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several more reactive aldehydes (Jung and Pohnert
2001
; Jung et al.
2002
).
Caulerpenyne is deterrent against some but not all herbivores (Jung et al.
2002
;
Baumgartner et al.
2009
) as it significantly reduces food quality due to protein
cross-linking. Due to their chemical characteristics, the aldehydes derived from
caulerpenyne activation presumably have greater bioactivity but this has been
difficult to test because of their instability. Other green algal species, particularly
in
Ulva
and related genera, produce high levels of dimethylsulfoniopropionate
(DMSP) which has multiple roles in algae (Van Alstyne
2008
). Upon wounding,
several algae convert DMSP into dimethylsulfide and acrylic acid, both of which
are feeding deterrents to sea urchins and likely act as activated defenses (reviewed
by Van Alstyne
2008
).
Fucoid brown algae have served as important models in understanding induced
defenses in seaweeds, particularly
Ascophyllum nodosum
and
Fucus
spp. which
elaborate phlorotannins as chemical defenses (Amsler and Fairhead
2006
). How-
ever, interactions involving induction of defenses vary with species and even
population.
A. nodosum
has been shown to increase phlorotannin production and
become more unpalatable when under attack by snails but not by isopods (Pavia and
Toth
2000
) while in some (but not all) populations
Fucus vesiculosus
induces
defenses (measured as palatability) when attacked by either snails or isopods
(Rohde et al.
2004
). Mechanical wounding alone does not induce defenses in
A. nodosum
(Pavia and Toth
2000
) and the induction appears to be a response to
digestive enzymes present in the snails' saliva (Coleman et al.
2007b
). Snails grazing
on
A. nodosum
result in waterborne cues being released that enable neighboring
A.
nodosum
individuals to induce defenses even in the absence of being wounded
themselves (Toth and Pavia
2000
) and also can attract predators of the snails (see
defenses in neighboring individuals are produced in response to predation by isopods
but not snails (Rohde et al.
2004
). However, although predation by isopods also
directly induces defense production in two other species of
Fucus
, in these other
species there is no induction from waterborne cues (Rohde and Wahl
2008
).
9.3.2 Defenses Against Pathogens
Seaweed pathogens include bacteria, fungi, and filamentous algal endophytes.
These relationships and the seaweeds' chemical defenses against pathogens have
been the subject of several recent reviews (Weinberger
2007
; Lane and Kubanek
2008
; Potin
2008
; Goecke et al.
2010
; see also Chap.
11
by Potin).
One seaweed defense that has been studied with both bacterial and filamentous
algal endophyte pathogens is the production of reactive oxygen species
(e.g., hydrogen peroxide). These are produced in response to breakdown products
from the algal cell walls during pathogen entry (Weinberger
2007
; Potin
2008
). Such
defenses are referred to as
oxidative burst responses
. In some cases, particularly with
bacteria, the ability of the reactive oxygen species to oxidize a wide variety of