Biology Reference
In-Depth Information
profile of the alga and in an increased resistance against spores of the epiphytic red
alga
Acrochaetium
sp. Semiquantitative RT-PCR and enzyme assays demonstrated
that this metabolic response occurs in succession of an upregulation of
lipoxygenases and phospholipase A2 activity. Even if this strongly suggests the
involvement of regulatory oxylipins, the known plant hormone jasmonic acid and
the algal metabolite prostaglandin E2 do not trigger comparable defense responses.
Interestingly also,
C. crispus
gametophytes synthesize ultraviolet absorbing
compounds around the sites of penetration of zoospores of the green endophyte
A. operculata
, whereas this response is absent in the sensitive generation (Bouarab
et al
.
2004
). These findings are reminiscent of the deposition of phenolic
compounds in higher plant-fungus or oomycete interactions (McLusky et al
.
1999
). Although the exact structure of these ultraviolet absorbing compounds
remains to be elucidated, their aromatic nature suggests that the perception of the
pathogen induces a pathway related to the phenylpropanoid metabolism of terres-
trial plants. Indeed,
A. operculata
extracts activated two key enzymes of this
pathway, shikimate dehydrogenase and phenylalanine ammonia-lyase (Bouarab
et al.
2004
) that are also activated by oxylipins.
The only very clear case of nonphlorotannin brown algal secondary metabolites
acting as antifoulants in nature that we are aware of is in
Dictyota menstrualis
(Order Dictyotales). Schmitt et al
.
(
1995
) reported that
D. menstrualis
was notice-
ably less fouled in nature than other macroalgae and that bryozoan larvae would not
settle on
D. menstrualis
in laboratory bioassays even though they would contact its
surface and would settle on several other species of brown and red macroalgae.
Extracts of
Lobophora variegata
(Order Dictyotales) from the Bahamas have
particularly strong antifungal bioactivity and Lane et al. (
2009
) identified the cyclic
lactone, lobophorolide, as a defensive compound with significant bioactivity at
exceptionally low concentrations against both pathogenic and saprophytic sympat-
ric fungi. Although
L. variegata
from 46 of 51 samples collected at ten sites in the
Bahamas contained measurable, bioactive concentrations of lobophorolide, it was
not present in two collections from the Red Sea even though crude extracts of the
Red Sea algae did have antifungal bioactivity. This indicates that although
L. variegata
from different regions may be chemically defended against pathogens,
the same specific compounds may not be involved.
Although the vast majority of the investigations into the defensive chemical
ecology of macroalgae have focused on roles of secondary metabolites (see Chap.
9
by Amsler), recent work has demonstrated that both red and brown algae employ
oxidative burst defenses against pathogens, a defensive mechanism that has been
recognized in terrestrial plants for a number of years (reviewed by Lamb and Dixon
1997
; Wojtaszek
1997
; Mahalingam and Fedoroff
2003
; see also Chap.
6
by
Bischof and Rautenberger).
This was first discovered in red algae, where cell wall degradation products elicit
a rapid release of H
2
O
2
that is toxic to epiphytic bacteria (Weinberger et al.
1999
,
2001
; Weinberger and Friedlander
2000
). K
upper et al
.
(
2001
) demonstrated an
analogous response in
Laminaria digitata
(Order Laminariales), which produces a
rapid oxidative burst of H
2
O
2
and O
2
—in response to oligomeric degradation
€
