title: Photo-fermentative bacteria aggregation triggered by L-cysteine during hydrogen production.
abstract: BACKGROUND
  Hydrogen recovered from organic wastes and solar energy by photo-fermentative bacteria (PFB) has been suggested as a promising bioenergy strategy. However, the use of PFB for hydrogen production generally suffers from a serious biomass washout from photobioreactor, due to poor flocculation of PFB. In the continuous operation, PFB cells cannot be efficiently separated from supernatant and rush out with effluent from reactor continuously, which increased the effluent turbidity, meanwhile led to increases in pollutants. Moreover, to replenish the biomass washout, substrate was continuously utilized for cell growth rather than hydrogen production. Consequently, the poor flocculability not only deteriorated the effluent quality, but also decreased the potential yield of hydrogen from substrate. Therefore, enhancing the flocculability of PFB is urgent necessary to further develop photo-fermentative process.
  
  
  RESULTS
  Here, we demonstrated that L-cysteine could improve hydrogen production of Rhodopseudomonas faecalis RLD-53, and more importantly, simultaneously trigger remarkable aggregation of PFB. Experiments showed that L-cysteine greatly promoted the production of extracellular polymeric substances, especially secretion of protein containing more disulfide bonds, and help for enhancement stability of floc of PFB. Through formation of disulfide bonds, L-cysteine not only promoted production of EPS, in particular the secretion of protein, but also stabilized the final confirmation of protein in EPS. In addition, the cell surface elements and functional groups, especially surface charged groups, have also been changed by L-cysteine. Consequently, absolute zeta potential reached a minimum value at 1.0 g/l of L-cysteine, which obviously decreased electrostatic repulsion interaction energy based on DLVO theory. Total interaction energy barrier decreased from 389.77 KT at 0.0 g/l of L-cysteine to 127.21 kT at 1.0 g/l.
  
  
  CONCLUSIONS
  Thus, the strain RLD-53 overcame the total energy barrier and flocculated effectively. After a short settlement, the biomass rush out will be significantly reduced and the effluent quality will be greatly improved in the continuous operation. Furthermore, aggregation of PFB could enable high biomass hold-up of photobioreactor, which allows the photobioreactor to operate at low hydraulic retention time and high organic loading rate. Therefore, the described flocculation behaviour during photo-hydrogen production is potentially suitable for practicable application.
authors: State Key Laboratory of Urban Water Resource and Environment, Harbin Institute of Technology, 73 Huanghe Road, P,O, Box 2614, Harbin, 150090, China. lbf@hit.edu.cn.
authors: Guo-Jun
authors: GJ
authors: Xie
authors: Bing-Feng
authors: BF
authors: Liu
authors: De-Feng
authors: DF
authors: Xing
authors: Jun
authors: J
authors: Nan
authors: Jie
authors: J
authors: Ding
authors: Nan-Qi
authors: NQ
authors: Ren
references: Environ Sci Technol. 2007 Jul 1;41(13):4620-5
references: 17695906
references: J Gen Microbiol. 1962 Sep;29:149-55
references: 13859412
references: Water Res. 2002 Feb;36(3):527-38
references: 11827315
references: J Biol Chem. 1983 Nov 25;258(22):13486-92
references: 6580291
references: Biotechnol Bioeng. 2006 Mar 5;93(4):771-8
references: 16255058
references: Biomacromolecules. 2004 Jul-Aug;5(4):1219-30
references: 15244434
references: Protein Sci. 1994 Jun;3(6):883-7
references: 8069219
references: J Am Chem Soc. 2005 Jun 8;127(22):8168-73
references: 15926845
references: Langmuir. 2004 Nov 23;20(24):10639-47
references: 15544396
references: J Microbiol Methods. 2001 Jan;43(3):153-64
references: 11118650
references: Biotechnol Biofuels. 2012 May 22;5(1):35
references: 22616621
references: Bioresour Technol. 2009 Jan;100(1):484-7
references: 18573657
references: Cell. 1991 Nov 1;67(3):581-9
references: 1934062
references: Biotechnol Adv. 2010 Nov-Dec;28(6):882-94
references: 20705128
references: Water Res. 2001 Feb;35(2):339-50
references: 11228985
references: J Mol Biol. 1991 Jan 20;217(2):389-98
references: 1992169
references: Biotechnol Biofuels. 2012 Feb 13;5:6
references: 22330180
references: Microbiology (Reading). 1999 Jun;145 ( Pt 6):1491-1497
references: 10411276
references: Water Res. 2011 Jan;45(2):509-20
references: 20875910
references: J Bacteriol. 2005 Jan;187(2):405-14
references: 15629911
references: Curr Opin Biotechnol. 2010 Jun;21(3):244-51
references: 20303737
references: Biochemistry. 1998 Feb 3;37(5):1264-73
references: 9477952
references: Appl Microbiol Biotechnol. 1995 Aug-Sep;43(4):755-61
references: 7546613
references: Environ Sci Technol. 2002 Apr 15;36(8):1720-7
references: 11993869
references: Arch Microbiol. 1977 Oct 24;115(1):19-23
references: 337919
references: Colloids Surf B Biointerfaces. 2009 Oct 1;73(1):1-9
references: 19493661
references: Environ Sci Technol. 2010 Jun 1;44(11):4355-60
references: 20446688
references: Methods Mol Biol. 1995;40:219-52
references: 7633524
references: J Microbiol Methods. 2011 Jun;85(3):199-205
references: 21397639
references: J Gen Microbiol. 1964 Apr;35:61-8
references: 14167649
references: Water Res. 2002 Nov;36(18):4583-91
references: 12418661
references: Colloids Surf B Biointerfaces. 2012 Apr 1;92:74-83
references: 22153836
references: Biopolymers. 1986 Mar;25(3):469-87
references: 3697478