What is nitrogen fixation?
Nitrogen fixation is no more nor no less than the use of
di-nitrogen (N2) as the source of nitrogen atoms for growth. But why is the
use of N2 to get nitrogen given a special name to distinguish it from the
use of NO3 or NH4? To answer this we need to briefly consider the nature of
N2.
The mouthful of air you are currently breathing in is made
up of about 78% di-nitrogen, 21% oxygen and 1% argon. The Earth's
atmosphere is mostly N2. N2 is also very abundant in seawater, because some
of the enormous amount in the atmosphere diffuses across the sea surface and
into the water.
But yet, somewhat incredibly, most phytoplankon in the
ocean are unable to use this abundantly available form of nitrogen. This is
because the two atoms in the N2 molecule are very firmly bound together,
with lots of energy being required to break them apart. Nitrogen in the
oceans that occurs as the dissolved ions nitrate (NO3) and ammonium (NH4),
on the other hand, is much more convenient to use because less energy
expenditure is required. It is as if the N atoms in an N2 molecule are held
together using chains, whereas linked into NO3 and NH4 molecules using only
string. (another analogy)
So N2 is super-abundant but difficult to use, whereas NO3
and NH4 are easy to use but inconveniently occasionally run out. The inert
nature of N2 molecules leads to them only being used as a last resort, hence
the special name of nitrogen-fixation for N2-usage.
Why is nitrogen-fixation important?
The real magic of nitrogen-fixation is not just that it
allows some plants to continue growing even when nitrate and ammonium have
run out, but also that it replenishes those other forms of nitrogen,
fertilising the ocean so that the NO3- and NH4-using phytoplankton can grow
once more. Returning to the farming analogy, nitrogen-fixation is a free and
natural source of nitrogen fertiliser. Nitrogen-fixers act as a conduit
of the inert N2 to the more readily usable forms of NO3 and NH4.
The rate of supply of this natural nitrogen fertiliser can
be so important that in many regions of the sea it controls how many
phytoplankton grow there. This in turn controls how many fish, whales and
other animals can live there, since phytoplankton are the base of marine
food chains. It also determines how much carbon dioxide that part of the
ocean `sucks down' from the atmosphere.
Which phytoplankton can fix nitrogen?
The few members of the phytoplankton which can use N2 are
exclusively members of an ancient division of life, the cyanobacteria.
Cyanobacteria first evolved more than 2 billion years ago, when the Earth's
atmosphere was rich in carbon dioxide but devoid of oxygen. This type of
organism is at least half as old as the Earth itself.
These descendants of survivors from an oxygen-less ocean
still today show signs of their ancient lineage. The enzyme they use to fix
nitrogen is deactivated by oxygen. Because the water they live in today is
always highly oxygenated, the cyanobacteria have had to create special
conditions inside their cells in order to isolate the enzyme from the oxygen
everywhere outside.
Most phytoplankton are microscopic isolated dots, each one
floating alone. But In some species the individual cells congregate
together to form chains or spirals or other
shapes, thus passing their short lives in a more sociable
fashion.
In the main nitrogen-fixing species in the ocean,
Trichodesmium, the individual cells often join together in a line to
form a filament of about 100 cells. These filaments themselves also join
together, in a second level of organisation, this time to form bundles of
filaments known as colonies, each one being made up of 100 or so filaments,
i.e. ~10,000 cells in total. This lifestyle may make it easier to
protect the nitrogen-fixing enzyme from oxygen, if many cells are adjacent
to other cells rather than always adjacent to water on all sides.
Some other nitrogen-fixing cyanobacteria found in the
oceans and seas are: Richelia
intracellularis as a symbiont within Rhizosolenia or
Hemialus, Nodularia spumigena and Aphanizimenon, with the latter two restricted
to fresh or coastal waters such as the Baltic Sea. Some smaller and less
conspicuous nitrogen-fixers may be out there still awaiting
discovery.
Where do nitrogen-fixers live in the ocean?
The distribution of N2-fixers is patchy in both time and
space, but in the open ocean they are restricted to low latitude waters,
within about 30 or 40 degrees of the equator, and can occur at any time of
year. The Sargasso Sea, the Caribbean, the ocean west of west Africa, the
east and north coasts of Australia, the seas around Fiji, the Red Sea and
the Arabian Sea are some areas of the highest concentrations of
Trichodesmium. In the Baltic Sea
nitrogen-fixers occur everywhere except for the northern-most part (Bothnian
Bay), and they occur mainly in the summer.
Being photosynthetic organisms nitrogen-fixing
phytoplankton are restricted to the sunlit zone, that is the top 100 m or
so, below which insufficient sunlight penetrates. The question of why
nitrogen-fixers are constrained to low latitudes in the open ocean, and why
they flourish in some low latitude areas but not others, is an exciting area
of current investigation.
T.Tyrrell@noc.soton.ac.uk.