Summary

Our summer project was designed to improve the process of nitrogen removal in the tertiary sewage treatment stage of raw sewage processing. Currently, a fairly common design is to spatially separate the processes of nitrification and denitrification so that distinct bacterial populations within activated sludge can perform their respective biochemical processes independently. Our project proposes that both processes be temporally separated and housed under a single organism. To achieve this, we attempted to transform the genes encoding for proteins responsible for nitrification into Paracoccus denitrificans, a known denitrifier in active sludge.

By the end of summer experimentation, we were able to transform most, but not all of the nitrification genes into Pc. denitrificans. Our central composite circuit consists of genes for HaoA and HaoB, proteins that catalyze the oxidation of hydroxylamine into nitrite ions. It also includes CycA and CycX, genes encoding for two c-type cytochromes involved in shuttling electrons from the ammonia oxidation reactions to terminal electron acceptors. We were, however, unable to prepare the genes for AmoA, B, and C into Pc. denitrificans. These genes would have encoded for a heterotrimeric protein complex that would have catalyzed the oxidation of ammonia into hydroxylamine.

In addition to the ammonia oxidation circuit we developed, we also designed a series of reporter circuits to test the function of several promoters in P. denitrificans compared to E. coli. We were able to transform Pc. denitrificans with a composite part consisting of Mn-SOD, an oxygen-inducible promoter, and GFP. With this reporter circuit, we attempted to characterize promoter activity in response to different dissolved oxygen levels by measuring relative fluorescence at the absorbance spectra of GFP compared to control untransformed cells.

In addition to Mn-SOD, a new promoter added to iGEM, we also tested the promoter activities of VhB and T7 in transformed E. coli cells. These promoters were connected to the coding sequences for GFP in a composite part. Again, all promoters were tested at various dissolved oxygen levels. These were tested against untransformed E. coli cells. Finally, we accounted for the fact that different oxygen levels may lead to different growth rates, so we divided RFU values by the OD600 of a given sample that was measured.

Paracoccus denitrificans

While Mn-SOD has been described in literature as an oxygen-inducible promoter element in E. coli, the results of experimentation described under protocols (citation, link) have been inconclusive. Following the previously described protocol, bacterial cell cultures were grown from single colonies for four hours to reach early- to mid-exponential growth phase (shown to be ~4 hours in the literature and our own results). After four hours, colonies were transferred into 14 mL tubes. Because equipment to modulate the dissolved oxygen environment was not available, tubes were covered after a specified amount of time and kept closed until the end of experimentation to simulate differential oxygen levels. Bacteria in tubes that are covered for longer periods of time will consume the available oxygen over time, resulting in lower dissolved oxygen levels. After the experimental window closes, all relevant parameters were measured (DO, OD600, and RFU output from the optical plate reader) simultaneously.

The first trial was conducted on 9/25/2017 and cultures were grown for a total of four hours before being transferred into 14 mL falcon tubes. At four hours, tubes were opened and tested for DO with a Vernier Instruments DO probe and for OD600 with a Genesys 2 spectrophotometer in the order from most hours spent open to least. The reasoning behind this is that the measurement process was time-consuming and exposing cultures that had spent more time open to atmospheric oxygen would minimize the bias caused by re-exposure, as they would be less sensitive than cultures that had more time in an anaerobic environment to changes caused by unsealing tubes. Immediately after all tubes were opened and samples were taken from each strain and time point, the well plate was sealed with an an optical adhesive cover and measured by a BioRad MyiQ optical plate reader.

When plotted against hours spent open and exposed to open atmosphere, a steady increase in the relative fluorescence can be observed. When compared against the DO content (in mg/L), however, the opposite trend is observed. For the relationship between DO and RFU, the coefficient of determination is 0.445, corresponding to a correlation coefficient of -0.667. This indicates that the observed RFU of bacteria containing the Mn-SOD reporter biobrick is negatively correlated with DO with a linear relationship of moderate strength. The relationship between the RFU and time spent exposed to atmospheric oxygen, while positive, was much weaker. The coefficient of determination (R2) was only 0.023, corresponding to a correlation coefficient (R) of 0.152. This is indicative of a weak linear relationship.

Figure 1: OD600 normalized RFU values between all measurement replicates over time and DO saturation/content (mg/L).

The same pattern is also observed in results taken from the second trial on 10/22. This time, bacterial cell cultures were grown at 37 C in a shaking incubator (220 rpm) for four hours before being transferred into smaller 3.5 mL culture volumes. All aliquots were grown and a new tube was covered every hour as described above, except cultures were grown for a total of six hours. As in the DH5a strain tested on 9/25, the strain transformed with the Mn-SOD reporter plasmid showed increasing fluorescence over time but a decreasing RFU value as the observed DO content increased. The coefficient of determination and correlation coefficient for the relationship between RFU value and time spend open were 0.503 and 0.709 each. This indicates that the relationship is positive and moderately strong. The relationship between DO and RFU values was negative and the coefficient of determination and correlation coefficient were 0.374 and -0.612. This is indicative of a moderately strong negative linear relationship.

Figure 2: Results from the trial performed over 10/22

Again, the observation is made that as the duration spent opened increases, the level of relative fluorescence picked up by the optical reader does as well. It is also evident in Figure 2 that as the dissolved oxygen content increases, the relative fluorescence decreases.These results both contradict and support our original hypothesis and indicate that an unmeasured effect is causing the DO content to decrease in cultures that spend more time exposed to atmospheric oxygen.

Figure 3: Aggregated data for Mn-SOD in Pc. denitrificans across both trials

Combining both trials yields stronger evidence for the trends described above, as a positive linear trend is also observed when plotting relative fluorescence against time spent exposed to atmospheric oxygen. In addition, the plot of RFU versus DO shows weaker linear relationship with coefficient of determination and correlation coefficient of only 0.085 and 0.292 when both trials are aggregated into a single dataset. This is weaker than the observed relationship between RFU value and time spent open, which had corresponding R2 and R values of 0.341 and 0.584. This indicates a moderate linear relationship between RFU value and time spent open.

In addition, Mn-SOD was tested in Pc. denitrificans and the results corroborated claims in the literature and results found by experimentation. The R and R2 values for the relationship between RFU value and time spent open were 0.198 and 0.445. The relationship between DO content and RFU value had R and R2 values of 0 and 0. These results were mildly positive and indicate to us that Mn-SOD generally behaves the same way in Pc. denitrificans as it is reported to in E. coli.

Figure 4: Scatterplots of relative fluorescence versus time and dissolved oxygen content