A Mechanistic Study Of Bacteria-algae Aggregation Processes Under Different Na~+ Ionic Strengths

Photosynthetic autotrophic algae and heterotrophic bacteria are important primary producers and consumers in the natural environment.The interaction between these two types of microorganisms is important for the elemental cycle of aquatic and terrestrial systems,and ecological health.In the natural environment,these two microorganisms often adhere to each other to form multicellular aggregates.In agglomerates,close contact between cells promotes interspecies interactions.Therefore,the aggregation process of algae and bacteria plays an important role in its growth,metabolism,related environmental and ecological functions.Previous studies on the interaction of bacteria and algae mostly focused on the characteristics of microbial growth rate,nutrient cycling,metabolites and gene expression under coexistence conditions.However,little research was done on the process of aggregation of algae and its determinants.Since ionic strength is one of the main environmental factors determining the interfacial reaction of microorganisms,this paper mainly studies the difference of aggregation process of E.coli K12 and Chlamydomonas reinhardtii under different ionic strengths(1 mM NaCl,10 mM NaCl,100 mM NaCl),by means of inverted fluorescence microscopy(FM),laser confocal microscopy(CLSM),scanning electron microscopy(SEM)and other dynamic aggregation process;using attenuated total reflection Fourier transform infrared spectroscopy(ATR-FTIR),contact angle,Zeta potential instrument and other analytical techniques,combined with biology Research methods(constructing mutant strains,extracting EPS,enzymatic treatment)clarify the aggregation mechanism of bacteria and algae.The major findings were summarized as follows:(1)In-situ observation of bacteria and algae interactions by microscopic techniques revealed large differences in interactions at different ionic strengths.When the ionic strength was 1 mM NaCl and 100 mM NaCl,no obvious aggregation was observed,and the bacteria and algae were dispersed and evenly distributed.In 10 mM NaCl environment,the interaction between bacteria and algae is dense and agglomerated.The phenomenon of adsorption aggregation is that a large number of bacterial cells directly adsorb on the surface of algae cells or form agglomerates around algae cells.This agglomerate will accelerate the growth rate of Chlamydomonas.(2)Based on ATR-FTIR,contact angle,Zeta potential analyzer and other interfacial reactions,traditional research techniques are used to characterize the surface properties of bacteria and algae under different ionic strengths.Previous studies have shown that the adsorption between E.coli and soil minerals increases with the increase of ionic strength(0 mM NaCl-100 mM NaCl)and increases with the increase of zeta.In the interaction,although the zeta potential values of different ionic strength bacteria and algae are consistent with the bacterial-solid interface reaction,the adsorption aggregation occurs in a 10 mM NaCl environment.It is indicated that the interaction between bacteria and algae in different environmental ionic strengths has a more important substance or force to play a leading role.(3)Combining gene knockout,EPS extraction,and enzymatic treatment biological research methods reveal the mechanism of aggregation of bacteria and algae.Enzyme treatment of agglomerates formed by bacteria and algae and quantitative analysis of algae EPS showed that the disintegration rate of protease-treated agglomerates was as high as 95%.The pre-treatment of algae and bacteria could not form agglomerates.In addition,the surface of the algae cells was tightly bound to the EPS,The protein content in a 10 mM NaCl aggregation environment is three times that of a non-agglomerated environmental protein of 100 mM NaCl,it is fully proved that the secretion of tightly bound proteins on the surface of algae cells is the main driving factor for aggregation.