302
15.1 Introduction
Macroalgae secrete a variety of organic nutrients in surrounding environment that
are utilized as a nutrient source and surface settlement cues by diverse microorganisms (Bengtsson and Ovreas 2010; Goecke et al. 2010; Bengtsson et al. 2012;
Godinho et al. 2013). Microbial communities living on the macroalgal surface are
highly dynamic and include bacteria, fungi, diatoms, protozoa, spores and larvae of
marine invertebrates (Holmstrom et al. 2002; Tujula et al. 2010; Burke et al. 2011a).
Macroalgae encompass intra- or extracellular bacteria and eukaryotes that constitute the endo- and ectophycosphere, respectively (Hollants et al. 2011, 2013; Singh
et al. 2015). These associated bacteria can exhibit a beneficial interaction with their
host by assisting with morphogenesis, growth and reproduction (Matsuo et al. 2005;
Marshall et al. 2006; Singh et al. 2011a, b; Spoerner et al. 2012). The first evidence
for a role of bacteria in algal development was reported in the middle of the twentieth century when researchers observed that axenic cultures of the green alga, Ulva,
species displayed an abnormal morphology (Provasoli and Pintner 1953, 1980;
Provasoli 1958). These observations have since been repeated in other algal species.
For example, in the case of the green macroalga Monostroma oxyspermum, a specific marine bacterial strain, YM2-23, has been shown to be essential for the normal
morphological development of the host (Matsuo et al. 2005). Likewise, the morphogenesis and growth of Gracilaria dura are impaired unless Gram-positive bacteria
(e.g. Bacillus sp.) are present (Singh et al. 2011a).
Associated microbial communities can also provide vitamins and essential nutrients to the algal host, thereby benefiting their growth. For example, nitrogen fixation
activity of some associated bacterial strains significantly influences growth of the
green and red seaweeds (Chisholm et al. 1996; Singh et al. 2011a). Bacterial biofilm
and their extracellular polymeric substance and chemical compounds are found to
be important for settling zoospores in Ulvaceae (Tait et al. 2005; Singh et al. 2013)
and releasing spores in Gracilariaceae (Weinberger et al. 2007; Singh et al. 2015).
Moreover, bacteria are thought to protect macroalgal surfaces from biofouling pressure via the production of both general and specific biological active chemical
metabolites (i.e. bioactives) (Wahl et al. 2012). Among other chemical compounds,
bacterial signalling or quorum sensing (QS) systems have gained attention for their
involvement in the bidirectional communication between bacteria and hosts (Venturi
and Fuqua 2013).
QS systems of bacteria modulate the coordinated expression of genes involved in
a diverse set of bacterial phenotypes including biofilm formation, motility, antibiotic production, virulence gene expression and exchange of genetic material
(Williams 2007). Gram-negative bacteria use a variety of different QS signalling
molecules; among them, N-acyl homoserine lactone (AHL) is arguably the most
widely studied molecule that controls several physiological responses of bacteria
(Fuqua et al. 2001; Venturi and Fuqua 2013). Investigation of AHL-mediated QS
began with the study of bioluminescence in the Hawaiian bobtail squid Euprymna
scolopes. One bacterium, Aliivibrio fischeri (formally Vibrio fischeri), is present in
R.P. Singh et al.
15.1 Introduction
Macroalgae secrete a variety of organic nutrients in surrounding environment that
are utilized as a nutrient source and surface settlement cues by diverse microorganisms (Bengtsson and Ovreas 2010; Goecke et al. 2010; Bengtsson et al. 2012;
Godinho et al. 2013). Microbial communities living on the macroalgal surface are
highly dynamic and include bacteria, fungi, diatoms, protozoa, spores and larvae of
marine invertebrates (Holmstrom et al. 2002; Tujula et al. 2010; Burke et al. 2011a).
Macroalgae encompass intra- or extracellular bacteria and eukaryotes that constitute the endo- and ectophycosphere, respectively (Hollants et al. 2011, 2013; Singh
et al. 2015). These associated bacteria can exhibit a beneficial interaction with their
host by assisting with morphogenesis, growth and reproduction (Matsuo et al. 2005;
Marshall et al. 2006; Singh et al. 2011a, b; Spoerner et al. 2012). The first evidence
for a role of bacteria in algal development was reported in the middle of the twentieth century when researchers observed that axenic cultures of the green alga, Ulva,
species displayed an abnormal morphology (Provasoli and Pintner 1953, 1980;
Provasoli 1958). These observations have since been repeated in other algal species.
For example, in the case of the green macroalga Monostroma oxyspermum, a specific marine bacterial strain, YM2-23, has been shown to be essential for the normal
morphological development of the host (Matsuo et al. 2005). Likewise, the morphogenesis and growth of Gracilaria dura are impaired unless Gram-positive bacteria
(e.g. Bacillus sp.) are present (Singh et al. 2011a).
Associated microbial communities can also provide vitamins and essential nutrients to the algal host, thereby benefiting their growth. For example, nitrogen fixation
activity of some associated bacterial strains significantly influences growth of the
green and red seaweeds (Chisholm et al. 1996; Singh et al. 2011a). Bacterial biofilm
and their extracellular polymeric substance and chemical compounds are found to
be important for settling zoospores in Ulvaceae (Tait et al. 2005; Singh et al. 2013)
and releasing spores in Gracilariaceae (Weinberger et al. 2007; Singh et al. 2015).
Moreover, bacteria are thought to protect macroalgal surfaces from biofouling pressure via the production of both general and specific biological active chemical
metabolites (i.e. bioactives) (Wahl et al. 2012). Among other chemical compounds,
bacterial signalling or quorum sensing (QS) systems have gained attention for their
involvement in the bidirectional communication between bacteria and hosts (Venturi
and Fuqua 2013).
QS systems of bacteria modulate the coordinated expression of genes involved in
a diverse set of bacterial phenotypes including biofilm formation, motility, antibiotic production, virulence gene expression and exchange of genetic material
(Williams 2007). Gram-negative bacteria use a variety of different QS signalling
molecules; among them, N-acyl homoserine lactone (AHL) is arguably the most
widely studied molecule that controls several physiological responses of bacteria
(Fuqua et al. 2001; Venturi and Fuqua 2013). Investigation of AHL-mediated QS
began with the study of bioluminescence in the Hawaiian bobtail squid Euprymna
scolopes. One bacterium, Aliivibrio fischeri (formally Vibrio fischeri), is present in
R.P. Singh et al.