Microbiology and Immunology

WORLD OF MICROBIOLOGY AND IMMUNOLOGY Dictyostelium

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box. The singular algal diatom cell lives protected inside the

frustule halves like a pair of shoes snuggled within a shoebox.

Frustules are ornate, having intricate designs delineated

by patterns of holes or pores. The pores that perforate the frus-

tules allow gases, nutrients, and metabolic waste products to

be exchanged between the watery environment and the algal

cell. The frustules themselves may exhibit bilateral symmetry

or radial symmetry. Bilaterally symmetric diatoms are like

human beings, having a single plane through which halves are

mirror images of one another. Bilaterally symmetric diatoms

are elongated. Radially symmetric diatom frustules have many

mirror image planes. No matter which diameter is used to

divide the cell into two halves, each half is a mirror image of

the other. The combination of symmetry and perforation pat-

terns of diatom frustules make them beautiful biological struc-

tures that also are useful in identifying different species.

Because they are composed of silica, an inert material, diatom

frustules remain well preserved over vast periods of time

within geologic sediments.

Diatom frustules found in sedimentary rock are micro-

fossils. Because they are so easily preserved, diatoms have an

extensive fossil record. Specimens of diatom algae extend

back to the Cretaceous Period, over 135 million years ago.

Some kinds of rock are formed nearly entirely of fossilized

diatom frustules. Considering the fact that they are micro-

scopic organisms, the sheer numbers of diatoms required to

produce rock of any thickness is staggering. Rock that has rich

concentrations of diatom fossils is known as diatomaceous

earth, or diatomite. Diatomaceous earth, existing today as

large deposits of chalky white material, is mined for commer-

cial use in abrasives and in filters. The fine abrasive quality of

diatomite is useful in cleansers, like bathtub scrubbing pow-

der. Also, many toothpaste products contain fossil diatoms.

The fine porosity of frustules also makes refined diatoma-

ceous earth useful in fine water filters, acting like microscopic

sieves that catch very tiny particles suspended in solution.

Fossilized diatom collections also tell scientists a lot

about the environmental conditions of past eras. It is known

that diatom deposits can occur in layers that correspond to

environmental cycles. Certain conditions favor mass deaths of

diatoms. Over many years, changes in diatom deposition rates

in sediments, then, are preserved as diatomite, providing clues

about prehistoric climates.

Diatom cells within frustules contain chloroplasts, the

organelles in which photosynthesis occurs. Chloroplasts con-

tain chlorophyll, the pigment molecule that allows plants and

other photosynthetic organisms to capture solar energy and

convert it into usable chemical energy in the form of simple

sugars. Because of this, and because they are extremely abun-

dant occupants of freshwater and saltwater habitats, diatoms

are among the most important microorganismson Earth.

Some estimates calculate diatoms as contributing 20–25% of

all carbon fixation on Earth. Carbon fixation is a term describ-

ing the photosynthetic process of removing atmospheric car-

bon in the form of carbon dioxide and converting it to organic

carbon in the form of sugar. Due to this, diatoms are essential

components of aquatic food chains. They are a major food

source for many microorganisms, aquatic animal larvae, and

grazing animals like mollusks (snails). Diatoms are even

found living on land. Some species can be found in moist soil

or on mosses. Contributing to the abundance of diatoms is

their primary mode of reproduction, simple asexual cell divi-

sion. Diatoms divide asexually by mitosis. During division,

diatoms construct new frustule cell walls. After a cell divides,

the epitheca and hypotheca separate, one remaining with each

new daughter cell. The two cells then produce a new

hypotheca. Diatoms do reproduce sexually, but not with the

same frequency.

See alsoAutotrophic bacteria; Fossilization of bacteria;

Photosynthesis; Photosynthetic microorganisms; Plankton and

planktonic bacteria

DDictyosteliumICTYOSTELIUM

Dictyostelium discoideum, also know as slime mold, are sin-

gle-celled soil amoeba which naturally occur amongst decay-

ing leaves on the forest floor. Their natural food sources are

bacteriathat are engulfed by phagocytosis. Amoeba are

eukaryotic organisms, that is, they organize their genes onto

chromosomes. Dictyosteliummay be either haploid (the vast

majority) or diploid (approximately 1 in 10,000 cells).

There is no true sexual phase of development, although

two haploid cells occasionally coalesce into a diploid organ-

ism. Diploid cells may lose chromosomes one by one to tran-

sition back to a haploid state. When food sources are plentiful,

D. discoideumreproduces by duplicating its genome and

dividing into two identical diploid daughter cells. Under star-

vation conditions, Dictyostelium enter an extraordinary alter-

nate life cycle in which large populations of cells

spontaneously aggregate and begin to behave much like a

multicellular organism. Aggregation is initiated when a small

proportion of cells emit pulses of cyclic AMP drawing in cells

in the immediate vicinity. In this phase of the life cycle, groups

of 100,000 cells coalesce and develop a surface sheath to form

well-defined slugs (pseudoplasmodia), which can migrate

together as a unit. As the pseudoplasmodium phase nears its

end, cells near the tip of the slug begin to produce large quan-

tities of cellulose that aids the slug in standing erect. This new

phase is called culmination. At this stage, cells from the under-

lying mound move upward toward the vertical tip where they

are encapsulated into spores forming the fruiting body. Spores

then are dispersed into the environment where they can remain

dormant until favorable conditions arise to resume the primary

life mode as independent organisms. Spores are resistant to

heat, dehydration, and lack of food sources. When a source of

amino acids is detected in the environment, spores open lon-

gitudinally, releasing a small but normal functioning amoeba.

Dictyosteliumare valuable biological model organisms

for studying the principals of morphological development and

signaling pathways.

See alsoMicrobial genetics

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