Geoscience Reference
In-Depth Information
a long series of ponds where it is continuously evaporated away in the
desert warmth and wind. In the end, salt (NaCl) precipitates from the
concentrated brine. In the middle stages of this process, when the salti-
ness of the water is about three times that of seawater, vast expanses
of spectacular cyanobacterial mats develop (
plate 7)
. On modern Earth,
cyanobacterial mats often form at high salinities, because really salty
water eliminates the majority of the animals who would otherwise feed
on and disrupt the cyanobacteria. On ancient Earth, after cyanobac-
teria evolved, but long before the evolution of grazing animals, the vast
portions of the shallow seafloor bathed in sunlight would have made a
suitable habitat. These regions, along with the bottoms of shallow lakes,
rivers, and ponds, must have looked much like the bottoms of these
present-day ponds in Baja, Mexico. On ancient Earth, cyanobacteria
often formed layered structures known as stromatolites, as they some-
explore the modern Baja mats, measure the activity level of the cyano-
bacteria, and unravel the ecology of the environment in which they live.
Our ultimate goal was to understand how cyanobacterial mats on an-
cient Earth might have functioned.
We explored many aspects of these mats, but I will focus here on
the cyanobacteria themselves. We studied a species known as
Microcoleus
chthonoplastes
, which predominates in the mats.
Microcoleus chthonoplastes
is
a so-called filamentous cyanobacterium, in which cells are attached end-
to-end in long strings called trichomes. Anywhere from two to dozens
of the trichomes are bundled together in sheaths fixed within the mats.
The cyanobacterial filaments move up and down the sheaths in response
to a variety of stimuli including light, oxygen, and sulfide levels. In the
Baja mats, these sheaths are packed together in a network that some-
what resembles tofu in consistency. Dave Des Marais made a nice draw-
ing of the Baja mat (
ig. 4.1)
based on transmission electron microscope
images. In looking at this, we see that the cyanobacteria in this Baja mat
(and similar to many other microbial mats in other places as well) are
concentrated within a layer less than 1 mm thick. They are concentrated
here, because by 1 mm depth in these mats, the visible light they use has
all been absorbed.
6
As the light diminishes, cyanobacterial abundance
decreases, and at the base of the layer we see a series of much smaller fila-
ments oriented mostly horizontally in the mat. These are green in color