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- 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--Iocated in
Guerrero Negro, Baja California Sur, Mexico. Hy-
persaline ponds belonging to the salt works harbor
extensive cyanobacterial mats, which are extraordi-
narily diverse, complex, and highly organized eco-
systems. 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 envi-
ronmental changes and how they influence sedi-
mentation 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 chthono-
plastes, 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 ab-
sorbed, and the oxygen is rapidly consumed by
heterotrophs, so an aphotic, anoxic zone develops,
beginning at a depth of only about 15 mm (the dark
region near the bottom of the section). Meanwhile,
anaerobic organisms generate H2S, which diffuses
upward, but is consumed by photosynthetic bacte-
ria (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 con-
centration falls, and motile organisms may move
upwards. 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 gra-
dients, 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 compo-
nents 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 crys-
talline 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 fac-
tors, microbial metabolic processes, maintenance of
microbial population structures in nature, and mi-
crobial 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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