other cyanobacteria is intense, and the oxygen generated diffuses downward. But the light is strongly absorbed, and the oxygen is rapidly consumed by heterotrophs, so an aphotic, anoxic zone develops, beginning at a depth of only about 1.5 mm (the dark region near the bottom of the section). Meanwhile, anaerobic organisms generate H2S, which diffuses upward, but is consumed by photosynthetic bacteria (which function in very dim, infrared light) and chemoautotrophs like Beggiatoa (the beaded filaments visible adjacent to the dark region). The dark layer thus marks the interface between the diminished concentrations of oxygen and sulfide. At night, photosynthesis ceases, the upper levels of the mat become sulfidic as the oxygen concentration falls, and motile organisms may move upward. Thus, the position of a microorganism in the mat is determined by many factors, including the steep gradients of light, oxygen, and sulfide, physiological adaptations to changes in those gradients, trophic mechanisms, and relationships with other organisms at higher and lower levels in the system.
In a related paper in this issue of The Biological Bulletin, John R. Spear and colleagues from the laboratory of Norman R. Pace (p. 168) report on molecular approaches to identifying the components of mat communities in Guerrero Negro and thus quantifying the extent of diversity. They also characterize, partially, a novel, relatively simple, laminated microbial community that occurs in crystalline gypsum; this finding documents further the enormous diversity of microorganisms at this site.
The articles by Des Marais and Spear et al. are both part of a workshop entitled Outcomes of Genome-Genome Interactions (p. 155). This meeting was meant to establish links among biogeochemical factors, microbial metabolic processes, maintenance of microbial population structures in nature, and microbial symbioses with multicellular hosts. The workshop was held at Woods Hole, Massachusetts (May 1-3, 2002) and was sponsored by the Center for Advanced Studies in the Space Life Sciences at the Marine Biological Laboratory (MBL).
The large, background image of the microbial mats in situ was taken by John R. Spear (University of Colorado, Boulder), and the Nomarsky image was taken by Jack D. Farmer (Arizona State University, Tempe). The composite picture on the cover was produced by Beth Liles at the MBL.
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Copyright © 2003 by the Marine Biological Laboratory.
