Sedimenticola-like species. They also observed lower abundances of Kistimonaslike and Spirochaeta-like gammaproteobacteria (13% and 0.2%, respectively).
Transcript clusters belonging to Sedimenticola-like species revealed genes involved
in sulfur and hydrogen oxidation, and carbon fixation. Metatranscriptomics also
revealed potential host-microbiota interactions involved in the establishment and
maintenance of the symbiosis, including transfer of nutrients (carbon, vitamins and
cofactors) from symbiont to host via host lysosomal digestion. Authors also detected
high abundance of lysozyme-encoding transcripts that may indicate the presence of
active lysosomes. Thus, the selection of symbiotic bacteria may be determined by
the bactericidal lysozyme secretion and other related compounds. Symbiont related
transcripts pointed out the expression of a hypothetical filamentous hemagglutinin
responsible for adhesion to host tissues or genes encoding virulence and bacterial
secretion. Despite the recent evidences, the significance of chemosymbiosis remains
unclear and further studies are required to fully understand the role of bactericidal,
adhesion and virulence factors in host selection and microbiome persistence.
24.4 Vibrios and Bivalve Molluscs, the Eternal Cold War
Vibrios are amongst the most common and widespread prokaryotes in temperate
marine environments, representing the major culturable fraction of the marine
microbial community (Ceccarelli et al. 2019). These marine bacteria possess high
colonisation potential, so they are frequently found associated with marine animals,
plants, algae and zooplankton (Takemura et al. 2014; Le Roux and Blokesch 2018)
constituting a very good example of a microbial loop leading to the assembly of
eco-holobionts (Fig. 24.1). Intimate associations, as the one presented above, with
mutualistic interactions are well documented, however Vibrio associations with
bivalves may also be neutral. Parasitic interactions are the most documented Vibrio-bivalve associations, as they cause important damages to the host (Romalde and
Barja 2010; Goudenège et al. 2015; Lemire et al. 2015; Bruto et al. 2018; Dias et al.
2018), thus lead to significant economic loses to shellfish farming industry (Travers
et al. 2015).
Bivalve molluscs accumulate exogenous bacteria as a result of their filter-feeding
habit. But these bacteria do not always colonise their tissues, instead they are
expelled from the mollusc, and as such, this bacterial fraction is also known as
transient microbiota. The stable fraction or resident microbiota is, in general, composed of complex Vibrio populations (Romalde et al. 2014; Chen et al. 2016; Bruto
et al. 2017) found both in healthy and diseased animals, where the microbes can
reach concentrations close to 100-fold higher than those in seawater (Shen et al.
2009). It is still not clear whether the presence of vibrios in bivalves results from
stable associations or from non-specific uptake from the surrounding environment.
Hunt et al. (2008a, b) found similar diversity of Vibrio populations in mussels as
compared to water samples, suggesting that the host tissues do not represent a high
selective habitat. In contrast, Bruto et al. (2017) showed that some populations were
518
A. Lasa and J. L. Romalde
Transcript clusters belonging to Sedimenticola-like species revealed genes involved
in sulfur and hydrogen oxidation, and carbon fixation. Metatranscriptomics also
revealed potential host-microbiota interactions involved in the establishment and
maintenance of the symbiosis, including transfer of nutrients (carbon, vitamins and
cofactors) from symbiont to host via host lysosomal digestion. Authors also detected
high abundance of lysozyme-encoding transcripts that may indicate the presence of
active lysosomes. Thus, the selection of symbiotic bacteria may be determined by
the bactericidal lysozyme secretion and other related compounds. Symbiont related
transcripts pointed out the expression of a hypothetical filamentous hemagglutinin
responsible for adhesion to host tissues or genes encoding virulence and bacterial
secretion. Despite the recent evidences, the significance of chemosymbiosis remains
unclear and further studies are required to fully understand the role of bactericidal,
adhesion and virulence factors in host selection and microbiome persistence.
24.4 Vibrios and Bivalve Molluscs, the Eternal Cold War
Vibrios are amongst the most common and widespread prokaryotes in temperate
marine environments, representing the major culturable fraction of the marine
microbial community (Ceccarelli et al. 2019). These marine bacteria possess high
colonisation potential, so they are frequently found associated with marine animals,
plants, algae and zooplankton (Takemura et al. 2014; Le Roux and Blokesch 2018)
constituting a very good example of a microbial loop leading to the assembly of
eco-holobionts (Fig. 24.1). Intimate associations, as the one presented above, with
mutualistic interactions are well documented, however Vibrio associations with
bivalves may also be neutral. Parasitic interactions are the most documented Vibrio-bivalve associations, as they cause important damages to the host (Romalde and
Barja 2010; Goudenège et al. 2015; Lemire et al. 2015; Bruto et al. 2018; Dias et al.
2018), thus lead to significant economic loses to shellfish farming industry (Travers
et al. 2015).
Bivalve molluscs accumulate exogenous bacteria as a result of their filter-feeding
habit. But these bacteria do not always colonise their tissues, instead they are
expelled from the mollusc, and as such, this bacterial fraction is also known as
transient microbiota. The stable fraction or resident microbiota is, in general, composed of complex Vibrio populations (Romalde et al. 2014; Chen et al. 2016; Bruto
et al. 2017) found both in healthy and diseased animals, where the microbes can
reach concentrations close to 100-fold higher than those in seawater (Shen et al.
2009). It is still not clear whether the presence of vibrios in bivalves results from
stable associations or from non-specific uptake from the surrounding environment.
Hunt et al. (2008a, b) found similar diversity of Vibrio populations in mussels as
compared to water samples, suggesting that the host tissues do not represent a high
selective habitat. In contrast, Bruto et al. (2017) showed that some populations were
518
A. Lasa and J. L. Romalde