PhD defence: Maya S. Dodhia

Maya S. Dodhia defends her thesis,

The Biogeochemically-Active Plastisphere: Microbes, metals and minerals on coastal plastic

Supervisors:
Associate Professor Nicole Posth, IGN
Assistant professor Mikkel Fruergaard, IGN

Assessment Committee:
Professor Elena Gorokhova, Stockholm University
Professor Linda Amaral-Zettler, NIOZ, NL
Associate Professor Søren Jessen (chair), IGN

Abstract:
Plastic polymers both a blessing and curse, define modern living. Their durability, moldability, cost of production and ease of transport amongst many other benefits have made this a material we can’t live without. Yet, the characteristics that make it such a useful material are also the reason it has become a globally ubiquitous and pervasive pollutant. The plastisphere forms the interface between plastic and its surrounding environment. As such, plastisphere interactions with plastics and the environment indelibly play a role in plastic degradation, transport and uptake into food webs and biogeochemical cycles. To estimate and track the role of plastic in these processes, it is therefore imperative to define plastisphere composition. Especially in the sediment environment where we still lack a strong understanding of plastisphere changes and interactions with the sediment, particularly as plastics are buried and incorporated into the anoxic layer. Since sediment is a major sink for plastics, this thesis explores microbial and metal composition in sediment matrices, and focuses on constraining their composition on plastic surfaces across redox and salinity regimes in pristine and anthropogenically-influenced settings. This thesis work has tackled three key aims: First, identifying the microbial composition on plastics across a redox transition from water column to anoxic sediment and across a river-estuary transect; Second, exploring the interfacial chemistry of the plastisphere in relation to accumulated metals and minerals; and third, investigating the genetic potential of plastisphere communities to interact and partake in various element cycles (N, S, Fe and As) to further explore the role of plastics in biogeochemical cycles. Samples from Denmark and Tanzania have been used to investigate the plastisphere. Findings show that there is a diverse plastisphere community that differs from the environment. Further, the ecosystem setting surrounding the plastic is the most important factor for determining what is found on the plastics. All samples had acquired metals from the environment but minerals were also commonly visually identified on the plastic surface, therefore the concept of the plastisphere was expanded to include them. Automated quantitative mineralogy (AQM) is applied to explore the mineral composition of the plastisphere matrix. We therefore provide a new tool for visualising the orientation of abiotic and biotic components of the plastisphere, providing information on the metals and the phase they may be present in on the surface. Biogeochemical functional potential of the plastisphere was also investigated by studying the genes related to cycling of nitrogen, sulphur, iron and arsenic reduction, finding that genes encoding for these redox pathways exist across plastisphere communities. This provides a basis for future investigations of plastisphere interactions with biogeochemical cycles. Together these studies improve understanding of the plastisphere interfacial chemistry, add knowledge to the plastisphere composition and expand the concept of the plastisphere beyond the microbial community to include minerals and metals. These studies further corroborate the role the ecosystem and redox plays in governing its composition, and expand on the role the plastisphere may play in relation to biogeochemical cycles. Through exploring the complex architecture of the plastisphere in coastal settings, we have gleaned new information for investigating plastic interactions with the environment that will move us towards understanding the impacts of plastic over time.

A digital version of the PhD thesis can be obtained from the PhD secretary at phd@ign.ku.dk before the defence. After the defence the thesis will become available from the Royal Danish Library Royal Danish Library | kb.dk