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ENVIRONMENTAL CONDITIONS 151

• Mitochondria and chloroplasts contain ribosomes,
  which closely resemble the ribosomes of prokaryotes
  in their sensitivity to antibacterial antibiotics and
  in the DNA sequence homology of the ribosomal
  RNA genes.

The origin of hydrogenosomes has remained an

unsolved problem until recently. They are bounded

by double membranes, have an inner membrane with

cristae-like projections, and contain ribosome-like

particles resembling those of methanogenic archaea.

Some of these features indicate a similarity with

mitochondria, but until recently no hydrogenosomal

genes had ever been found. However, in 1998 the first

evidence of hydrogenosomal genes was found in an

anaerobic ciliate, Nyctotherus ovalis, which occurs in

the hindgut of cockroaches. In electron micrograph

sections of this organism the hydrogenosomes showed

immunogold labeling when treated with a commercial

antiserum against DNA (Fig. 8.9). This evidence was

reinforced by using the polymerase chain reaction,

with primers directed against conserved regions of

the genes coding for mitochondrial small subunit

(SSU) ribosomal RNA. A homologous SSU rRNA gene

was obtained from the anaerobic ciliate, N. ovalis,

indicating commonality between hydrogenosomes

and mitochondria.

Physiology of oxygen tolerance

The existence of strictly anaerobic organismssuch

as rumen chytrids and ciliated protozoa indicates that

oxygen can be toxic – it kills these organisms when they

are exposed to even low levels of oxygen. The reason

for this is well known: several highly reactive forms of

oxygen such as O 2 −(superoxide anion), H 2 O 2 (hydro-

gen peroxide), and OH· (hydroxyl radical) are pro-

duced inadvertently when oxygen reacts with some

of the common cellular constituents such as flavo-

proteins and quinones. These reactive oxygen species

would, ordinarily, damage cellular components such

as macromolecules, in the same way as the peroxides

of common disinfectants are used to kill micro-

organisms. So, all organisms that grow in the presence

of oxygen need mechanisms for coping with the

toxic effects of oxygen, and this is achieved in ways

described below.

Superoxide is converted to hydrogen peroxide by

the enzyme superoxide dismutase, according to the

following equation:

O 2 −+O 2 −+2H+→2H 2 O 2 +O 2

Hydrogen peroxide is then converted to water and

oxygen by the enzyme catalase, according to the fol-

lowing equation:

H 2 O 2 +H 2 O 2 →2H 2 O +O 2

The combined effect of superoxide and catalase is,

therefore, to convert the reactive oxygen species to

water. All obligate anaerobeslack one or both of

these enzymes. For example, Neocallimastixhas super-

oxide dismutase but not catalase, so its inability to

deal with peroxides probably accounts for its failure to

tolerate the presence of oxygen.

The hydroxyl radical is the most toxic of all, but

occurs only transiently – mainly as a consequence of

ionizing radiation. The small amounts that are produced

Fig. 8.9Electron micrograph section
of a cell of the anaerobic ciliate,
Nyctotherus ovalis, showing: macronu-
cleus (N), micronucleus (n), vacuoles
(V), and hydrogenosomes (H). The
small black dots within the hydro-
genosomes are immunogold particles
that were tagged to bind to DNA.
(Reproduced from Akhmanova et al.
1998, with permission; © Macmillan
Publishing.)