| Among the green algae that have the ability to produce hydrogen,C.reinhardtii(Chlamydomonas reinhardtii)is considered to be the most promising algae species in biohydrogen production,primarily owing to its ability to grow with acetate as its sole carbon source,and it has the ability to rapidly respond to various environmental stimuli,and has strong tolerance to anaerobic stress.But a major obstruction toward the hydrogen production of C.reinhardtii is the extreme oxygen sensitivity of the hydrogenase.As a result of this sensitivity,H2 production mostly appears as a transient phenomenon,and the hydrogen production efficiency is very low.In order to increase the hydrogen production of C.reinhardtii,the most widely used method is sulphur-deficient culture of C.reinhardtii,which inhibits photosynthetic oxygen evolution of PSII,and makes the culture system rapidly reach anaerobic state,thereby increasing hydrogen production.However,sulfur-deficient culture also inhibits the production of electrons from photolyzed water,so it effectively improves the hydrogen production of C.reinhardtii,in addition to the need to establish anaerobic conditions,but also need to provide rich electrons for the reduction of protons by hydrogenase.Carbon-based nanomaterials(CBNs),as the most widely investigated and used nanomaterials,have unique absorption spectroscopy and good electrical conductivity,which can effectively absorb light energy and produce abundant photoelectrons.This study explored the co-cultivation of different types of CBNs with C.reinhardtii cells in order to increase the supply of electrons under hydrogen production conditions by CBNs,thereby achieving efficient hydrogen production.To this end,we selected three CBNs with good water compatibility:the single-walled carbon nanotubes modified by ssDNA(ssDNA/SWCNTs),multi-walled carbon nanotubes modified by carboxyl groups(MWCNTs)and the graphene oxide(GO).They were separately co-cultured with algae cells to analyze the effects of three CBNs on the growth and photosynthetic hydrogen production of C.reinhardtii cells.The specific results are as follows:(1)Characterization experiments were separately carried out on ssDNA/SWCNTs,MWCNTs and GO suspensions respectively,and their physical and chemical properties such as morphology and diameter,carbon structure and zeta potential were analyzed.It was found that the carbon tubes were interlaced to form a network-like structure,and GO was in a sheet-like structure.And its purity ratio is:ssDNA/SWCNTs>GO>MWCNTs.Moreover,it was found by dispersive characterization that CBNs had strong electronegativity and could be stably dispersed in water,the order is ssDNA/SWCNTs>GO>MWCNTs.(2)Three CBNs suspensions(1,2,10,50 or 100 ?g·mL-1)were separately co-cultured with C.reinhardtii,and observed the effects of different concentrations of CBNs on their growth.The results showed that the inhibition of CBNs on C.reinhardtii increased with the increase of its concentration.The cell morphology and its localization in the cells were observed by co-cultured of 10 ?g·mL-1 CBNs suspension with algae cells.The results showed that all three CBNs could enter the cytoplasm of C.reinhardtii;GO could induce the cells to aggregate with them in the pre-growth stage of C.reinhardtii.The above results indicated that the three CBNs had low toxicity to C.reinhardtii,and the toxicity was GO>ssDNA/SWCNTs>MWCNTs.(3)C.reinhardtii were treated separately using three CBNs,and then the closed sulfur-deficient hydrogen production culture is carried out.We measured the hydrogen production,net photosynthetic oxygen evolution rate,PSII photochemical quantum yield(Fv/Fm)and electron transport rate(ETR).The results showed that ssDNA/SWCNTs,MWCNTs and GO further reduced the Fv/Fm and ETR of C.reinhardtii cells in co-cultures.The measured net photosynthetic oxygen evolution activity data showed a negative value,indicated that under these conditions,the respiration rate of co-cultured algae cells and control algae cells was greater than the photosynthetic oxygen evolution rate,and the net photosynthetic oxygen evolution rate of C.reinhardtii cells with added GO was significantly different from that of the control.This series of effects caused an increase in hydrogen production.In particular,the co-cultures were cultured on the seventh day under hydrogen production conditions,the hydrogen production of co-cultures supplemented with GO,ssDNA/SWCNTs or MWCNTs was separately 2.6 times,2 times or 1.6 times than of the control.In summary,based on the characterization of the physical and chemical properties of three CBNs,this study specifically analyzed the toxicity mechanism of C.reinhardtii.The nanometer size,morphology and dispersibility of the three CBNs are different,and the toxicity to algae cells is also different.The order of toxic effects is:GO>ssDNA/SWCNTs>MWCNTs.It can be seen that the outer diameter,shape,concentration,dispersibility and other properties of nanomaterials may be the reasons that affect the growth of algae.We focused on the effects of three CBNs on PSII photosynthetic activity and photosynthetic hydrogen production of C.reinhardtii.The results showed that the three CBNs could further reduce the activity of PSII,limited the photosynthetic oxygen release capacity,and induced the hypoxia state of cells to promote the photosynthetic hydrogen production of C.reinhardtii.The promotion effect is:GO>ssDNA/SWCNTs>MWCNTs.This study laid the foundation for further finding suitable nanomaterials and greatly increasing the hydrogen production of C.reinhardtii. |
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