Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Spectrophotometer

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SOIL FORMATION, INVOLVEMENT OF

MICROORGANISMSSoil formation, involvement of microorganisms

Microorganismsare essential to soil formation and soil ecol-

ogy because they control the flux of nutrients to plants (i.e.,

control of carbon, nitrogen, and sulfur cycles,), promote nitro-

gen fixation, and promote soil detoxification of inorganic and

naturally occurring organic pollutants. Soil microorganisms

are also part of several food chains, thus serving as source

nutrients to one another, and frequently serve as the primary

members of food chains in soil biota.

The roots of plants are also part of soil biota and some

fungi. Many bacterialive in symbiotic relation to plant roots,

around which there is an area of elevated microbial activity,

known as rhizosphere. The Animalia kingdom is also repre-

sented in soil biota by Nematodes, Earthworms, Mollusks,

Acarina, Collembola, as well as several insects and larvae that

feed mostly on decaying organic matter. They all take part in the

soil food chain and help to promote the conversion of organic

matter into bacterial and fungal biomass. Soil microbiology is a

relatively recent discipline and it is estimated that about only

one percent of soil microorganisms are so far identified.

The soil ecosystem is composed of inorganic matter (cal-

cium, iron oxide, nitrates, sulfur, phosphates, ash, and stone

particles), substrates (fallen leaves, dead organisms, rotten

wood, dead roots), organisms (microbes, animals, and plants),

air, and water. Bacteria and fungi are mostly heterotrophic

organisms that feed on the existing organic matter by decom-

posing them in order to absorb the resultant micronutrients and

minerals. Therefore, they are essential to the recycling process

of nutrients that keeps soils in good condition for plant growth.

The community of microorganisms in a given type of soil dif-

fers from that belonging to another soil type. They are highly

dependent on environmental factors such as levels of carbon

dioxide, oxygen, hydrogen, soil pH(whether acid, alkaline, or

neutral), types of substrates, amounts of available substrates,

levels of moisture, and temperature. Each community is highly

complex, and so far, little is known about the succession of

microorganisms in the food chains and the interconnected food

webs they form, or about the sequence of events in the cycling

pathways of soil ecosystem.

The arrival of new substrate in the soil increases bacte-

rial populations that feed on them, thus recycling in the

process, nutrients important to both plants and other soil

organisms. Bacterial expansion leads to a second event,

known as succession, which is the growth of protozoapopula-

tions that predate bacteria. The expansion of protozoa popula-

tions triggers the activity of mites, which feed on protozoans.

Substrate arrival triggers as well the activity and expansion of

fungi populations, which are also decomposers. Some fungal

species compete with other fungal species for the same sub-

strates, such as the Pisolithusand the Fusarium.Nematodes

are triggered into action and feed on both fungi and other

species of nematodes. Some fungi are able to entrap and feed

on nematodes too. In the rhizospheres, these populations are

more active than in other parts of the soil and atmospheric fac-

tors may influence rhizospheres biota. An American research

group is studying the response of soil biota in California grass-

lands to determine the long-term effect of increased levels of

carbon dioxide on soil biota dynamics and on plant growth.

They found that in a carbon dioxide enriched atmospheric

environment, the colonization of plant roots by fungi is aug-

mented, which facilitates carbon and nutrient exchange

between the host plants and the fungi (i.e., symbiosis), thus

favoring fungi colonies to expand within the soil, as well

favoring the growth of grass. Consequently, the number of soil

micro arthropods has also increased, since many of them feed

on fungi colonies. However, after six years of experimental

carbon dioxide atmospheric enrichment, significant increases

on bacterial populations were not recorded. Therefore, the

experiment succeeded in illustrating one portion of the food

chain in grassland soil, and supplied evidence that the induced

enhancement of natural-occurring symbiotic relationships in

the rhizosphere may be useful for agricultural productive pur-

poses.

See alsoBacterial kingdoms; Composting, microbiological

aspects; Microbial symbiosis; Microbial taxonomy;

Photosynthetic microorganisms; Protozoa; Slime molds

SPACE MICROBIOLOGY•seeEXTRATERRESTRIAL

MICROBIOLOGY

SSpectrophotometerPECTROPHOTOMETER

A spectrophotometer is an optical device that can determine

the concentration of a compound or particles in a solution or

suspension.

Light of a pre-selected wavelength is shone through a

chamber that houses the sample. The sample particles, bacte-

riafor example, will absorb some of the light. The amount of

light that is absorbed increases with increasing numbers of

bacteria in a predictable way. The relationship between

absorbance and the number of absorbing sample molecules is

expressed mathematically as the Beer-Lambert Law.

The absorbance of light can also be described as the

optical density of the sample solution or suspension.

The percent of light that has been absorbed can be

determined and, by comparing this absorption to a graph of

the absorption of known numbers of bacteria, the concentra-

tion of bacteria in the suspension can be computed. In a

microbiology laboratory, such measurements are routinely

used in bacterial growthstudies, to determine the number of

bacteria growing in a cultureat certain times based on the

absorbance of the suspension. A standard curve can be con-

structed that relates the various measured optical densities to

the resulting number of living bacteria, as determined by the

number of bacteria from a defined portion of the suspensions

that grows on agarmedium.

Some spectrophotometers are equipped with a single

measuring chamber. For these so-called single-beam instru-

ments, the absorbance of a sample is taken, followed by the

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