Microbial Communities and Chemical Pollutants: Exposure Related Adaptations in Environmental Microbiomes and Their Potential for Bioremediation

Bioremediation is a treatment strategy that involves the removal of chemical pollutants using biological agents. When compared to physico-chemical treatment approaches, bioremediation causes less site disturbance and offers a range of other economic and ecological benefits. Yet, this treatment approach is not routinely selected mainly because of the unpredictability of the biological agents in the heterogeneous environments encountered at sites. Generally, three main approaches are utilized to improve bioremediation efficacy. These approaches consist of: 1) allowing native bacteria to degrade the pollutant (bioattenuation); 2) stimulating indigenous bacteria to improve their natural degradative capacity (biostimulation); or 3) supplying exogenous microorganisms that are known to degrade a specific contaminant if no known degraders are present at the site (bioaugmentation). Genetic bioaugmentation is another bioremediation treatment strategy, which relies on the well-studied mechanism of horizontal gene transfer (HGT) in which plasmids from exogenous donors are transferred to indigenous recipients. This strategy circumvents the need for the exogenous strain to compete with indigenous microorganisms under site conditions, a challenge that is difficult to overcome. HGT is a widespread natural phenomenon that readily occurs especially under harsh environmental conditions such as that in heavily contaminated environments.

Although all of these approaches show promise, in general, little is known about how organisms assemble in contaminated environments, limiting the implementation of bioremediation treatment strategies under field conditions. The work presented in this dissertation aims to address this challenge by characterizing and engineering prokaryotic microbiomes in soils contaminated with polycyclic aromatic hydrocarbons (PAHs). The overarching goals of this dissertation were to first utilize next-generation sequencing (NGS) to characterize pollutant-exposed environmental microbiomes and then use these data to develop a generalizable framework, which combines metagenomic and physico-chemical data to inform bioremediation treatment strategy selection. Then, in order to effectively validate this framework, a method for monitoring catabolic plasmid conjugation was developed. This dissertation was broadly broken down into four objectives as described below.

The first objective was to investigate environmentally relevant adaptation events in sediment microbial communities and gut-associated microbial communities in Atlantic killifish exposed to PAHs. Sediment and gut samples were obtained from the Republic Creosote Co. site located along the Elizabeth River (ER) and the Kings Creek (KC) reference site and their microbiomes were comparatively analyzed using high-throughput sequencing. In addition, because it is known gut microbes regulate host metabolism, the gut-associated metabolome was also characterized. Overall, significant community shifts were identified between sites, suggesting an environmental microbiome evolved to withstand high levels of PAHs. Specifically, of the OTUs identified, 9 species were different between KC guts and KC sediment while 176 were different between Republic guts and Republic sediment. These data suggest that factors other than dietary influence affect the microbiota colonized in the Republic fish gut. With respect to the metabolome, the amino acid (AA) concentrations were found to be higher for 19 out of 21 AAs in the Kings Creek samples when compared to the Republic samples. This indicates both a potential consequence of the microbial shifts and impact on metabolism between in the PAH-exposed fish sub-populations. Overall, this work provides insight into chemical-associated microbial community and metabolomics shifts and some of the potential resulting impacts.

The second objective was to develop a framework for precision bioremediation in which optimal microbial taxa could be identified for biostimulation, bioaugmentation and genetic bioaugmentation at a given site. Here, we developed an approach that combined Illumina Miseq high throughput sequencing, chemical profiling and Spearman correlation analyses. This framework was developed using samples obtained from the Holcomb Creosote Co. Superfund site, which is contaminated with both PAHs and heavy metals. Using this framework, Geobacter was identified as a biostimulation target while Mycobacterium and Sphingomonas were identified as strong potential biostimulation and genetic bioaugmentation targets, respectively. Based on availability and sequencing data, a consortium of Mycobacterium fredericksbergense, Sphingomonas aromaticivorans F199 (containing the pNL1 catabolic plasmid) was selected as a possible bioaugmentation cocktail to treat the Holcomb Creosote Superfund soils for PAHs. Though it is possible to identify prospective bioremediation targets using this methodology, this approach remains limited mainly due to the inadequate amount of fully sequenced site-specific environmental strains and catabolic plasmids available in databases. Additional research is needed especially using shotgun metagenomic sequencing, rather than amplicon-based sequencing, to increase the availability of environmental microbiome data and thereby improve the identification of potential donor strains and catabolic plasmids.

The third objective consisted of developing a method to effectively monitor genetic bioaugmentation and validate the method in a series of lab scale precision bioremediation scenarios. Previous methods used to monitor for HGT events have relied on using either fluorescent labeling or culturing. However, these monitoring techniques have been shown to be ineffective in complex matrices and for large-scale field applications. Herein, qPCR probes were designed to effectively monitor plasmid conjugation in complex matrices on a large scale.

In the final inclusive objective, the microbial cocktail formed by the framework developed in Objective 2 and the monitoring technique developed in Objective 3 were validated in lab scale reactors using a realistic PAH-contaminated soil matrix obtained from the Holcomb Creosote Co. Superfund site. Overall, it was found that the targeted microbial consortium was able to improve bioremediation by constructing an engineered environmental microbiome capable of increasing the rate of PAH biodegradation with a minimal long-term impact on the communities. In particular, there were overall increases in the class Bacilli and decreases in Betaproteobacteria sustained over time. In addition, the efforts to monitor genetic bioaugmentation were successful and HGT through plasmid conjugation was quantified. Specifically, the probes were able to detect conjugation of the NAH7 plasmid immediately following genetic bioaugmentation. Not surprisingly, the Inc-P9 plasmid was not maintained in the community, as it did not provide a strong enough consistent benefit towards survival to justify the metabolic load. The probes were also successful in quantifying the pNL1 plasmid, though no sustained HGT events were detected.

Overall, this dissertation provides significant advancements to the field of precision bioremediation. In particular, this dissertation work begins to integrate metagenomic and chemical measurements using statistical methods to effectively identify bioremediation targets and providing tools to monitor bioremediation progress under field relevant conditions. It is anticipated that as environmental microbiome databases continue to be populated, the use of frameworks such as that outlined in this dissertation work will be instrumental for the identification of targeted bioremediation treatment strategies.