Team:UUlm/Engineering

Engineering success

Introduction

For the years 2020 and 2021 we, the team UUlm, have dedicated our project to the biological degradation of polystyrene. Polystyrene, from here on referred to as PS, is a synthetic polymer with versatile applications in our modern society. Like most man-made polymers, PS degrades very slowly in nature. Even in environments with high microbial activity the polymer is highly recalcitrant to biological decay.[1],[2],[3]
However, one environment has been identified where the degradation rate of PS is comparably high: the guts of the larvae of the beetle Tenebrio molitor.[3],[4] Responsible for this capability of the larvae are several strains of intestinal bacteria which were able to degrade PS.[5] With a genetic modification to hese bacteria, we intend to improve their ability to degraded PS.

History of polystyrene degredation research

Early investigations into the biodegradability of PS date back to the 1970s. James E. Guillet et al. belong to the pioneers of plastics biodegradation research. In 1974 they used 14C styrene monomers to produce a radioactively marked PS vinyl ketone copolymer. After mixing the polymer into soil, mud, sludge, etc., its concentration could be determined by the dose of radioactive irradiation from the decay of 14C. Based on this principle, the amount of non- and partially degraded polymer in the soil mixtures was measured, as well as the amount of 14CO2 from the sealed environment which was collected by a CO2 absorbent.[1] The process of measuring and understanding the degradation has been refined ever since. Already in 1979, Kaplan et al. compared the ability of fungi, invertebrates and microbes to degrade PS. Their study showed that of the tested organisms only fungi and microbes had limited abilities of degrading PS.[2] In 2015, Yang et al. investigated the larvae of Tenebrio molitor Linneaus, which offer an environment with comparably high PS degradation rates. They could show with 13C labelled PS that it was indeed used metabolically. This realization stemmed from an increase in 13C fatty acids. They also showed this ability of the worms to be dependent on bacteria in their intestinal biome by feeding them antibiotics.[3],[4]

Possible improvement of polystyrene degradation

Currently, the most promising basis for a more effective biological PS degradation lays in the identified intestinal bacteria from T. molitor larvae. Inspired by the principle of exoenzymes secreted by bacteria, a possible way of improving the degradation efficiency was reasoned. The macromolecules that make up the polymer form dense structures and leave a relatively small surface area that can be attacked by the bacteria. If this surface area could be increased, in theory the bacteria could degrade PS more efficiently. As the stability of the supramolecular assembly of PS is only due to van der Waals forces and π-π interactions[7], it can be strongly influenced by the solvent system.[8] Acetone, for example, can dissolve PS.[9] Coincidentally reliable methods of acetone production by bacteria have been shown.[10] Consequently the respective bacteria, genetically modified to produce acetone, could have a significant advantage in breaking down PS. As the bacteria themselves would produce the acetone, the process would not be dependent on the addition of any other adjuvants. As acetone is just a solvent and would theoretically not be used up, it could be dissipated and used elsewhere. A two-bacteria-system would be conceivable and could avoid the possible obstacles of gene transfer into the wildtype intestinal bacteria. However, the stability and reliability of the bacterial culture could be assured by implementing the acetone production into the same bacteria which degrade the PS. As they would have evolutionary benefits over other bacteria when living on a pure PS substrate, they could be relatively resistant against invasion of other (pathogenic) bacteria.

Population dynamics study

To verify the PS degradation by T. molitor larvae and to assess the actual rate of degradation, a population dynamics study was conducted with groups of 50 g of larvae. Two test groups fed with 20 g of expanded PS (EPS) foam flakes (Ø 1 cm3) was compared to a control group fed with 90 g of oatmeal. The EPS foam was obtained as standard packaging material and the oatmeal was obtained as standard grocery product from a local supermarket.
All larvae were cultivated in a two-barrier containment. The inner barrier consisted of a small high-density polypropylene (HD PP) drawer cabinet from the hardware store. This plastic compound was chosen for two main aspects: mealworms have been reported to feed on a variety of plastics. The HD PP was chosen to offer a longer durability when being attacked by the larvae compared to non-high-density plastics.[20] Also, the inside of the drawer was checked for smooth surfaces. This measure ensured the lack of structural weak points that would be readily accessible by the larvae’s mouthparts. The structural integrity of the drawer cabinet was checked with every measurement. The second aspect was the material property offered by HD PP to be resistant against most household chemicals and solvents such as acetone. The outer barrier was put in place for any event of failure of the first barrier. This second barrier consisted of a fully closed acrylic glass cabinet. As this cabinet was completly sealed to prevent any outbreak of the larvae, the ventilation of the entire unit had to be done manually by opening the door of the outer container.
The four groups (two test groups, two control groups) were separated into four drawers with altering sequence (from top to bottom: control 1, test 1, control 2, test 2) to exclude possible local differences of external factors. As both substrates contained little to no water, a small dish (HD PP) filled with demineralized water was placed into each drawer and steadily refilled to increase the humidity. After the first 24 h of the experiment, the humidity was identified as problematic. The larvae gathered around the water bowl where the relative humidity was higher than in the rest of the drawer. After spraying the content of the drawers evenly with demineralized water, the larvae spread over the entire content of the drawers over the course of one hour. As the water shortage could harm the larvae and lower their productivity, a humidifier was designed to keep the humidity constantly at a high level. The humidifier was constructed as shown in Fig. 1 and placed inside the outer containment. The time switch was adjusted to power the unit every 3 h for a duration of five minutes. The drawers were provided with small holes on the upper side of the walls to ensure air circulation. Furthermore, humidity sensors were placed in each drawer to monitor the humidity.

FIG. 1. Air humidifier. Components: (1) outer beaker, (2) inner beaker, (3) water level, (4) ultrasonic “fogger” [power: 24 V * 1 A, fog production: 400 ml/h], (5) 24 V, 0.3 A fan and air inlet, (6) air outlet, (7) 24 V power supply with a time switch.

After the installation of the humidifier on the fourth day of the experiment, an increase in humidity from approximately 30 % to 50 % relative humidity was displayed by the humidity sensors. Also, the larvae no longer gathered around the water bowls. The humidity remained at 50 ± 5 % during the rest of the experiment.
The relatively consistent humidity can be explained by the regular airing of the outer container. The humidifier itself was designed to be self-regulating: if the humidity is already high, the air becomes oversaturated more quickly and the formed fog is “heavier”. This heavy fog deposits within the lower portion of the inner beaker and flows back into the outer beaker before it can reach the upper portion of the inner beaker and leaves through the outlet.

FIG. 2. Population dynamics of Tenebrio molitor larvae. Control group (A) fed with oatmeal and test group (B) fed with extruded polystyrene. Mass fractions in stacked format: substrate (oats + faeces or extruded polystyrene), faeces, larvae of T. molitor, pupae of T. molitor, and dead mass including shed, parts of larvae or pupae and dead larvae or pupae.

Over the course of 33 days, six measures were taken to determine the momentary composition of the drawers’ contents. Therefore, sieves were used. The contents were separated into larvae, pupae, dead material (including shed, parts of larvae and pupae, dead larvae and pupae), faeces, and the substrate, being EPS or oatmeal. As the oatmeal disintegrated into particles of similar size as the faeces, they could not be separated and therefore are stated as one component. The masses of these fractions are depicted in Fig. 2. Besides the composition, the total mass of the drawers’ contents was measured by weighing the drawer and subtracting its empty mass. These measurements together with a summed-up biomass (larvae, pupae, dead mass, faeces) and substrate mass (polystyrene or oatmeal, faeces) are shown in Fig. 3.

FIG. 3. Population dynamics of Tenebrio molitor larvae. Control group (A) fed with oatmeal and test group (B) fed with extruded polystyrene. Total mass is determined by measuring the container and subtracting empty mass. Biomass as a sum of larvae, pupae, and dead material (A) and larvae, pupae, dead material, and faeces (B). Oatmeal includes faeces (A).

From the resulting data, characteristic rates could be determined for the test and control groups. Therefore, a regression of linear area of the curves was performed to calculate the gradient. This gradient equals the respective change rates of the mass fractions. From this change rate, a relative change rate was determined by dividing the change rate through the initial mass of the used larvae, being 50 g for each group. The substrate change rate for the control group includes the change rate of the faeces, as they could not be physically divided and were measured together. Besides substrate change rates and larvae mass change rates, change rates for the total biomass was calculated by adding up the mass change rates of the larvae, the pupae and dead mass. These rates are stated in Tab. 1.

Tab. 1. Characteristic change rates of control groups and test groups
Control group Test group
Substrate change rate [g/d] -1.135 -0.082
Relative Substrate change rate [1/d] -0.022 -0.002
Larvae mass change rate [g/d] -2.03 -1.321
Relative Larvae mass change rate [1/d] -0.04 -0.026
Biomass change rate [g/d] -0.015 -0.709
Relative Biomass change rate [1/d] -0.000[3] -0.014

Acetone synthesis plasmid construction

To test if acetone has a positive impact on the degradation ability of mealworms (or rather their intestinal bacteria), certain modifications are planned. A recombinant Escherichia coli strain is to be created, containing an acetone synthesis plasmid. This plasmid bears the required genes for the acetone production: a thiolase A (thlA), an acetate/butyrate:acetoacetatyl-CoA transferase with subunits A (ctfA) and B (ctfB), and an acetoacetate decarboxylase (adc). Acetone is produced from acetyl-CoA.[10] The first step of this conversion is catalysed by thiolase A. Two molecules of acetyl-CoA are converted into one molecule of acetoacetyl-CoA. Next follows the transfer of CoA by the butyrate-acetoacetate CoA transferase, which results in acetoacetate. The last step is carried out by the acetoacetate decarboxylase and is the elimination of CO2. The product of this is acetone. The pathway is shown in Fig 4.

FIG. 4. Acetone synthesis pathway. The synthesis of acetone starts with two molecules of acetoacetyl-CoA. CoA standing for coenzyme A.

The three genes mentioned above were obtained from two different plasmids ([pJIR750_ac2t2] and [pJIR750_ac3t3]), which were already assembled by our project instructor M. Sc. Teresa Schoch. These donor plasmids are variations of the gene constellations and gene origins on the same backbone. M. Sc. Teresa Schoch also designed the acetone synthesis plasmids for us, which we are very grateful for. For those designated plasmids, a [pMTL83151] shuttle vector[19] was chosen as backbone. This backbone was originally obtained from Prof. Minton, University of Nottingham. It already contains resistance genes against chloramphenicol (catP), an origin of replication (repH), genes for replication in Gram-negative bacteria (ColE1), and genes enabling conjugation (traJ). The three genes obtained from the donor plasmids were combined with a PthlA promoter and inserted into the shuttle vector. The resulting plasmids are shown in Fig. 5. Via transformation, the two plasmids were transformed into E. coli XL1 Blue MFR’. After the transformed bacteria were plated on agar plates containing chloramphenicol and colonies were picked. An analytical digest was performed with the restriction enzymes SalI and EcoRI on plasmids isolated from the picked colonies , see notebook. The samples were sent to a commercial sequencing facility. After sequencing, the targeted genes could not be identified on the isolated plasmids. This experiment needs to be repeated. The source of failure has not been identified yet. One possible explanation is the partial destruction of the genes during the transformation.

FIG. 5. Acetone synthesis plasmid construction. Two plasmids with different gene constellations and gene origins. Genes assembled: PthlA promotor (PthlA), thiolase A (thlA), CoA transferase subunits A (ctfA) and B (ctfB), acetoacetate decarboxylase (adc), an origin of replication (repH), chloramphenicol resistance gene (catP), genes for replication in Gram-negative bacteria (ColE1) and genes enabling conjugation (traJ).

Future plans

A possible next step is to isolate one or more bacterial strains with the ability to degrade polystyrene from the mealworm intestines. Tang et al. (2017) showed a very practical method for doing this.[5] To enrich the bacteria in the intestines, the mealworms were fed with polystyrene as a sole diet for 3 weeks. Then the guts were washed with basal medium. This suspension was plated on basal medium and incubated at 37 °C for 24 h under aerobic and anaerobic conditions. The microbes were transferred onto PS agar plates with yeast extract and incubated under the conditions stated above. The microbes were collected, and the pure colonies were preserved for the following experiments. To measure the degradation rate of polystyrene by these cultures a turbidity assay was used.[11] This is a qualitative and quantitative method because it can measure whether PS is degraded and how fast the degradation takes place. An overnight culture was used and some additional medium containing a PS suspension was added. The resulting culture was incubated at 37 °C while simultaneously the PS concentration could be measured spectrophotometrically at 600 nm. To observe the degradation of PS, the measurement was repeated in defined intervals.
Also, the remaining PS concentration in larvae, pupae, dead material, and faeces needs to be measured. This is necessary to gain more information about the actual rate of PS degradation and for a better understanding of variations between differently treated larvae and the bacteria themselves. Besides the measurement of radioactively marked PS or isotopic distribution measurements, either gel-permeation-chromatography (GPC) or turbidity systems were used by other research groups to determine these residues. As the analytical method needs to be reliable, easy, and safe to perform lots of measurements, radioactively and isotopically labelled PS as well as GPC are unsuitable. The turbidity system is fairly convenient, but offers no exclusive sensitivity to PS. Therefore, the concentration is to be measured fluorometrically. PS fluoresces with a characteristic peak at 335 nm when dissolved in 1,2-dichloroethane and excited with ultraviolet light.[14] The reliability of this method with biological contaminants has to be tested.

Polystyrene degradation pathway

The ability to degrade synthetic polymers such as PS is most often shared by those bacteria which already degrade natural polymers such as lignin and cellulose.[15] Especially lignin shows similarities regarding aromaticity. The first step of degrading a polymer often is formation of monomers. This most certainly happens in the case of polystyrene too. How exactly the bacteria perform the cleavage of the PS polymer into monomeric units is not fully understood yet. Nakamiya et al. (1997) showed that the hydroquinone peroxidase from Azotobacter beijerinckii depolymerises PS in an artificial two-phase system.[15] This cleavage is depicted in Fig. 6.

FIG. 6. Suggested depolymerization of PS with hydroquinone peroxidase.[15]

For the degradation of styrene however, two pathways are largely known. One pathway leads from phenylacetic acid to acetyl-CoA and succinyl-CoA.[12],[13] This way is shown in Fig. 7. The second pathway proceeds via 3-vinylcatechol and ends with 2-vinylmuconate or with pyruvate and acetaldehyde.[13] This pathway is depicted in Fig. 8 and goes back to Mooney et al. (2006).[16]

FIG. 7. Suggested aerobic styrene degradation pathway by Teufel et al. [13] The enzymes shown are styrene monooxygenase (SMO), styrene oxide isomerase (SOI), phenylacetaldehyde dehydrogenase (PAALDH), phenylacetate-CoA ligase (PCL), phenylacetyl-CoA 1,2-epoxidase (PaaABCDE), 2-(1,2-epoxy-1,2-dihydrophenyl)acetyl-CoA isomerase (PaaG), oxepin-CoA hydrolase (PaaZ-ECH), 3-oxo-5,6-dehydrosuberyl-CoA semialdehyde dehydrogenase (PaaZ-ALDH), 3-oxo-5,6-didehydrosuberyl-CoA thiolase (PaaJ), 2,3-dehydroadipyl-CoA hydratase (PaaF), 3-hydroxyacyl-CoA dehydrogenase (PaaH).
FIG. 8. Suggested styrene degradation pathway by Mooney et al. [16] The enzymes shown are styrene 2,3-dioxygenase (SDO), styrene 2,3-dihydrodiol dehydrogenase (SDHDD), 2,3-vinylcatechol extradiol dioxygenase (VCEDO) and 2,3-vinylcatechol intra-diol dioxygenase(VCIDO).

If the isolation of a bacterium from T. molitor fails, a second approach would be to build a bacterium that can degrade PS from scratch. In 2010, Teufel et al. showed that E. coli possesses the ability to degrade phenylacetic acid to acetyl-CoA and succinyl-CoA.[17] Pseudomonas fluorescens possesses the genes for the conversion of styrene over styrene oxide and phenylacetaldehyde to phenylacetic acid.[18] Thus, if it would be possible to transfer these genes from P. fluorescens to an E. coli strain, this E. coli would be capable of converting styrene into acetyl-CoA and succinyl-CoA. The only enzyme missing for the complete degradation of PS in this pathway would be one with the ability of depolymerising PS into styrene. A. beijerinckii posesses a hydroquinone peroxidase which can depolymerise PS.[15] With the respective gene transferred into E. coli, the resulting bacteria srain would theoretically have the ability of degrading PS completely.

References

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