Geoscience Reference
In-Depth Information
this, I join a cruise onboard the
Vidal Gomez
, a rickety old American
oceanographic ship recommissioned into the Chilean Navy. We are
traveling due west from Iquique, Chile, to a site some 20 kilometers
offshore. Dolphins are playing in the bow wake, and pelicans are search-
ing for fish in the distance. We arrive at the study site, and the dolphins
disperse; it's no fun when the ship is standing still. I relax and enjoy
the view, but am startled by a school of anchovies breaking the surface
by the side of the boat. As the ripples settle, I look down and think that
just 100 meters below my feet the waters are completely oxygen free. We
are studying one the great oxygen-minimum zones (OMZs) in the world's
oceans. These are found of the western coasts of Peru and northern
Chile, and to the north, of the coasts of Central America and Mexico,
as well as in the Arabian Sea in the Indian Ocean. In these oxygen-free
areas, microbes convert the nitrate in seawater to N
2
gas and are respon-
sible for about one-third of all nitrate removal from the oceans. Without
replenishment, nitrate, a key nutrient in algal production, would be de-
pleted from the ocean within about five thousand years.
Luckily, replenishment occurs, and it does so in many places over the
Earth surface as nitrate loss to N
2
gas is not just a marine phenomenon.
The process of replenishment is known as nitrogen fixation, where many
different types of microbes (more specifically prokaryotes
9
) convert N
2
gas to ammonium for use by the cell. This is a very energy-intensive and
complicated process conducted by the enzyme complex known as ni-
trogenase. On land, nitrogen-ixing prokaryotes often live in symbiotic
association with plant roots; the roots of legumes like soybeans are a
good example. In aquatic environments, cyanobacteria are the main
nitrogen fixers. This makes sense because cyanobacteria obtain ample
energy from the Sun to run the nitrogen fixation process. Rather para-
doxically, oxygen, the main product of cyanobacteria, also poisons the
nitrogenase enzyme. Evolution has produced a number of very clever
solutions to this apparent dilemma, and I will outline a few of these.
A number of filamentous cyanobacterial types have developed spe-
cial cells called heterocysts. These are spaced along the filament at semi-
regular intervals, and nitrogen fixation occurs in these cells. Heterocysts
contain multiple cell-wall layers that restrict the diffusion of oxygen into
the cell, and while they contain photosystem I, which produces the en-
ergy to drive nitrogen fixation, they have no photosystem II. Therefore,
