As an important resource, hydrogen is attracting worldwide attentions not only for its essential function in fertilizer and petroleum industries but as a promising fuel candidate . Hydrogen can be generated through various methods, among which biological dark fermentation owns advantages due to its environmental compatibility and high hydrogen production rates . Performance of microorganisms present in a dark fermentation system has a crucial effect on its successful operation. Mixed cultures own advantages over pure cultures for its easier operation and control, furthermore, actual wastes can be better degraded and utilized by mixed cultures. To enrich hydrogen producers and suppress hydrogen consumers coexist in a mixed culture, different pretreatment methods have been explored. In our previous study[3, 4], gamma irradiation was proved to be an effective pretreatment method for enriching hydrogen-producing microorganisms while inhibit methanogens, the pretreated inoculum showed excellent performance both in cumulative hydrogen production (2.65L/L) and hydrogen yield (2.14 mol/mol glucose). However, the effect of gamma irradiation on microbial diversity was not determined, and the mechanism of good hydrogen production was not explored in depth. In this study, experiments were conducted in different operating conditions to maximize hydrogen production from glucose, and the metabolic processes under optimized conditions were explored. Furthermore, microbial distribution in digested sludge at different stage (raw, gamma irradiation pretreated and after dark fermentation) were analyzed by using the pyrosequencing tool.
Optimization of fermentative hydrogen production process by response surface methodology, most suitable conditions were determined to be 31 ℃，pH 7.5 and glucose concentration of 8 g/L. Periodically analysis of metabolites during hydrogen evolution process provided helpful information that indicate different fermentation types in reaction different stage. Microbial analysis shows that the microbial community of raw digested sludge was very diverse and sequences belong to more 20 phylum were examined, all of which showed kinds of average distribution. However, after 5 kGy gamma irradiation, microbial community was dominated by Firmicutes (40.74%) and Proteobacteria (35.92%). When it comes to the consortium after fermentation, 99.66% of microbes were occupied by Firmicutes, among which, an unclassified clostridium and clostridium butyricum made up 50.34% and 39.14, respectively.
As a newly developed pretreatment method, gamma irradiation showed its excellent effect on enriching hydrogen producers (clostridium) from mixed cultures. As the upmost microbe present in the dark fermentation system was defined as an unclassified clostridium, which may be a mutant strain caused by gamma irradiation, further work relating to its isolation and identification is deserved.
. Das, D.Veziroglu T.N., Advances in biological hydrogen production processes. Int J Hydrogen Energ 2008; 33: 6046-6057.
. Das, D.Veziroǧlu T.N., Hydrogen production by biological processes: a survey of literature. Int J Hydrogen Energ 2001; 26: 13-28.
. Yin, Y., Hu J.Wang J., Gamma irradiation as a pretreatment method for enriching hydrogen-producing bacteria from digested sludge. Int J Hydrogen Energ 2014; 39: 13543-13549.
. Yin, Y., Hu J.Wang J., Enriching hydrogen-producing bacteria from digested sludge by different pretreatment methods. Int J Hydrogen Energ 2014; 39: 13550-13556.