Introduction to Fungi, Third Edition

HETEROBASIDIOMYCETE YEASTS
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widely used to prevent the graft rejection
reaction after organ transplantations. In vitro,
growth of C. neoformans is inhibited by cyclosporin
A at 37°C but not at lower temperatures.
The use of cyclosporin A for human antifungal
therapy is impossible because its immunosuppressive
activity outweighs the antifungal effect.
However, Cruz et al. (2000) have reported cyclosporin
derivatives which are antifungal but do
not have immunosuppressive properties, i.e. they
appear to interfere with fungal calcineurin
signalling but not with the equivalent human
signalling chain involved in the immune
response. It remains to be seen whether these
substances are sufficiently specific for use as new
antifungal drugs.
24.2.2 Phaffia and Xanthophyllomyces
Phaffia rhodozyma and its putative teleomorph
Xanthophyllomyces dendrorhous have aroused
considerable interest because they are among
very few fungi producing the commercially
valuable carotenoid pigment astaxanthin (see
Fig. 24.8), and probably the only ones to do so in
pure culture. Astaxanthin is of importance in the
fish farming industry because salmonid fish
(salmon and trout) require a minimum level of
astaxanthin in their food for healthy growth.
One reason for this may be that these fish
contain high levels of polyunsaturated fatty
acids which are susceptible to peroxidation by
reactive oxygen species such as the hydroxyl
radical (HO) or superoxide radical (O 2 ).
Astaxanthin and the related pigment canthaxanthin
are also responsible for the orange
pigmentation of salmon steak. In nature, astaxanthin
travels the food chain phytoplankton !
zooplankton ! larger crustaceans ! salmon.
Simple methods to extract and analyse astaxanthin
from Phaffia and salmon have been
described by Weber and Davoli (2003). In view
of the correlation between oxidative processes
and cancer or degenerative diseases related to
ageing, astaxanthin is also gaining popularity
as a ‘nutraceutical’, i.e. an additive to the
human diet.
Astaxanthin is now produced commercially
by total chemical synthesis, by extracting it from
the exoskeletons of shellfish, or by microbial
fermentation using Phaffia or the alga
Haematococcus pluvialis. Whereas wild-type strains
of Phaffia synthesize a limited amount of astaxanthin
(typically less than 300 mgg 1 dry
weight), the astaxanthin levels in strains used
for industrial production are at least 10-fold
higher. In-depth reviews of biotechnological
aspects of astaxanthin production have been
written by Johnson and An (1991) and Johnson
and Schroeder (1995a,b).
It is unclear why, among the numerous red
yeasts, Phaffia and Xanthophyllomyces are the only
ones as yet known to synthesize astaxanthin.
Part of the answer may lie in their unusual
habitat, all strains known to date having been
isolated from the slime fluxes of broad-leaved
trees, especially birch (Betula spp.) in Alaska,
Japan, Scandinavia and Russia (Phaff, 1990).
Schroeder and Johnson (1995) have presented
evidence that birch sap contains a photosensitizer,
i.e. a substance that becomes energized by
UV light. When this passes on its excitation
energy, it can transform ground-state oxygen
(triplet oxygen,
3 O 2 ) to the reactive singlet
oxygen ( 1 O 2 ) state. This can be returned to its
ground state under dissipation of the excess
energy as heat by astaxanthin and other carotenoids.
Astaxanthin appears to be necessary for
the survival of Phaffia under the oxidizing
conditions prevalent in birch sap. In fact,
cultivation and mutant studies have shown
that astaxanthin is able to protect its producing
organism against a wide range of oxidative
stresses caused by H 2 O 2 , singlet oxygen and the
hydroxyl and superoxide radicals, and that
astaxanthin biosynthesis is increased under
such conditions (Schroeder & Johnson, 1993).
In contrast, b-carotene is the main pigment at
low oxygen partial pressure, i.e. Phaffia cultures
grown under reduced aeration appear yellow
instead of orange (see Fig. 24.8).
The original isolate of Phaffia is a purely
asexual strain, but other isolates show sexual
reproduction under suitable conditions characterized
by nitrogen starvation and the presence
of polyols (Kucsera et al., 1998). This perfect state
was named Xanthophyllomyces dendrorhous, and
there is still disagreement as to whether Phaffia