Microbial interactions with marine plastic debris
The ultimate fate of plastic in the oceans
Despite growing awareness, over 275 million tonnes of plastic waste are still generated annually and mostly not recycled. It is estimated that 3-5% of global plastic waste (4.8 to 12.7 million metric tonnes) will inevitably be washed into the oceans every year. The problem is far from reaching its summit as a 10-fold increase from current levels is estimated for 2025 and peak waste will not be reached before 2100. Hence, this problem has been flagged as a 'planetary crisis' during the recent UN environment summit (Nairobi, December 2017). Polyethylene (PE), polypropylene (PP) and polystyrene (PS) are three of the most abundantly manufactured polymers, representing over 55% of global synthetic polymer production (29.6%, 18.9% and 7.1%, respectively) and, being the main polymers used in packaging, they are the main materials that go to waste (i.e. representing up to 70% of the plastic waste stream). Contrary to most other plastic polymers, PE, PP and some forms of PS are less dense than water and therefore have a high dispersal through rivers and oceans if they escape waste management, being the main polymers found in surface marine plastic debris. Due to their chemical structure, PE, PP and PS are some of the most refractory, durable and inert polymers.
Despite some studies have claimed microbial biodegradation for some of these materials, none has given clear irrefutable proof for this process (i.e. complete microbial mineralisation) for all these materials and, hence, intense weathering that embrittles these materials causing fragmentation into smaller fragments is the only know form of degradation in marine environments. Furthermore, large speculation was raised by a recent marine circumnavigation expedition (the Malaspina cruise) claiming that fragments <2 mm in size were largely depleted from ocean surface waters and, what was most disturbing, that only 1% of the expected marine plastic debris could be found, awakening controversy on the fate of small buoyant microplastics in marine ecosystems (e.g. sinking, microbial biodegradation or fauna ingestion amongst other possibilities).
What is the ultimate fate of plastics in marine ecosystems? This question has driven this part of our group's research since we started working on this topic in 2015. In order to answer this question we are working towards the understanding of i) the ecology of marine plastic debris and ii) their biodegradability in these ecosystems, both requiring a better understanding of microbe-plastic interactions. To learn more about these, click on the corresponding box.
The ecology of marine plastic debris
Our research is addressing this topic that goes all the way from studying the physical nature of microbial colonisation of marine plastic debris to the influence of microbial colonisation on microplastic ingestion by macrofauna and ecotoxicity. But first of all we considered of uttermost importance to develop a method to easily track small microplastics (<1 mm in size) and, hence, recently pioneered a novel method to effectively detect the smaller fraction of buoyant microplastics. The use of a fluorescent dye that stains plastics was applied in preliminary fieldwork trials allowing the efficient detection of the predicted fragmentation pattern of large plastics due to weathering or degradation, ending the current speculation about the "apparent" loss of this smaller fraction from surface waters.
Publication issued from this work:
Erni-Cassola, G., Gibson, M.I., Thompson, R.C., Christie-Oleza, J.A. 2017 Lost, but found with Nile Red: a novel method for detecting and quantifying small microplastics (1 mm to 20 μm) in environmental samples. Environmental Science & Technology, 51: 13641-13648
Figure 1. Microscope images of processed sand samples demonstrating selective Nile Red fluorescent staining of synthetic polymers. (a) excitation-emission 460-522. (b) composite image of excitation/emission 460/522 nm and brightfield. Figure adapted from Erni-Cassola 2017 Environmental Science & Technology.
Our lab has an interest in both natural and synthetic polymers. Synthetic polymers, commonly known as plastics, are inert materials and highly recalcitrant to biodegradation. The biodegradation of polyethylene terephthalate (PET) plastic bottles by a single microorganism was recently published [Yoshida 2016 Science], although the report is not devoid of controversy. Aliphatic polyesters are readily mineralized as the ester bonds can be hydrolysed by esterase enzymes releasing monomers/oligomers for mineralisation. Nevertheless, the breakdown of more refractory polymers (e.g. those containing an aliphatic backbone such as PE, PP or PS) is not obvious as it is too slow to be visualised through traditional microbiology methods and, furthermore, there are no known enzymes that carry out the initial step of depolymerisation. Other processes that may speed up biodegradation is the physical erosion by the chewing of larger organisms (e.g. the caterpillars of the wax moth Galleria mellonella [Bombelli 2017 Current Biology]). However, refractory plastic degradation is thought to come as a consequence of chemical change of the polymer chain (oxidation), a reduction of its molecular weight (scission) and, ultimately, complete mineralisation. Since the mechanical integrity of plastic depends on the length of the polymer chains that constitute it, degradation causes embrittlement and fracture of the plastic due to a reduction in its tensile extensibility. Abiotic weathering processes (i.e. photodegradation via UV radiation, temperature and hydrolysis) added to the mechanical action of water is the main cause of large plastic fragmentation in marine systems. Nevertheless, complete mineralisation of plastics into CO2 and H2O can only be accomplished by combustion or biodegradation. Environmental plastic debris exposed to humidity and sunlight rapidly develop a diverse microbial biofilm, coined the ‘plastisphere’. The plastisphere in marine environments is clearly distinct from its surrounding planktonic microbiome and, more interestingly, it usually contains hydrocarbon-degrading microbes. Mathematical models have suggested plastic biodegradation and considerable speculation has been generated on refractory plastic biodegradation in the environment, but the lack of reliable tools to clearly assess whether this process can be carried out by environmental microbes has hampered clear conclusions and, to date, this biological process remains controversial. We are tackling this issue via i) microbial selection and evolution of microbial consortia that can biodegrade such polymers, and ii) the development of novel materials to assess unequivocally polymer biodegradation. This part of our group’s research is being carried out in close collaboration with the polymer chemist Dr Gibson (Department of Chemistry, University of Warwick).