Box 15.2 (continued)
evolutionary and ecological strategies of these
microorganisms. Thus, the genome of M. aeruginosa is
characterized by a very high plasticity, which would give
this species the ability to colonize and proliferate in very
diverse environments (Frangeul et al. 2008).
The consequences of these blooms affect both the
functioning of the ecosystems in which they occur and
their uses. The high biomass generated during these
proliferation events are associated to a decrease in
diversity in the phytoplankton community (one or
two species becoming dominant), mainly due to strong
competition for access to light. This decrease in diversity may affect the entire food webs because colonial
and filamentous cyanobacteria are poorly consumed
by grazers. In addition, the large amounts of organic
matter generated during these blooms are nutritional
resource for heterotrophic bacteria, which results in
an increase in respiratory activity and therefore a
decrease in oxygen concentrations in the water column
up to anoxia. The consequences of this anoxia, which
moves progressively from the bottom to the upper
layer of the water body, are twofold. On the one
hand, it can promote the release of phosphorus
trapped in sediments and make it available to
cyanobacteria, and, on the other hand, it can cause
mass mortality of fishes. Regarding the use of water,
cyanobacterial blooms disrupt recreational activities
in water bodies, drinking water production, and fishing. Recent guidelines in many countries have also set
thresholds in cyanobacteria cell and microcystin
concentrations beyond which bathing should be
suspended or the production of drinking water should
be stopped.
This regulation on cyanobacteria is justified by
the fact that many types of cyanobacteria can
produce toxins harmful to human and animal health.
These toxins are of different chemical natures, and
they have different targets. The most frequently
encountered are hepatotoxins, especially microcystins
(Box Fig. 15.3a) and nodularins, which are small
cyclic peptides, as well as cylindrospermopsin
(Box Fig. 15.3b) and its analogues which are alkaloids.
They are followed by neurotoxins, such as anatoxins
(Box Fig. 15.3c), saxitoxin and its derivatives, and
β-N-methylamino-L-alanine (BMAA), which is a
non-proteinogenic amino acid. The last category of
cyanotoxins is dermatotoxins/irritant toxins, like the
lyngbyatoxin-a (Box Fig. 15.3d), the aplysiatoxin,
or the debromoaplysiatoxins which are alkaloids
or lipopolysaccharides constituting the wall of
cyanobacteria and other Gram-negative bacteria.
(continued)
Box 15.2 (continued)
These toxins have been involved, on all continents, in
many cases of intoxication of animals and humans. However, if there are numerous cases of animal’s death due to
cyanotoxins, fortunately few cases of human deaths due
to cyanotoxins have been reported because of water
treatment processes, which allow removing a very large
fraction of these toxins in drinking water treatment plants.
Many questions remain about these toxins, and, among
them, two are currently attracting a lot of work.
The first of these issues relates to the observation that,
when a proliferation of cyanobacteria occurs, it is very
difficult to predict what kind of toxin will be produced
and how much it will be. For microcystins, it has been
shown, for example, that all species of cyanobacteria
were not able to synthesize them, some of them having
lost the genes necessary for the biosynthesis of these
molecules during their evolution. In the same way, for
species known to be able to produce microcystins, it has
been shown that their populations contain a mix of cells
having the genes for the synthesis of this toxin and of cells
that have lost these genes or have nonfunctional genes. In
some ecosystems, it has been found, for two very frequent
bloom-forming species, Planktothrix agardhii and M.
aeruginosa, that the proportions of microcystinproducing cells could vary significantly during the
course of the bloom (between 20 and 80 %). Finally,
by using an experimental approach based on co-cultures
of microcystin-producing and non microcystinproducing strains, it has been shown that under limiting
conditions for the growth of cyanobacteria, the fitness of
toxic strains was better than that of the nontoxic ones,
and, generally, the inverse was observed in non-limiting
conditions for growth (Box Fig. 15.4). All these results
suggest that the cost/benefit ratio to produce microcystins varies with environmental conditions encountered by the cyanobacteria.
These results lead us to the second question,
which concerns the identification of the functional
role of cyanobacterial toxins, knowing that this issue
is common to almost all secondary metabolites
produced by bacteria. For microcystins, some authors
have suggested that these molecules could:
1. Play a major role in defense against predators,
zooplankton, or have allelopathic functions
2. Constitute carbon or nitrogen reserves
3. Be involved in the iron intracellular metabolism
4. Be signal molecules involved in cell communication (quorum sensing)
None of these assumptions is presently validated.
Very recent works, based on a comparative transcriptomic approach between a microcystin-producing
(continued)
626
P. Lebaron et al.
evolutionary and ecological strategies of these
microorganisms. Thus, the genome of M. aeruginosa is
characterized by a very high plasticity, which would give
this species the ability to colonize and proliferate in very
diverse environments (Frangeul et al. 2008).
The consequences of these blooms affect both the
functioning of the ecosystems in which they occur and
their uses. The high biomass generated during these
proliferation events are associated to a decrease in
diversity in the phytoplankton community (one or
two species becoming dominant), mainly due to strong
competition for access to light. This decrease in diversity may affect the entire food webs because colonial
and filamentous cyanobacteria are poorly consumed
by grazers. In addition, the large amounts of organic
matter generated during these blooms are nutritional
resource for heterotrophic bacteria, which results in
an increase in respiratory activity and therefore a
decrease in oxygen concentrations in the water column
up to anoxia. The consequences of this anoxia, which
moves progressively from the bottom to the upper
layer of the water body, are twofold. On the one
hand, it can promote the release of phosphorus
trapped in sediments and make it available to
cyanobacteria, and, on the other hand, it can cause
mass mortality of fishes. Regarding the use of water,
cyanobacterial blooms disrupt recreational activities
in water bodies, drinking water production, and fishing. Recent guidelines in many countries have also set
thresholds in cyanobacteria cell and microcystin
concentrations beyond which bathing should be
suspended or the production of drinking water should
be stopped.
This regulation on cyanobacteria is justified by
the fact that many types of cyanobacteria can
produce toxins harmful to human and animal health.
These toxins are of different chemical natures, and
they have different targets. The most frequently
encountered are hepatotoxins, especially microcystins
(Box Fig. 15.3a) and nodularins, which are small
cyclic peptides, as well as cylindrospermopsin
(Box Fig. 15.3b) and its analogues which are alkaloids.
They are followed by neurotoxins, such as anatoxins
(Box Fig. 15.3c), saxitoxin and its derivatives, and
β-N-methylamino-L-alanine (BMAA), which is a
non-proteinogenic amino acid. The last category of
cyanotoxins is dermatotoxins/irritant toxins, like the
lyngbyatoxin-a (Box Fig. 15.3d), the aplysiatoxin,
or the debromoaplysiatoxins which are alkaloids
or lipopolysaccharides constituting the wall of
cyanobacteria and other Gram-negative bacteria.
(continued)
Box 15.2 (continued)
These toxins have been involved, on all continents, in
many cases of intoxication of animals and humans. However, if there are numerous cases of animal’s death due to
cyanotoxins, fortunately few cases of human deaths due
to cyanotoxins have been reported because of water
treatment processes, which allow removing a very large
fraction of these toxins in drinking water treatment plants.
Many questions remain about these toxins, and, among
them, two are currently attracting a lot of work.
The first of these issues relates to the observation that,
when a proliferation of cyanobacteria occurs, it is very
difficult to predict what kind of toxin will be produced
and how much it will be. For microcystins, it has been
shown, for example, that all species of cyanobacteria
were not able to synthesize them, some of them having
lost the genes necessary for the biosynthesis of these
molecules during their evolution. In the same way, for
species known to be able to produce microcystins, it has
been shown that their populations contain a mix of cells
having the genes for the synthesis of this toxin and of cells
that have lost these genes or have nonfunctional genes. In
some ecosystems, it has been found, for two very frequent
bloom-forming species, Planktothrix agardhii and M.
aeruginosa, that the proportions of microcystinproducing cells could vary significantly during the
course of the bloom (between 20 and 80 %). Finally,
by using an experimental approach based on co-cultures
of microcystin-producing and non microcystinproducing strains, it has been shown that under limiting
conditions for the growth of cyanobacteria, the fitness of
toxic strains was better than that of the nontoxic ones,
and, generally, the inverse was observed in non-limiting
conditions for growth (Box Fig. 15.4). All these results
suggest that the cost/benefit ratio to produce microcystins varies with environmental conditions encountered by the cyanobacteria.
These results lead us to the second question,
which concerns the identification of the functional
role of cyanobacterial toxins, knowing that this issue
is common to almost all secondary metabolites
produced by bacteria. For microcystins, some authors
have suggested that these molecules could:
1. Play a major role in defense against predators,
zooplankton, or have allelopathic functions
2. Constitute carbon or nitrogen reserves
3. Be involved in the iron intracellular metabolism
4. Be signal molecules involved in cell communication (quorum sensing)
None of these assumptions is presently validated.
Very recent works, based on a comparative transcriptomic approach between a microcystin-producing
(continued)
626
P. Lebaron et al.