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The! genetic! basis! of! microcystin! production! is! complex.! It! has! been!

observed!that!all!the!blooms!containing!microcystin?producing!strains!also!contain!
related!strains!that!lack!the!ability!to!produce!this!toxin!(24).!In!addition,!usually,!
patches! of! high?! and! low?toxin! concentration! occur! simultaneously! in! the! same!
water!body!(27,!28).!In!short,!Carrillo!et!al.!(29)!and!Martín!et!al.!(30)!documented!
a! great! genetic! variability! in! microcystin! production! in! different! M.) aeruginosa!
strains! isolated! from! the! same! waterbloom,! very! toxic! to! non?toxic! strains!
occurring! together! were! documented.! It! is! thought! that! the! difference! between!
microcystin! producing! (toxic)! and! nonproducing! (nontoxic)! strains! of!
cyanobacteria! lies! primarily! in! the! presence! or! absence! of! microcystin! synthetase!
gene! cluster! (31,! 32).! ! Finally,! it! is! tested! to! that! the! production! of! toxin! in! a! M.)
aeruginosa)bloom!also!varies!with!time!(24,!28).!!

Some! studies! attribute! the! higher! or! lower! production! of! toxin! to!
environmental! factors! such! as! nitrogen! and! phosphorus! concentration,! light!
intensity! or! temperature! (24,! 33).! However,! laboratory! studies! have! shown! that!
environmental!factors!induce!changes!in!the!concentration!of!toxins,!but!to!a!lesser!
magnitude! than! those! observed! between! different! strains! grown! under! the! same!
environmental!conditions!(reviewed!by!20).!!

Combinations! of! experiments! and! statistics! procedures! were! employed! for!
estimating!the!proportion!of!genetic!versus!phenotypic!variability!(heritability)!in!
microcystin! net! production! on! 18! M.) aeruginosa! strains.! Results! indicated! that!
more! than! 70%! of! microcystin! net! production! variability! is! due! to! genetic!
differences! among! strains! (34).! Consequently,! genetic! factors! appear! to! be! the!
main! cause! of! the! spatial?temporal! heterogeneity! observed! in! microcystin!
production.!!

Additionally,! most! of! the! phenotypic! variability! for! the! most! important!
quantitative! traits! of! M.) aeruginosa! (i.e.! growth! rate,! respiration,! photosynthetic!
efficiency,!use!of!inorganic!carbon,!pigment!concentration,!photochemical!and!non!
photochemical! quenching,! morphology! and! others)! is! also! due! to! genetic!
differences!among!strains!and!not!to!the!environment!(35?37).!!

Since! asexual! reproduction! prevails! blooms! of! M.) aeruginosa! could! exist! as!
large! clonal! families,! and! consequently! clones! of! ancient! divergence! can! be! quite!
different!from!one!another!by!mutations.!Due!to!importance!of!genetic!differences!
for!the!most!significant!quantitative!traits!of!M.)aeruginosa!(including!microcystin!
production)! analyze! and! characterize! the! different! strains! forming! a! bloom! has! a!
significant!environmental!relevance.!

Nowadays,!differentiation!of!cyanobacteria!species!and!strains!are!based!on!
modern! procedures! of! molecular! genetics! (38).! PCR! procedures! focused! to!
identification! of! microcystin?producing! genotypes! of! different! cyanobacteria!

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