Effect Of Carbon Source,Electrolytic Hydrogen Production And Saline Conditions On Enrichment Of HOB

Hydrogen oxidizing bacteria（HOB）are facultative autotrophic microorganisms.It can recycle H₂,nitrogen sources,and CO₂into economically valuable products:bioproteins,bioplastics,and biofertilizers.Biofertilizer-type HOB are promising.But existing enrichment methods are limited by low biomass and poor hydrogen mass transfer coefficients.In addition,there is a lack of simulated realistic culture environments to verify the availability of HOB.Firstly,based on the facultative autotrophic nature of HOB,this study proposed a mixotrophism through low concentrations of additional glucose to increase the biomass.Moreover,an electrolytic water in-situ culture system was constructed to address the problem of low hydrogen transfer efficiency during HOB culture.By optimising the electrodes and the applied voltage,the capacity of the system to culture HOB was improved.Finally,the application of HOB were extended by cultivating it in a simulated saline environment.There are implications for expanding the range of applications for HOB.With these elements in mind,the main findings of this study are as follows:（1）Low glucose concentrations（50 mg/L）were able to maintain a high growth rate of the flora for a long time and didn’t affect the metabolism of hydrogen by the flora under batch conditions.The feasibility of low-concentration glucose（50 mg/L）mixotrophic culture for HOB were verified by running the continuous reactor over a long period.The nitrogen utilization and biomass of HOB were increased by 1.48 and 1.5 times when compared to autotrophic culture,which achieved the aim to increase biomass.The composition of the flora enriched by autotrophic and mixotrophic conditions differed.The flora obtained from the autotrophic condition was dominated by Hydrogenophaga（44.01%）,a typical biofertilizer-type HOB.The flora was found to have ACC degradation capacity（1.28 mg/L/d）and phosphate solubilisation capacity（6.36-8.97 mg/L）.The dominant genus in the mixotrophic culture was Xanthobacter（55.76%）,with an ACC degradation rate of 1.84 mg/L/d and dissolved phosphate in the system reaching 9.84-15.11 mg/L.It was confirmed that the ability of the mixotrophic flora to metabolize hydrogen was not reduced compared to the autotrophic flora.（2）The pure titanium,pure nickel,pure copper and stainless steel were used as cathodes for in situ culturing of HOB.The nickel electrode had a maximum total nitrogen conversion rate of 9.71 mg N/g VSS/d,the hydrogen production performance of the nickel and the electrochemical characteristics of the microorganisms obtained from the culture were also in the best level compared to other electrode materials.It was found that the Pseudonocardia（37.54%）was the main component of the flora composition when nickel was used as the cathode.Based on this result,the nickel electrode was the most suitable for the system.Comparing the effects of incubation at different voltages,the total nitrogen conversion rate of the microorganisms at 2.0 V was 63.18%,47.47%,15.01%and 6.10%higher than the other voltages respectively.The flora was dominated by Pseudonocardia（22.17%）,Bradyrhizobium（16.96%）and Pseudomonas（9.48%）as the main constituents.This meant that an electrolytic hydrogen production in situ culture system with a nickel electrode as cathode and an applied voltage of2.0 V is favourable for the culture of HOB.（3）The batch test revealed that HOB were well tolerated in environments where Na Cl didn’t exceed 12.4 g/L.Moreover,pH didn’t further deteriorate microbial activity and nitrogen utilization capacity.The reactor running with Na Cl=12.4 g/L revealed that the activity of HOB almost returned to the pre-stress level after 30 d of Na Cl shock.The average ammonium conversion rate（ACR）of the reactor at this time was 0.25mg N/mg VSS/d（before was 0.30 mg N/mg VSS/d）.The bacterial population changed drastically by the impact of Na Cl,from containing mainly Xanthobacter（50.23%）and Rhodocyclaceae（11.58%）to being dominated by Paracoccus（58.51%）and Methyloversatilis（6.72%）.After the reactor was changed to pH=9.0,HOB were significantly inhibited and the ACR decreased to 0.21 mg N/mg VSS/d.The flora changed again at this stage,with Paracoccus（32.51%）,Stappia（39.3%）and Halomonas（10.68%）as the main microorganisms.Function validation showed that both salt shocked and pH shocked colonies had the abilities of ACC degradation and phosphate solubilization.Validation by seed germination revealed that the flora obtained by pH shock increased seed germination by 22%.