Chemical Ecology of Seaweeds - Seaweed Biology - page 174

gametes releasing the C 11 pheromone lamoxirene [reviewed by Maier and M € uller

( 1986 ) and M uller ( 1989 )], a behavior that can be described as a true chemotaxis

(cf. Amsler and Iken 2001 ). Lamoxirene also stimulates release of male gametes
from male gametophytes' antheridia (Maier 1982 ).
Unintentional forms of chemical communication, at least from the perspective of
the signal sender, occur between spores of the green seaweed Ulva spp. and biofilm
bacteria as well as in bacterial stimulation of spore release of the red alga
Acrochaetium sp. (see also Chap. 11 by Potin and Chap. 10 by Friedrich). It has
been known for some time that biofilm composition can influence Ulva spore
settlement (e.g., Thomas and Allsopp 1983 ) and that bacterial metabolites are neces-
sary for normal development of Ulva germlings (e.g., Provasoli and Pinter 1980 ).
Joint et al. ( 2000 ) reported that within biofilms, the spores preferentially settle on or
next to bacterial cells. This settlement pattern is the result of spores “eavesdropping”
on N -acylhomoserine lactone (AHL) signals (Joint et al. 2002 ; Tait et al. 2005 ) which
are produced by many bacteria for quorum sensing, an intraspecific form of chemical
communication involved in multiple bacterial processes (Chhabra et al. 2005 ). The
mechanism is a form of chemokinesis in which the spores drastically reduce their
swimming speed when close to the AHL source, but this only occurs if they are in
contact with the surface of the substrate (Wheeler et al. 2006 ).
AHLs can also be involved in seaweed spore release. The filamentous red alga
Acrochaetium sp. is a common epiphyte on the larger red seaweed Gracilaria
chilensis and spore release in Acrochaetium sp. appears to be induced by AHLs
from bacteria growing epiphytically on G. chilensis (Weinberger et al. 2007 ; see
also Chap. 11 by Potin). However, no such effect of AHLs was observed with
another (although not closely related) filamentous red alga, Sahlingia subintegra ,
which also grows epiphytically on G. chilensis (Weinberger et al. 2007 ).
As discussed later, some seaweeds release waterborne chemical cues when
under attack by some (but not all) herbivores. These cue adjacent conspecifics to
induce the production of defensive chemical compounds even without being
attacked themselves (Toth and Pavia 2000 ; Toth 2007 ). Predators of herbivorous
snails preying on the seaweeds can also sense these cues, another example of
communication between species. Coleman et al. ( 2007a ) reported that fish predators
can be attracted by the scent of snails feeding on other organisms but significantly
more so if the snails are grazing on Ascophyllum nodosum , an alga known to induce
its own defenses when neighboring conspecifics are under attack (Toth and Pavia
2000 ). Crab predators of the snails were attracted by A. nodosum that had been
grazed by snails even if the snails had been removed (Coleman et al. 2007a ).

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9.2.2 Environmental Sensing

Seaweed spores are known to respond to physiochemical cues, in particular to surface
hydrophobicity. Spores from several species of brown and green algae settle prefer-
entially on hydrophobic surfaces in the laboratory, although the adaptive significance