1. What experimental conditions and experimental approaches need to be standardized to allow this field to move forward?
Experimental work on the ingestion and impact of microplastics on marine detrital and suspension feeding organisms must use particles that have been conditioned in natural seawater to allow a microbial community to form on their surfaces, and use environmentally relevant concentrations such as 1 to 100 MP particles per m3. Standards that need to be established include: MP sampling approaches as a function of particle size and location in the water column and sediment, MP extraction from sediment and biological tissues, units of measurement, size and shape classification, analytical methods for polymer and additive classification and exposure treatments in toxicological experiments.
To add to the confusion, the current definition of microplastics are plastics between 5 mm and 1 µm. While this definition is being used in most reports it, makes no real sense given the we use the metric scale for measurement in science. A re-definition would be megaplastics (10 cm to meters), macroplastics (1 mm to 10 cm), microplastics (1 to 1000 µm), and nanoplastics (< 1 µm). The size of MPs is another area of inconsistent descriptions. Some authors use surface area (mm2), volume (mm3), length (mm) and subsequently report concentration as area per unit volume (mm2/m3), volume per unit volume (mm3 or cm3/m3), length per unit volume (mm/m3) or just particles per unit volume (particles/m3). It is time we settled on one notation. Since the overall governing factors controlling MP vertical distribution is their density (mass per unit volume [kg/m3]) and their drag (D) through the water (D = ½ pSU2 Re, where p is the kinematic viscosity, S is the wetted or frontal surface area U is the velocity and Re is the Reynolds Number or ratio of inertia to viscous forces), then it would make sense that MPs should be reported in volume, mass and some shape factor to take account of frontal surface area. We suggest use of the shape factor: 4/perimeter2. If the MP cannot be resolved in 3D then it should be reported in area and maximum and minimum axes and depth or thickness. Other properties used in colloid science to evaluate particle characteristics- such as morphology, shape, ferret diameter, radius of gyration, equivalent spherical diameter (ESD), and Stokes radius, which is a measure of how particle shape influences its drag at low Re, should all be considered given the purpose of the analysis, but at least with area and max/min dimensions, one can calculate mass given the polymer type and its density.
Another area in need of standardization are methods for determination of polymer type. Pyrolysis GCMS is the gold standard for identification of organic molecules. However, while this technique is extremely costly and tedious and limited to a fairly large sample size (several mg), it is the only method available to identify other additive compounds such as plasticizers, fire retardants, anti-fragmentation materials, and the like. Vibrational spectroscopy, both Raman and FTIR (Fourier Transform Infrared) are used routinely to identify polymers. FTIR requires dry material since water absorbs IR light quickly and its sample size is limited to greater than 10 µm by the diffraction limit of the wavelength it uses, namely ~800 nm and above. Raman spectroscopy on the other hand can provide a distinguishing signal directly through water since water is essentially Raman inactive, and its sampled area can be close to 1 µm2 when a laser wavelength less than 600 nm is used. Laser excitation at wavelengths less than 600 nm are used to achieve better chemical resolution, but fluorescence can dominate the signal at these high energy, short wavelengths. Mitigating fluorescence requires extended Raman techniques such as spectral modulation, time-domain and time gated Raman spectroscopy. Our laboratory are working on the latter technique for rapid, in situ measurements of polymers. Hand-held Raman spectrometers are a reasonable solution for rapid identification of polymer type and are now available in the rage of $12,000 to $15,000 and should come down in price in the near future.
Inter-calibrations between laboratories are needed to provide consistency among researchers. For example, by exchanging samples between labs that have been processed using one or more different methods, we begin to see the issues associated with the use of multiple techniques for sample collection, MP extraction from a matrix, such as water, sediment and biological tissue. Currently, there is no one acceptable procedure to accomplish this, but communication between labs is essential to move forward quickly.
Toxicological studies require defined exposure to contaminants such as duration and concentration. For example, standards need to be developed using environmentally realistic sizes and concentrations as previously stated, but also exposure to biofilm-producing microbes that alter the surface properties of the particles.
2. What is required to quantify the abundance and distribution of MPs in the world ocean?
Only a small fraction of the estimated mass of plastics released into the ocean has been accounted for by sampling that has been done to date, which has focused mainly on surface waters. There is an urgent need to locate this “missing plastic” and understand its distribution in the oceans.
Since on average, MP concentration is likely to be very low (< 1 particle per m3), very large volumes of water on order 10s to 100s of m3 must be processed to statistically describe the distribution of polymers resulting from fragmentation of larger plastic sources. Therefore, new instrumentation and the use of autonomous vehicles are needed to sample, process and classify MPs in high volume. Currently, there is no one approach to sample and classify MPs from the ocean surface to the deep sea. Nets can only efficiently sample to ~100 µm and depth profiles are challenging. One solution may be depth-specific pumping combined with size screening and filtration. Filters may then be processed using spectroscopic analysis of polymer type, where particles remaining on a filter are imaged with Infrared, Raman, or hyperspectral imaging techniques. Another approach may be large volume flow-through imaging and spectroscopic analysis. While the latter approach could provide real-time information, several hurdles must be overcome: Spectroscopic techniques such a Raman and Fourier Transform Infrared (FTIR) are relatively slow and require seconds to obtain a signal from a target such as a potential MP particle. Higher power lasers with either or both acoustic focusing and laser tweezers might solve this issue but is technically difficult and expensive to implement. Further technology development needs to occur before we are ready to use any of these techniques in the field.
Moreover, these new instruments need to be deployed on Autonomous Underwater Vehicles (AUVs) such as unmanned powered vehicles, Lagrangian floats and gliders, as well as on moorings. Some low hanging fruit would be to install MP sensors on ships of opportunity such as ferries, ocean cargo ships, research and fishing vessels in their seawater cooling system emanating from the sea chest. This could quickly establish and accelerate a data acquisition stream from near-surface water on a global scale. However, since it has been shown that the bulk of the MPs are not necessarily at the surface and therefore cannot be captured using neuston nets, we need to target key locations in the ocean where the probability of elevated concentrations of MPs is higher, such as at the pycnocline and other density discontinuities. One advanced instrument currently in development at WHOI is the MesoBot (http://www.whoi.edu/main/mesobot), a deep-diving robot that seeks density interfaces within the mesopelagic zone (200 to 1000 m depth) and follows and tracks particles and/or organisms for an extended period of time. A representative number of these key locations of elevated MP concentrations should be sampled. Once the water column has been sampled, benthic sediment will need to be processed in both areas of high and low plastics at the surface. Together with water column and sediment analysis underway we may begin to assemble a full mass budget for the world ocean.
3. What kinds of numerical models do we need to understand and predict the distribution and fate of MPs?
The sea surface distribution of MPs is known to be influenced by large-scale wind patterns, as these winds drive the convergence of Ekman flow that accumulates MPs in the center of the subtropical gyres. While this process can explain features such as the so-called Great Pacific garbage patch, the surface distribution of MPs depends on other large-scale processes as well. The zonal asymmetry in North Atlantic currents has helped form an MPs “front” that is advancing into the Arctic Ocean with high concentration on the Barents Shelf, but much lower concentrations elsewhere. It is likely that this front is a transient feature resulting from the transport of European plastics that represents a timely opportunity to track ocean currents with this tracer before the MPs are more widely dispersed. The orientation of the MPs front could also be modified by changes in climatic quantities such as air-sea heat and freshwater fluxes and thus may be related to the greater Atlantic overturning circulation. As MPs are strongly influenced by ocean circulation, then new observations of both surface and subsurface MP concentration promise to put constraints on the circulation that could be leveraged to improve estimates of the underlying physical processes.
We need to assess and utilize an even wider range of physical and chemical models to simulate the behavior of oceanic MPs, however, if the goal is to improve current assessments. Upper-ocean MPs are subject to the same physical processes that affect surface drifters and phytoplankton, including fronts and vortices formed by submesoscale (<10 km) variability. Such processes are the bridge between injection of MPs from rivers and coasts and the ultimate large-scale distributions, and are likely crucial for setting the rates and pathways of dispersion. The impact of mixing at the Kolmogorov scale that is ubiquitously detected by microstructure measurements must also be assessed. Simulations of vertical transport based on changing density as a function of microbial colonization and weathering could reveal critical concentration regions at large density gradients such as the seasonal or permanent pycnocline. Thus, the same physical processes that induce aggregation of plankton cause concentration of MPs and increase contact rates and the probability for ingestion by plankton. Ultimately, these spatial scales that are much smaller than the ocean gyres may turn out to be the most important for the physical-biological interactions.
4. How does microbial activity affect the physical and chemical properties of MPs?
Although studies have shown that microbial communities can colonize MPs, very little is known about the role of microbial communities in modifying the physical and chemical properties of different plastics. Highly controlled laboratory experiments need to be conducted with defined levels of colonization and compared to similarly controlled levels of UV illumination and physical abrasion, as well as realistic physical marine conditions.
These experiments could establish a “weathering index” that could be used to age MPs sampled from the ocean. One possibility is the use of Raman spectroscopy to identify specific peaks associated with crystalline versus amorphous domains. Since amorphous domains typically leach out more quickly than crystalline domains, the ratio of crystalline to amorphous material in plastics will indicate the degree of degradation.
We could capitalize on the low cost of sequencing technology to better understand the composition and genomic and transcriptomic characteristics of MP-dwelling cells and their potential to degrade particles. In addition, developing “model-type” systems to study bacterial degradation, such as polyethylene-degrading bacteria within the guts of some invertebrates, could provide a valuable link into the degradation of different polymers within marine life.
We can ask: Does the colonization of MPs influence their buoyancy and therefore their vertical position in the water column? Do oceanic algal blooms change the flux of MPs through the water column to the sea floor? Can a new type of polymer be developed that attracts specific microbes that enhance their degradation?
5. What are the ecological impacts of MPs on marine organisms, populations and ecosystems?
MPs have been found in a variety of suspension feeders, including ciliates, salps, and larvaceans as well as edible shellfish and fish. MPs ingested by organisms can cause gastrointestinal obstruction, poor nutrition; tissue inflammation and oxidative stress, physiological responses to toxic compounds such as phthalates, PCBs, and PAHs, they may act as vectors for pathogens such as Vibrio sp. Chlamydomonas, sp., Chlostridium sp., Cryptosporidium, E. Coli, Hepatitis A, etc., and may directly impact gut microbiomes. MPs can adsorb hydrophobic chemicals including a variety of persistent organic pollutants (POPs) and may carry additives that are strong oxidizers in seawater that can, under some circumstances, be transferred to animals consuming the MPs.
Physiological effects on marine organisms include inhibition of feeding activity, nutritional dilution of normal diets causing depletion of lipid reserves, heightened immune inflammatory response, and reduction of fitness leading to loss of reproduction. An increase in susceptibility of coral communities to pathogen infections in response to microplastics has also been shown. Moreover, NPs have been shown to accumulate in the tissues of marine shellfish and be transported across epithelia and cell membranes.
What is the impact of MPs and their leachates on microplankton, macroplankton, fish, and larger vertebrates? What is the influence of MP and NP polymer type, size, shape, surface area, and other morphological and weathering conditions on their uptake, fate, and effects in organisms, including the bioavailability of adsorbed contaminants?
6. What are the impacts of MPs on Human health and the socioeconomic ramifications of MPs in the human food chain?
There are a growing number of reports of MPs in edible seafood. One of the major questions concerning micro- and nanoplastics (NPs) is the extent to which they pose a threat to human health, individually or relative to other sources of exposure to these materials, or in instances where the gut is consumed, such as with some shellfish, or where only the muscle is consumed. Potential human health risks may also arise from exposure to additives such as bisphenol A (BPA) and phthalates and from exposure to environmental contaminants such as PCBs and PBDEs sorbed to the surface of MPs or NPs, or from possible physical effects of large quantities of MP or NP particles accumulating in the gut or other organs.
Currently, there are no standard methods for sampling and measuring plastics in seafood, and no information to guide the interpretation of the data, e.g. regulatory limits or guidelines above which the seafood would be considered hazardous. In addition, we lack the critical information needed to assess the human health impacts of exposure to MPs and NPs from seafood compared to inhalation from the atmosphere.
Assessing the risks to human health involves two components: hazard assessment and exposure assessment, and oceanographic research can contribute to both of these. Hazard assessment requires experimental exposure studies to define the adverse effects of concern, the mechanisms by which they occur, and the dose-response relationships. Aquatic animal models can contribute to that effort by taking advantage of their rapid life cycles and accessible developmental stages. A key question is whether it is the plastics themselves, or the contaminants they carry, that is the primary hazard.
Exposure assessment requires information on the concentrations of MPs and NPs and their contaminants in seafood relative to other sources of exposure. Oceanographers can contribute by providing a quantitative understanding of MP-NP distribution and fate in marine animals, especially those consumed by humans and the dynamics of how plastics move through marine food webs. Hazard assessment and exposure assessment can then be combined into a human health risk assessment.
As we investigate the possible mechanisms and risks of human health risks from MPs, we should take advantage of synergies with ongoing work on health effects posed by plastic food packaging, nano-medicines, and nanoparticles. Consumption of seafood is not the only route by which humans are exposed to MPs and NPs; understanding the impacts of these materials will require a careful parsing of the total MP burden on humans, which is likely to vary with, among other things, the amount and type of seafood in the diet. Initial risk assessments might focus on differences in health outcomes, for example, in seafood-dependent communities consuming seafood from high vs. low MP level waters.
Beyond the assessment of human health risk and its economic ramifications, and those of other ecological effects of marine MPs, there are interesting policy questions concerning the cost-effective ways to reduce MP risks. These are likely to be different for point-source inputs of primary MPs to ocean waters and for secondary MPs degrading from macroplastics. Until the ecological and human health effects are better understood, it is difficult to make a convincing case that marine MPs are a problem that must be remediated with urgency, given the many other ecological and human health challenges we face today.
Conclusions and recommendations
As we are just beginning to realize that MPs are distributed widely throughout the world ocean, and are consumed by marine and freshwater organisms, many intended for the commercial fisheries, the dearth of information concerning their impact on those organisms, the food chain and on humans is astounding. However, to better understand the scope of the problem at hand without established best practices and heightened rigor for experimental conditions, reporting units, definitions, identification of optimal sampling and classification technologies, the quantification of distributional patterns of microplastic particles will be slow to emerge.
New technologies need to be developed or adapted to provide classification and quantification of polymers and their additives at all oceanographic scales to create a mass budget to compare against plastics sources. Numerical models must be developed to allow for “now” and “forecasts” of distributional patterns, impacts of MPs on physiological processes, and projections on ecosystem function. We need to understand how the marine microbial world interacts with MPs causing degradation, nucleating marine snow and enhancing carbon flux to the deep sea, and influences their distributional behavior in the water column and sediments.
Finally, humans are consuming MPs at an ever increasing rate. Should we ignore this fact or should we begin to understand if and how MPs are impacting human health? The current lack of a cohesive research effort on MPs is reminiscent of the notorious history of the impact of 1,1,1 trichloro-2,2-bis[p-chlorophenyl]ethane (known as DDT) and polychlorinated biphenyls (PCBs). Both of these contaminants were exposed for their impacts on the ecosystem and human health at least a decade before they were effectively controlled or taken off the market entirely. It is the social awareness of unintended consequences that forces change in our system. Social awareness of the impacts of microplastics cannot proceed rigorous scientific inquiry. Therefore, it is up to responsible scientists to take on this issue and provide the data necessary for making responsible decisions by management.