Inducible Defenses Of Zooplankton(Daphnia)and Cyanobacteria Under Declining Calcium Condition

Cyanobacteria and zooplankton are important components of food web and co-exist in many freshwater environment.They both have direct and indirect effects on each other and also on other freshwater organisms.Zooplankton grazing control cyanobacteria overgrowth while cyanobacteria inhibit zooplankton grazing.Cyanobacteria have long been reported to be poor food for zooplankton.Daphnia species are particularly sensitive to hindrance of the filtering mechanism caused by cyanobacteria,which form large colonies and produce different toxins.Cyanobacterial and zooplankton inducible defenses are important but understudied process that regulate the trophic interactions of freshwater ecosystem.Daphnia due to its large size is considered an important zooplankton with the highest potential to control bloom.Previously,maternal induction has been investigated in different Daphnia species without considering phenotypic plasticity of prey.In present study,we studied the interactive inducible defenses of both prey and predator to better understand their interactions in natural environment.Similarly,previously maternal induction in different Daph”ia species and clones have been investigated in controlled conditions.However,recently calcium decline has been reported in soft water lakes,which could co-occur with anthropogenic stressors such as cyanobacterial blooms,warming,and could reduce Daphnia growth and survival.So,here,we further explored that how calcium(Ca)concentration,which is an important nutrient for Daphnia growth and survival will affect these inducible defenses.A series of microcosm experiments were performed to investigate the interactive inducible defenses of two common Daphnia species,and its phytoplankton prey,Microcystis aeruginosaPCC7806(Ma)and Microcystis flos-aquae(Mf).Furthermore,role of calcium in maternal induction,and in Daphnia carapace rigidity and its decomposition was investigated.Cell density,chlorophyll a,colonies formation and Microcystin content of prey,as well as,Daphnia growth parameters and survival was determined according to standard protocols,The results are as follow:1.Understanding interactive inducible defenses of Daphnia and its phytoplankton prey.Laboratory experiments were performed to explore inducible defenses in Daphnia and its prey,in order to better understand their interactions.Two Daphnia species,Daphnia carinata and Daphnia magna were fed with either Microcystis aeruginosapCC78o6(Ma)or Microcystis flos-aquae(Mf)mixed with Chlorella vulgaris(Cv)(exposed Daphnia),and or pure Cv(unexposed Daphnia).Exposed prey cultures were produced by prior exposure to Daphnia infochemicals.Growth parameters and toxin production of exposed prey cultures were significantly different than that of control.Exposed DaPhnia fecundity and survival was higher as compared to unexposed Daphnia.Growth and reproduction was reduced in exposed Daphnia when fed with exposed prey as compared to those fed with unexposed prey.This study provides information on the interactive inducible defenses between cyanobacteria and its grazer under laboratory conditions and may increase our understanding of cyanobacteria and Daphnia interactions in the fresh water environment.2.The importance of Ca in improving resistance of Daphnia to Microcystis.A laboratory microcosm experiments were performed to examine maternal effects in Daphnia carinata and Daphnia pulex,and that how Ca concentration in maternal environment induce responses in their offspring.The results showed that growth and survival were increased in offspring from exposed Daphnia as compared to unexposed,when raised in high Ca and increasing M.aeruginosa concentration.Among exposed Daphnia,offspring from high Ca mothers,produced more neonates with large size and higher survival as compared to offspring from low maternal Ca.Exposed D.carinata and D.pulex offspring,when reared in Ca deficient medium and increasing M.aeruginosa concentration,time to first brood increased,size become large and total offspring decreased subsequently in three alternative broods in offspring from low maternal Ca.In contrast,growth and reproduction in offspring from high Ca exposed mothers were consistent in three alternative broods.This study demonstrates that Ca have role in Daphnia maternal induction against Microcystis,and recent Ca decline and increasing Microcystis concentration in lakes may decrease growth and survival.3.Rapid mineralization of Daphnia carcasses may boost cyanobacterial blooms.Two Daphnia species,Daphnia carinata and Daphnia pulex were grown in low and high calcium concentration and fed with either high or low dietary phosphorous(P).Daphnia grown for two weeks were taken and their mineralization was investigated in presence of natural bacteria and or natural bacteria mixed with specific bacteria at 24? and 28?.Filtrates of Daphnia carcasses were given to Microcystis and Chlorella cultures and their growth and colonies formation were recorded.Results indicate that under sufficient dietary P and environmental Ca,the Daphnia has more Ca and P and decomposition process was slow,while Daphnia carcasses decomposed rapidly in case of sufficient P and low Ca,as observed by growth of Microcystis and Chlorella.Under high Ca and low P and low Ca and low P,the concentration of P and Ca was not significantly different but decomposed slow and fast respectively.The carcasses decomposition rate was high at 28? as compared to 24?.Carcasses grown with specific bacterial strains enhanced colonies formation in Microcystis species while no colonies formation was observed in Chlorella and in natural micro-organisms treatment.Here,we showed that besides nitrogen and phosphorous eutrophication,Ca decline could enhance cyanobacterial blooms,due to Daphnia mortality and their rapid mineralization.Consequently,the maternal induction in two common Daphnia species against Microcystis toxicity was less in low Ca concentration as compared to high Ca.Daphnia with high Ca has better growth and survival as compared to low Ca.Recently,Ca decline in lakes and reduce growth of Daphnia species may increase cyanobacterial blooms.