| Microalgae are photosynthetic autotrophic unicellular or multicellular microrganisms,which are widely distributed in oceans,lakes and swamps.In natural environments,microalgae exist symbiotically with other organisms such as bacteria and fungi,forming algae-bacteria consortia instead of individual growth.Recently,algae-bacteria consortia have been used for the removal of nutrients such as nitrogen(N)and phosphorus(P),remediation of metal and metal(loid)s from water and degradation of organic pollutants from sewage.However,there are few reports regarding the accumulation and detoxification of arsenic(As)and cadmium(Cd)by consortia with different proportions of algae:bacteria.In this study,As and Cd tolerance and accumulation capacity of Chlorella salinaBacillus subtilis(C.salina-B.subtilis)consortia were evaluated.By setting different ratios of algae-bacteria cells,following aspects were investigated:(1)The effect of As(?)and As(?)on the growth of algae-bacteria consortia and the difference in the accumulation,adsorption,absorption and speciation transformation of As(?)and As(?);(2)Effects of Cd on the growth of algae-bacteria consortia and differences in the accumulation,adsorption,and absorption of Cd;(3)Changes in cell morphology,the content of intracellular phosphorus(P),glutathione(GSH),extracellular polymeric substances(EPS),and surface functional groups of algae-bacteria consortia treated with As and Cd.The results of this study are summarized as follows:1.Growth of consortia with various alga-bacterium ratios and As accumulation and speciation transformation under arsenic treatmentsIn this research,consortia of C.salina and B.subtilis was prepared by setting different ratios of alga-bacterium(1:0,1:1,1:2,1:3,1:4)and treated with a series of As(?)and As(?)concentrations for 7 days to determine the growth of the symbionts,the As accumulation,adsorption,absorption,and As speciation transformation in the consortia and culture medium.The results showed that the addition of different proportions of B.subtilis promoted the growth of C.salina.The chlorophyll content of C.salina was the highest under the ratio of 1:4.Under As(?)and As(?)treatment,the growth rate of chlorophyll content,dry weight and specific growth rate of algae-bacteria consortia were higher than that of single C.salina,indicating that the addition of B.subtilis increased the As tolerance of C.salina cells.Under the same As treatments,the higher proportion of B.subtilis increased the amount of As accumulation by algae-bacteria consortia,the accumulation changed from absorption to adsorption,the proportion of As(?)decreased and the proportion of As(?)increased in the symbionts,indicating that B.subtilis has a certain reduction ability.Only As(?)was present in the culture medium under As(?)treatment,while in the case of As(?),a small amount of methylated arsenic was present in the culture medium,indicating that different algal-bacteria ratios affected the As speciation transformation ability of symbionts.2.Growth of consortia with various alga-bacterium ratios and Cd accumulation and removal under Cd treatmentsThe algae-bacteria consortia with various alga-bacterium ratios(1:0,1:1,1:2,1:3,1:4)were treated by different concentration of Cd for 7 days.The growth of the symbionts,Cd accumulation,adsorption,absorption and removal rate by algae-bacteria consortia were determined.The results showed that,with the increase of B.subtilis ratio in the algal culture,the chlorophyll contents,dry weight and specific growth rate of the symbionts were increased under the Cd treatments.At 1:4 algae-bacteria ratio,higher rates were observed than those of the single C.salina,indicating that the addition of B.subtilis enhanced the Cd tolerance by C.salina.With the increasing Cd concentrations,the Cd accumulation by algae-bacteria consortia was increased,and the accumulation changed from adsorption to absorption.In conclusion,the increase of bacteria in algal culture enhanced the Cd tolerance and removal by C.salina cells.3.Physiological response of consortia with various alga-bacterium ratios under As?Cd treatmentsThe morphology of algae-bacteria consortia under different proportions of algaebacteria was observed by applying different As(?)and Cd treatments,and the contents of extracellular polymers(EPS),intracellular glutathione(GSH),changes in the surface functional group and intracellular and extracellular phosphorus(P)of algae-bacteria consortia were determined.The results showed that,under As(V)and Cd treatment,the morphology of 1:0 symbionts was more prone to deformation than that of 1:4 ones,and the effect of Cd was greater than that of As(V).Under As(V)and Cd treatments,the EPS contents of algaebacteria consortia were different.With increasing concentration of As(V)in the culture medium,the EPS contents of symbionts were slightly decreased,while in the case of Cd treatment,the EPS contents of algae-bacteria consortia were increased.Polysaccharides were the major component of the EPS accounting for 80%of the total EPS contents that may play a significant role in Cd accumulation and tolerance.FTIR analyses showed that NH,OH,P=O,and CO functional groups were present on the surface of algae-bacteria consortia and shifts in peaks after As and Cd treatments indicated that carboxyl,hydroxyl/amino,and phosphoryl groups of algae-bacteria consortia were involved in As and Cd accumulation on the cell surface.Increasing As and Cd levels enhanced the synthesis of GSH in algae-bacteria consortia,suggesting that GSH are important substances for the detoxifying of heavy metals for symbiotic systems.The increasing proportion of B.subtilis in algal culture also led to higher P contents in symbiotic or medium and subsequently reduced the intracellular accumulation of As(V)because of As(V)and P transportation competition.These findings suggested,more P accumulation in algae-bacteria consortia reduced As(V)and Cd toxicity and enhanced Cd adsorption.Altogether,this study demonstrated significant differences existed in the As/Cd accumulation and speciation transformation in the consortia with various ratios of C.salina-B.subtilis,and confirmed the metabolic processes associated with As,Cd detoxification inside and outside the algae-bacteria consortia.It was found that the difference in the proportion of algae and bacteria significantly affected the growth of the symbionts.The addition of B.subtilis in the C.salina culture changed the As and Cd resistance and their accumulation in the consortia.The As and Cd toxicities were reduced by processes such as increased synthesis of EPS,intracellular GSH,functional groups on the cell surface,and intracellular and extracellular P contents. |
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