Biology Reference
In-Depth Information
coralline crustose algae (Wieczorek and Todd
1998
; Steinberg and deNys
2002
; see
also Chap.
10
by Friedrich). Shifts in the community composition of alga-
associated bacterial communities may potentially result in changed availability of
such cues and subsequently in changed intensity of fouling (Steinberg and deNys
2002
).
Two recent reports show that the main determinant of the composition and
structure of epiphytic bacterial communities at the surfaces of various seaweeds
is the nature of the algal host (Hengst et al.
2010
; Lachnit et al.
2011
). Algae clearly
shape the structure and composition of microbial communities in their vicinity
(Lam and Harder
2007
; Lam et al.
2007
). On the other hand, they are also affected
by these microorganisms. Certain bacteria have been shown to provide their host
algae with growth factors, nutrients, or protection from settlement by other micro-
or macrofoulers (Armstrong et al.
2001
; Dobretsov and Qian
2002
; Matsuo et al.
2005
; Zheng et al.
2005
) and nonpalatable epiphytes potentially protect more
palatable hosts from grazing (Karez et al.
2000
). Quorum sensing (QS) is a cell-
to-cell communication mechanism that allows bacteria to coordinate swarming,
biofilm formation, stress resistance, and production of secondary metabolites in
response to an excess of the threshold of QS signals (Paul and Ritson-Williams
2008
; Dobretsov et al.
2009
). Gram-negative bacteria, such as
Pseudomonas
or
Vibrio
strains, produce
N
-acyl homoserine lactones (AHLs) as signaling
compounds.
Pseudomonas
spp. are also known to produce diketopiperazines acting
as QS signals (Dickschat
2010
). The signal molecules
-butyrolactones and
oligopeptides are known to be synthesized by Gram-positive bacteria, e.g.,
members of the genera
Streptomyces
or
Bacillus
(reviewed by Dobretsov et al.
2009
). The interaction between zoospores of eukaryotic green macroalgae
(Ulvales) with
Vibrio anguillarum
indicates algal susceptibility to quorum sensing
AHL molecules (Joint et al.
2002
; Wheeler et al.
2006
). Although the specific
mechanism regulating these responses to AHLs is not known, it was shown that the
AHL molecules affect the calcium influx into the spores of
Ulva
sp., affecting their
motility toward the surfaces where they eventually settle (Diggle et al.
2007
; Joint
et al.
2007
). In addition, it has been demonstrated that life cycle completion and
spore release in the red epiphytic alga
Acrochaetium
sp. strongly depend on AHLs,
which are produced by bacteria associated with the basiphyte
Gracilaria chilensis
(Weinberger et al.
2007
). These findings of AHL perception in green and red algae
confirm that AHL signaling is more widespread among eukaryotes than previously
thought. This indicates a more general importance of the associated microbial
communities in interactions with macroalgae.
During the past decade it has been shown that various macroalgae are able to
stimulate, inhibit, or inactivate QS signals in bacteria by producing QS inhibitors or
analogs thereof (Maximilien et al.
1998
; Joint et al.
2007
; Kanagasabhapathy et al.
2009
). The Australian red macroalga
Delisea pulchra
produces halogenated
furanones, structural analogs to
N
-acyl homoserine lactones. These furanones
protect the algal surfaces by interfering with AHL-regulated processes and selec-
tively inhibit bacterial colonization and biofilm formation (Maximilien et al.
1998
;
Rasmussen et al.
2000
; Manefield et al.
2002
). In addition to the furanones of
g
