Simulations:

The parameter values used in the overall model were the same as in the component models. The simulation was run starting with initial 108 cells and no enzymes present in 10 mL culture volume. The cells were allowed to culture on PET for 24 hours to ramp up their enzyme concentrations then, 108 cells are taken and cultured in 10 mL with 10 grams of PET till all the PET is degraded. The amount of PET that was selected is approximately the amount of plastic in a regular water bottle [1]. Since PET is a polymer solid, we converted the grams of given PET to number of moles of the repeat units. A concentration of repeat units was then calculated and used by the model. This assumes that the PET chain was a single long strand and had every repeat unit readily accessible. Assuming the PET chain was a single long strand was determined to be reasonable due to what we learned about plastic polymer behavior from plastic specialist Dr. Jared Tatum at Ampacet. Assuming that every unit is easily accessible is not entirely accurate but it was necessary for this simulation. The graphs below show the simulation results.

Figure 1 shows the concentration of PET repeat units and ethylene glycol over 125 hours. It is obvious that it takes some time for the concentration of enzymes to be high enough for the PET to be noticeably degraded. But once that threshold is reached, the plastic is degraded relatively quickly. The concentration of PET seems very high, but this is because of the conversion of solid PET to monomeric repeat units in that solid. This simulation was run on 10 grams of PET which translates to 52000 mM of repeat units. The ethylene glycol disappears due to the consumption of the cells as their carbon source. Figure 2 and Figure 3 better demonstrate this consumption.

Figure 1. Overall Model Consumption of PET and Production of EG

Figure 2 shows cell number increase over the same simulation as Figure 1. The cell numbers reach very high levels as they are growing nearly exponentially the entire time. Only once the Ethylene glycol is all consumed does the system stabilize as start to decay. Figure 3. shows the degradation of PET as a solid overall in grams, instead of repeat unit concentration like in Figure 1.

One way to improve this model in the future is by experimentally determining some of the estimated values of PETase and MHETase. Another way is if better methods for converting grams of PET to concentrations, without having to make the assumption of equal accessibility, would be found it could improve the accuracy of the model.

References:

[1] “Aspa and Public Joint Venture for Recycling Services-Appendix A,” American Samoa Power Authority Materials Management Office. June 2014. [Online]. Available: http://www.aspower.com/aspaweb/bids/RFP%20NO.%20ASPA14.1216%20ASPA%20AND%20PUBLIC%20JOINT%20VENTURE%20RECYCLING-Appendix%20A.pdf

Acknowledgements:

Thank you to Dr. Irene Reizman our Principle Investigator and Dr. Jared Tatum from Ampacet for their contributions and help informing the model.