The biofilm, whose subunit was CsgA engineered with HisTag on N-termial and SpyCachter-HisTag on C-terminal, was grown on microspheres, 25 micrometers in diameter for 48 hours. NR’s (7.72*10^-9 M) were then added and given 30 min to bind to the HisTag on CsgA subunit. (The engineered SpyCatcher was used for future pure hydrogenase binding.) The solution was centrifuged and the sediments contained biofilm beads covered with NR. This sediment was resuspended in PBS and was added to the reaction solution consisting of
with engineered hydrogenase (wet weight 100ug) resuspended in PBS, 150Mm NaCl, 100mM VitaminC, and mediator solution (5mM Paraquat dichloride, for mediating the electrons across the cell membrane). The whole solution including bacteria is adjusted to pH=4 by 100mM Tris-HCl(pH=7.0), given that the pH of 4 was reported to be an optimal environment.
was induced with IPTG overnight at room temperature.
In the activity assay of the hydrogenase in producing hydrogen, the system goes through three periods of “light-on and light-off”. The result (see below) shows the stability of the system and the reversible catalytic activity of the hydrogenase of the reaction, 2H+ + 2e- ⇿ H2 .
Figure 1 Apparatus of the hydrogen production assay.
It contains (1) an anaerobic reaction container which is a transparent circular cuvette that allows light to go through; (2) a light source in our hydrogen production assay acting as a substitute for the real sun. (We chose a high-power white LED light, set 28cm away from the reaction container for a even distribution of photons); (3) a hydrogen electrode linked to its inner sensor inserted into the reaction container to measure the realtime concentration of hydrogen; (4) a date hub; (5) a computer connected to the hub to record the data and generate the curve of concentration variation within a period of time.
Results
Figure 3 Hydrogen production with nano rods suspension replaced by nano rods bound to biofilm beads.
During the period with lighting, the hydrogen production increases, until we shut off the light at points that correspond to the tips. The curve then goes downward, showing that the hydrogen concentration is lowered, an evidence of the bidirectional catalytic activity of hydrogenase. It is noteworthy that the hydrogenase shows the greatest production rate at the beginning of lighting: a transient sharp rise can be observed at the valleys. It is also obvious that each period of “light-on light-off” gives similar curves, which implies that our hydrogenase is stable.
In our experiment, we find that despite the reported affected catalytic ability of FeFe hydrogenase due to oxygen, non-strict anaerobic and short-term exposure to oxygen does not cause detrimental effects on the enzyme activity of producing hydrogen. This can be explained by the high catalytic ability and the segregation layer from the atmosphere provided by the hydrogen it produces. Meanwhile, the electron sacrificial agent VitaminC also adds to the “protection layer” of the hydrogenase in our system.
Comparing the system with biofilm and with out biofilm Figure 3 and Figure 4
Before we tested the system will biofilm-anchored CdS nanorods, we tested ones with freely-flowing CdS nanorods. The result is shown below in Figure4.
During lighting period, the hydrogen production increases, until we shut off the light at points that correspond to the tips. The curve then goes downward, showing that the hydrogen concentration is lowered, an evidence of bidirectional catalytic activity of hydrogenase.
In the process of hydrogen generation without biofilm-anchored CdS, a stir bar with a necessary speed of 800 RPM was needed. But in Figure 3, the system with biofilm, a stir bar was not used. It is likely because the aggregates of NR have a bigger chance in colliding with
E. coli to transfer electrons. We therefore propose this model as our final model, although further optimization of the system is still under way, including deciding on the optimized material and size the microsphere for biofilm growth.
