Project Highlights:

  • Understand how marine microorganisms interact with micron-size plastic debris
  • Provide insights into the unexplained microplastic gap in the oceans
  • Develop interdisciplinary skills at the interface of biology and physics

We live in a plastic age where the consumption of synthetic polymers is relentlessly increasing every year. Most of the consumed plastic is of single use and, hence, will rapidly end up as waste. Mismanaged plastic waste inevitably enters aquatic systems and ultimately is washed into the oceans: recent estimates show that ~10 million metric tons were dumped in the seas just in 2010, with a predicted 10-fold increase by 2025 [1]. Most plastics are highly durable and therefore expected to persist in the environment for centuries.

The ultimate fate of marine plastics and its environmental impact is not well understood. Recent global surveys estimate that the plastic currently floating in the oceans accounts for only ~1% of the total expected marine plastic debris that has been dumped by humans during the past 50 years [2]. The fate of the remaining 99% is unknown.

Marine plastics fracture and break due to weathering, and therefore small debris should be much more abundant than large ones. This has indeed been observed, but only down to a size of ~2mm. Below that size a “microplastic gap” appears: a sharp cutoff at the millimetre scale with sub-millimetre size debris strikingly under-represented [2,3].

What processes are responsible for the development of the microplastic gap? Could these be the key to understand the fate of the missing plastic?
One interesting hypothesis is that the gap is a consequence of two possible biological processes: i) fast sinking due to changes in buoyancy following colonisation by microorganisms; and ii) direct uptake of microplastics by plankton. These two processes are most likely complementary, rather than mutually exclusive. The latter in particular has been partly supported by direct laboratory observations of copepods feeding on 20um-size plastic particulate [4]. At the same time, we have recently shown that motile microorganisms can easily come into close contact with micron-size passively drifting particulate while swimming. This would provide an easy route to microplastics uptake for the multitude of phagotrops in the oceans, providing a further direct route for plastic to enter the global food web.

Interactions between plastic debris and microorganisms. a) A 1µm polystyrene particle being entrained by a microalga; b) bacterial colonies (blue) on the surface of a piece of polyethylene (red).


This Ph.D. project will develop critically our understanding of the relation between sub-mm plastic particulate and marine microorganisms, with the aim of clarifying the mechanisms leading to the emergence of the microplastic gap. Your research will adopt a two-pronged approach, studying both the large and the small ends of the size spectrum. For large plastic objects (~1000-100um), the student will study quantitatively the dynamics of microorganismal colonisation depending on size and plastic type. Size has already been shown to have a strong effect e.g. on microbial colonisation and processing of oil microdroplets from platform spillages [5]. At the small end (<10um), the student will focus on the uptake of plastic microparticles by marine phagotrophs, already reported anecdotally [6]. The student will start from microfluidic and microscopy techniques already available in the Polin lab [7] and will further improve/refine them.

Training and Skills

Co-supervised by Dr. Marco Polin (Warwick, Physics Department) and Dr. Joseph Christie-Oleza (Warwick, School of Life Sciences), the project will require a combination of experimental (general microbiology wetlab) and theoretical/numerical skills (in particular quantitative data analysis and programming). The successful candidate will either already possess them or be willing to learn enthusiastically.

CENTA students are required to complete 45 days training throughout their PhD including a 10 day placement. In the first year, students will be trained as a single cohort on environmental science, research methods and core skills. Throughout the PhD, training will progress from core skills sets to master classes specific to CENTA research themes. 



Year 1: Familiarisation with existing experimental protocols, and establishment of new ones for: formation and weathering of microplastics; microbiology wetlab; microfluidics; microscopy. Establishment of protocols for experiments on plastic colonisation (large size) and ingestion (small size).

Year 2: Systematic experiments on colonisation of plastic debris as a function of size and surface features (e.g. roughness). Quantification of colonisation dynamics, species abundance and spatial heterogeneity of the biofilm.

Year 3: Establishment of protocols for ingestion experiments. Systematic characterisation of the dynamics of ingestion and retention; investigation of possible modification to physico-chemical properties of ingested plastic.

Partners and collaboration (including CASE)

The project has the potential to become CASE, by building links with industries involved in either plastic production or waste management.

Further Details

Enthusiastic applicants with 1st class BSc or MSci degrees in Biology, Physics, or related fields. Previous laboratory experience will be an advantage but is not required. Programming skills are necessary at least at a basic level.


Marco Polin: m.polin@warwick.ac.uk

Joseph Christie-Oleza: j.christie-oleza@warwick.ac.uk