Microbiology and Immunology

Nitrogen cycle in microorganisms WORLD OF MICROBIOLOGY AND IMMUNOLOGY

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elucidate many of the principles of genetics of higher organ-

isms. It is relatively easy to cultivate in the laboratory.

Neurosporaare eukaryotic organisms; that is, they organize

their genes onto chromosomes. They may exist as either

diploid cells (two copies of geneand chromosome) or haploid

(one copy of each gene and chromosome). Neurosporahas

both a sexual and an asexual reproductive cycle which allows

exploration of genetic processes more complex than those

found in bacteria.

The asexual cycle consists of a filamentous growth of

haploid mycelia. This stage is the vegetative stage. While the

nuclei in this stage are indeed haploid, the tubular filaments

contain multiple nuclei often without the distinction of indi-

vidual cells. Under conditions of sparse food resources, the fil-

aments (called hyphae) become segmented producing bright

orange colored macroconidia, asexual spores that can become

detached and are more readily dispersed throughout the envi-

ronment. Asexual spores can develop again into multicellular

hyphae, completing the cycle. Asexual spores can also func-

tion as male gametes in the sexual reproductive cycle.

The sexual part of the life cycle begins with the mature

fruiting body called the perithecium. These are sacs of sexual

spores (ascospores) resulting from meiotic division. The sex-

ual spores are discharged from the perithecium and can ger-

minate into haploid cultures or fuse with conidia of

complementary mating types. There are two genetically dis-

tinct mating types A and a. Neurosporacannot self fertilize,

rather haploid sexual spores of opposite mating types must be

joined at fertilization. Nuclear fusion of the male and female

gametes occurs setting the stage for meiotic division to form

ascospores. The diploid stage is brief as nuclear fusion quickly

gives way to two meiotic divisions that produce eight

ascospores. Ascospores are normally black and shaped like a

football. The physical position of the ascospores is linear and

corresponds to the physical position of the individual chromo-

somes during meiosis. In the absence of crossing over, the four

a-mating type ascospores are next to each other followed by

the four A-mating type ascospores.

The existence of a large collection of distinct mutant

strains of Neurosporaand the linear array of the products of

meiosis makes Neurosporaan ideal organism for studying

mutation, chromosomal rearrangements, and recombination.

As a relatively simple eukaryote, Neurosporahas permitted

study of the interactions of nuclear genes with mitochondrial

genes. Neurosporaalso exhibits a normal circadian rhythm in

response to light in the environment, and much of the funda-

mental genetics and biology of circadian clock cycles (chrono-

biology) have been elucidated through the careful study of

mutant cells which exhibit altered circadian cycles.

See alsoMicrobial genetics

NNitrogen cycle in microorganismsITROGEN CYCLE IN MICROORGANISMS

Nitrogen is a critically important nutrient for organisms,

including microorganisms. This element is one of the most

abundant elemental constituents of eukaryotic tissues and

prokaryotic cell walls, and is an integral component of amino

acids, proteins, and nucleic acids.

Most plants obtain their nitrogen by assimilating it from

their environment, mostly as nitrate or ammonium dissolved

in soil water that is taken up by roots, or as gaseous nitrogen

oxides that are taken up by plant leaves from the atmosphere.

However, some plants live in a symbiotic relationship with

microorganisms that have the ability to fix atmospheric nitro-

gen (which can also be called dinitrogen) into ammonia. Such

plants benefit from access to an increased supply of nitrogen.

As well, nitrogen-assimilating microorganisms are of

benefit to animals. Typically animals obtain their needed

nitrogen through the plants they ingest. The plant’s organic

forms of nitrogen are metabolized and used by the animal as

building blocks for their own necessary biochemicals.

However, some animals are able to utilize inorganic sources of

nitrogen. For example, ruminants, such as the cow, can utilize

urea or ammonia as a consequence of the metabolic action of

the microorganisms that reside in their forestomachs. These

microbes can assimilate nitrogen and urea and use them to

synthesize the amino acids and proteins, which are subse-

quently utilized by the cow.

Nitrogen (N) can occur in many organic and inorganic

forms in the environment. Organic nitrogen encompasses a

diversity of nitrogen-containing organic molecules, ranging

from simple amino acids, proteins, and nucleic acids to large

and complex molecules such as the humic substances that are

found in soil and water.

In the atmosphere, nitrogen exists as a diatomic gas

(N 2 ). The strong bond between the two nitrogen atoms of this

gas make the molecule nonreactive. Almost 80% of the vol-

ume of Earth’s atmosphere consists of diatomic nitrogen, but

because of its almost inert character, few organisms can

directly use this gas in their nutrition. Diatomic nitrogen must

be “fixed” into other forms by certain microorganisms before

it can be assimilated by most organisms.

Another form of nitrogen is called nitrate (chemically

displayed as NO 3 -). Nitrate is a negatively charged ion (or

anion), and as such is highly soluble in water.

Ammonia (NH3S) usually occurs as a gas, vapor, or liq-

uid. Addition of a hydrogen atom produces ammonium

(NH 4 +). Like nitrate, ammonium is soluble in water.

Ammonium is also electrochemically attracted to negatively

charged surfaces associated with clays and organic matter in

soil, and is therefore not as mobile as nitrate.

These, and the other forms of nitrogen are capable of

being transformed in what is known as the nitrogen cycle.

Nitrogen is both very abundant in the atmosphere and is

relatively inert and nonreactive. To be of use to plants, dinitro-

gen must be “fixed” into inorganic forms that can be taken up

by roots or leaves. While dinitrogen fixation can occur non-bio-

logically, biological fixation of dinitrogen is more prevalent.

A bacterial enzyme called nitrogenase is capable of

breaking the tenacious bond that holds the two nitrogen atoms

together. Examples of nitrogen-fixing bacteria include

Azotobacter, Beijerinkia, some species of Klebsiella,

Clostridium, Desulfovibrio, purple sulfur bacteria, purple non-

sulfur bacteria, and green sulfur bacteria.

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