| dc.description.abstract | Plastic is ubiquitous and recalcitrant within the marine environment, and as plastic pollution continues to grow, the bacteria which are associated with this debris are becoming a cause for concern due to the presence of potential pathogens. One of the noteworthy pathogens ever present on plastic are vibrios, whose global importance is paramount due to the increase in infections to both humans and animals worldwide. However, the biotic and abiotic processes driving their attachment and the importance of their presence on microplastic requires further investigation. The comprehensive aim of this thesis was to investigate the association of Vibrio parahaemolyticus with a range of microplastic particles in vitro and in situ. As well as this, the first instance of an in vivo model examining V. parahaemolyticus transfer from microplastic fibres into the commercially important bivalve, Crassostrea gigas, is developed.
In the first instance, Vibrio attachment to a range of polymers at different temperatures was examined using standard culturing techniques. These in vitro exposures provided the initial evidence on whether polymer type or temperature, a key ecological driver of environmental Vibrio populations, influence bacterial attachment. The data showed V. parahaemolyticus readily attached to all particles examined, including a glass bead control, albeit attachment across all temperatures and particle appeared to be stochastic. Because of this stochasticity, there was very little statistical difference across all temperatures and polymers examined. Upon normalisation of the data, a linear regression described temperature to influence V. parahaemolyticus significantly higher than that of the polymer type when comparing all polymers with a glass control. The data from this chapter will be used to inform methodological development for chapter 4.
The unique Fleet Lagoon, Dorset was used to examine vibrios presence within the plastisphere community compared to natural surfaces i.e., glass and the particle attached fraction of the water, when exposed in situ across a natural gradient of conditions. Here, sterile, virgin microplastic particles were deployed at three sites across the lagoon and sampled at three time points during a 4-week period. Throughout the lagoon the temperature, salinity and pH differed significantly, with colder, saline conditions at the eastern end (Ferry Bridge) and warmer, less saline conditions found at the Swannery end. Of note was the significantly higher pH in the middle of the lagoon at Langton Herring (pH 9.18). Vibrio were present on all plastic particles at some point throughout the exposure at two of the three sample sites yet almost completely absent at Langton Herring (max relative abundance of 0.016%). There was no significant difference in the abundance of vibrios between any of the particles, yet there were significantly higher numbers found in the particle attached fraction of the water (max abundance of 8.14%). There was a higher species diversity (Shannon and Chao1) on all plastic particles than within the surrounding water at all sample sites. Pathogens did not appear to proliferate on the microplastic, yet there was a selective enrichment of hydrocarbon degrading bacteria on microplastic compared to glass beads. Analysis revealed that it was the prevailing environmental conditions that had the most influence on the microbial communities both spatially and temporally.
Finally, a proof-of-concept in vivo study using Crassostrea gigas was employed to examine bacterial transfer from biofilmed microplastic into an organism. C. gigas were exposed to nylon microplastic fibres inoculated with V. parahaemolyticus as in chapter 2 at a lower (4298 fibres per beaker) and higher (8596 fibres per beaker) concentration. A concentration dependant transfer was discovered with a higher amount of microplastics during an exposure leading to 25% of oysters exhibiting V. parahaemolyticus within their tissues. Compare this to 0% at a lower concentration, there appears to be a concentration dependent transfer. Vibrios were also present in oyster tissues post depuration, providing empirical evidence on microplastics role as a vector of a bacterial pathogen into oyster tissues.
The findings in this thesis confer with the current literature that vibrios are associated with multiple polymer types, yet this association is largely dependent on the surrounding environmental conditions. Vibrios do not appear to be tightly associated within the multi-species biofilm that form on microplastic particles, with their abundance being stochastic, with no clear succession or dominance of vibrios within bacterial communities on plastic. There was not preferentially proliferation of vibrios on plastic per se, when compared to other natural surfaces and organic aggregates. However, a novel insight into microplastics role as a vector of a known pathogen, V. parahaemolyticus into an important aquaculture species, C. gigas is described. Thus, implying this multi-faceted paradigm requires further investigation. | en_GB |
