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wide range of secondary metabolites, and they are considered as a potential
source of molecules with therapeutic activities. Freshwater cyanobacteria
are better known as toxin producers although they certainly produce valu-
able compounds in terms of therapeutic potential. There is a growing inter-
est in studying terrestrial or symbiotic cyanobacteria, although not much is
known on these microorganisms, because they produce diverse secondary
metabolites with interesting biological activities, such as the cryptophycins.
3. OVERVIEW OF SECONDARY METABOLITES FROM
CYANOBACTERIA FOR WHICH BIOSYNTHETIC
GENES HAVE BEEN IDENTIFIED
While identification of secondary metabolites from cyanobacteria
has a long history, the identification of the biosynthetic genes responsible
for the production of secondary metabolites, in these microorganisms, is
relatively recent (after year 2000). We have summarized in
Table 6.1
essen-
tial data concerning the cyanobacterial secondary metabolites for which
the biosynthetic genes have been identified. Among all the cyanobacterial
metabolites isolated (probably over a thousand), we have only found about
30 cases for which the biosynthetic genes have been identified. The main
reason for this low number of clusters discovered is that the identification
of these genes is highly dependent on genomic data on the producer. In
fact, there are about 100 cyanobacterial genomes so far sequenced and the
majority of them are from unicellular species that do not produce second-
ary metabolites (
Hess, 2011
). Finally, the fact that the genetic manipulations
of cyanobacteria are difficult has also probably hampered the discovery of
biosynthetic genes implicated in secondary metabolism.
3.1. The Metabolites
It is not easy to classify the secondary metabolites from cyanobacteria because
they are very diverse in structure. We have followed a simple classification
based on the origin of the secondary metabolite: first, the cyanotoxins from
freshwater cyanobacteria, followed by other metabolites from freshwater
cyanobacteria, and then metabolites from terrestrial cyanobacteria, followed
by metabolites from marine cyanobacteria and finally, ribosomal peptides
(RPs) and other metabolites.
As already noted, the structural diversity in the metabolites described
in this review is very large. It is also interesting to note that for each class
of metabolite, there are almost always variants that have been detected.
