Geoscience Reference
In-Depth Information
and contain bacteriochlorophyll
a
and
c
. They represent a genus of
anoxygenic phototrophs known as
Chloroflexus,
, or informally, as green-
nonsulfur bacteria. These phototrophs use wavelengths of light that
penetrate below the cyanobacteria, and they use this light to oxidize the
sulfide produced by sulfate reducers in the dark, deeper, oxygen-free
layers of the mat. The ecology of this mat is much more complex than
my simple description indicates, but this overview introduces the major
players and shows, basically, how a cyanobacterial mat ecosystem can
be structured.
The dimensions of this mat are so compressed that to really under-
stand it we need tools that can probe the mat to at least a 0.1 mm resolu-
tion. Niels Peter Revsbech of Aarhus University, Denmark, faced this
challenge as part of his PhD project, and in the late 1970s, he developed
the first microelectrodes for measuring the distribution of oxygen, sul-
fide, and pH in nature. These tiny electrodes had tip diameters of mere
microns (
ig. 4.2)
and thus the necessary ine-scale resolution. Niels Peter
is a master of electrode design, and he has continued to develop these
and others for ecological use. These developments have spurred major
advances in microbial ecology by providing a window into the chemis-
try of microbial mats (and other natural ecosystems) at scales nearly
matching the size of the microbes themselves. We probed the Baja mats
with such electrodes, and an example of the distribution of oxygen is
shown in
igure 4.3.
To me, it is quite amazing that all the action with
oxygen takes place over a thickness of less than 2 mm. Separate your
forefinger and thumb by 2 mm and just imagine: in this short distance,
oxygen rises to about 4 times air saturation (that's nearly 1 bar of O
2
!)
and drops again to nothing. That's during the day. After the Sun goes
down, the peak in oxygen concentration quickly disappears.
Not long after developing the first oxygen microsensors, Niels Peter
also devised a clever way of determining rates of oxygen production
in microbial mats.
7
We applied this method to our mats, and the distri-
bution of oxygen production rates are shown together with the oxygen
profile in
igure 4.3.
These rates of oxygen production are huge, and on
a per volume basis, they are among the highest you can find anywhere
in nature. Even if we integrate over depth and determine rates of oxy-
gen production (which are equivalent to rates of organic matter produc-
tion by oxygenic photosynthesis
8
) on an areal basis (this means the rate
