Competitiveness of alga Microcystis aeruginosa co-cultivated with cyanobacterium Raphidiopsis raciborskii confirms its dominating position

Microcystis aeruginosa has always been regarded as the main culprit of cyanobacterial blooms in freshwater.However,in recent years,Raphidiopsis raciborskii has gradually replaced M.aeruginosa as the culprit of cyanobacterial blooms in some tropical and subtropical shallow lakes.To reveal which one plays a more dominant role,interactions between cylindrospermospin(CYN)-producing R.raciborskii and microcystins(MCs)-producing or non-MCs-producing M.aeruginosa strains were studied using bialgal cultures at different initial ratios of biomasses of the two species at 25℃.During the co-cultivation,the M.aeruginosa strains inhibited the growth and heterocyst formation of R.raciborskii filaments,and thus occupied a dominant position during the co-cultivation regardless of the initial biomass ratios in the cultures.In addition,the MCs-producing M aeruginosa strain contributed to a higher portion of the total biomass and exerted a stronger inhibitory effect on R.raciborskii compared with the non-MCs-producing strain.However,the growth of both MCs-producing and non-MCs-producing M.aeruginosa strains was stimulated by R.raciborskii in the co-cultures compared with M.aeruginosa monoculture,indicating that M.aeruginosa could outcompete R.raciborskii if given enough time,enabling it to develop into the dominant species even in very low initial concentration.To our best knowledge,this is the first report on the loss of heterocyst formation by a species of cyanobacteria that resulted from interactions between two different species of cyanobacteria.These findings indicate that it is difficult for R.raciborskii to replace the dominant position of M.aeruginosa under the experimental environmental condition,and the allelopathic effects of M.aeruginosa on R.raciborskii could significantly contribute to the success of M.aeruginosa.

Overview of the distribution and adaptation of a bloom-forming cyanobacterium Raphidiopsis raciborskii:integrating genomics,toxicity,and ecophysiology

Raphidiopsis raciborskii is a notorious bloom-forming and filamentous cyanobacterium that has been extensively investigated into its toxicity,phylogeny,and spreading potential.Studies have demonstrated that this species has spanned different climates from tropical zones to temperate regions,suggesting that R.raciborskii is becoming a cosmopolitan species in freshwater systems around the world.In fact,it has been proposed that several characteristics of R.raciborskii may explain its spread and dominance.In particular,R.raciborskii is known to display a high extent of physiological plasticity regarding nutrients,light regimes,and te mperature s.Moreover,this species illustrates different ecotype s with distinct environmental requirements.Here,we present an overview of R.raciborskii’s global distribution and adaptation strategy based on the recent findings from genome variance,toxicity,and ecophysiology.The expansion of its geographical distribution can be linked to its genome,toxicity,and ecophysiology.The variable genes are mainly associated with the stress response,phage defense,DNA repair,cell cycle control,and membrane transport,illustrating the species’adaptability in response to changing environments.In fact,the species shows rapid adaptability to low and/or variable nutrient availability,especially changing phosphorus availability.Moreover,the variabilities of strains within the population extend their flexibility to adapt and acclimate to ambient environment.In addition,cylindrospermopsins(CYN)appear to have a potential biological role in facilitating theirs dominance or bloom.These strategies of R.raciborskii make it a challenge to manage in a fre shwater system,reflecting the management of its bloom from further evidence of the complex ecophysiology,toxicity,and genome of this species.

Physiological and molecular responses of invasive cyanobacterium Raphidiopsis raciborskii to ambient phosphorus deficiency

Raphidiopsis raciborskii can cause harmful cyanobacterial blooms when concentrations of environme ntal pho sphorus(P)are very low,thus the physiological and molecular mechanisms involved in the acclimation to P need to be characterized better.The growth,chlorophyll fluorescence,alkaline phosphatase,and expression of genes directly involved in P assimilation were compared in the R.raciborskii FACHB1496 strain grown with and without inorganic P.The specific growth rate(μ),Chl a,and six fluorescence parameters(minimal fluorescence(F),maximal fluorescence(F),maximal variable fluorescence(F),electron transport flux(further than QA)per RC(ET/RC),quantum yield of the electron transport in PSⅡ(?),and the probability that an electron from a trapped exciton is moved into the electron transport chain beyond Q~-(Ψ))markedly decreased in R.raciborskii in response to experimental P-deficiency.In contrast,the relative variable fluorescence at the J-step(V),trapped energy flux(leading to QA reduction)per RC(TR/RC),and alkaline phosphatase activity significantly increased.In addition,gene expressions involved in the alkaline phosphatase(phoAl and phoA2),high-affinity inorganic P transporter(pstSl),phosphonate transporter and metabolism(phnD and phnM),and nucleotidase(nucH)were significantly upregulated under P deficiency.However,physiological and molecular responses were resumed rapidly after P re-supplementation following P-deficient conditions.Our results highlight that R.raciborskii can perform coordinated and complex cellular and physiological responses to cope with P deficiency,reflecting R.raciborskii’s multi-faceted machinery to respond to environmental P fluctuations.