Mitigating Contamination

Contamination, cross-contamination and loss of plastics are a major challenge for microplastics research. It is recommended that all laboratory processing should include steps for preventing or limiting airborne contamination, and procedural blanks used to account for this [124]. Additional processing may be utilized during microplastic isolation to improve the detectability or identification of microplastics; however, each additional step increases the opportunity for contamination. Where possible, plastic consumables should be avoided [167]. All samples should be preserved by freezing, desiccation or in filtered ethanol or formalin, although the latter may result in loss of some plastics. On research vessels, glassware may not be feasible, so plastic may be used following sufficient cleaning with filtered water.

Data Analysis

Researchers should be aware of biases in sampling environmentally exposed individuals. Firstly, the condition of organisms prior to capture is unknown, and linking microplastic (and co-contaminant) burden with condition is prohibitive. Secondly, sampling may lead to an underestimation of the microplastic burden in a population because highly contaminated individuals are dead or dying, or remain in shelters and burrows owing to reduced functionality. Thirdly, it is vital that sampling is spatially and/or temporally broad to ensure that observed levels of contamination are representative of the wider population. For example, Welden et al. [41] observed significant spatial variation in fiber contamination in three Scottish populations of Nephrops norvegicus.

Recommendations for Future Work

In reviewing the relevant literature, it is apparent that research is currently skewed toward vertebrates (Figure 8.4). The range of ecological functions carried out by invertebrates and the diverse niches which they occupy all suggest that the impacts of microplastics on these groups may have a marked effect on the environment. Further assessment is recommended of the uptake and impact of microplastics in these groups as this is essential if we are to predict the extent of the effects on biodiversity, ecosystems and ecological processes. A comparison of the relative uptake and retention of the different categories and shapes of microplastics is also required to determine which are the most harmful. Many laboratory studies of microplastic ingestion rely solely on pre-produced plastics, which are easily purchased from suppliers (e.g., polystyrene microbeads [10-15,17,26,47,105,117]); however, these are not representative of the diverse forms currently present in the

Laboratory and field studies investigating microplastic interactions with biota separated by Phyla

FIGURE 8.4 Laboratory and field studies investigating microplastic interactions with biota separated by Phyla. The value in parentheses above each bar indicates the number of species studied within the taxa. Total number of studies included in this review 120.

environment. Understanding which plastics are readily retained is necessary to determine the threat posed by the relative levels of environmental microplastics and to link these to evidence of negative effects in physiology and behavior, which may impair function at the ecosystem level.

In fact, a number of studies have considered the observable impacts of microplastics on organisms. In 11/120 studies reviewed here, researchers examined the relationship between microplastic uptake and changes in physiology and body chemistry. Endpoints have included organism behavior [104,168], lysosomal response [56], lipid content [169,170], protein content [170], population fitness [161], cellular population growth [171] and individual growth [15,170]. They also utilized ecotoxicological assays to monitor embryonic development [31,172] and the uptake of metals [94,173] and chemicals [162,163,174]. Laboratory studies must use environmentally realistic concentrations of microparticles to allow evaluation of potential harm to the individual as a result of field exposure. In wild populations, where the presence of confounding factors makes it difficult to attribute biological responses and condition directly to plastic exposure, direct observation of plastic type and abundance remains the most reliable method of determining microplastic impacts.

Microplastics may be selectively or nonselectively ingested or acquired through trophic transfer. Again, the artificially inflated levels of nonrepresentative plastics used in a number of currently available studies greatly increases the potential for chance transfer of plastics. The results of such studies, while useful in indicating the potential for transfer, display transfer in a way not feasible at normal contamination levels. In studies not focusing on the ecotoxicological impacts, we recommend inflated levels of microplastic should only be used in the presence of an ecologically valid control determined by reference to rigorous environmental sampling.

Lastly, few studies have addressed the movement of microplastics within ecosystems. Many rely on seeding plastic into non-natural food sources, in mesocosm experiments with no alternate food sources, and while these experiments have shown the transfer of plastic between food and organisms, there is a clear need for more robust information on the validity of these results in natura [175].

The need for rapid, accurate assessment of the levels of microplastic in wild populations is essential for determining baseline levels of contamination and assessing the risk of microplastics to organisms and ecosystems. The diverse physiology of the organisms covered in this review has necessitated the analysis of a range of protocols for microplastic extraction and enumeration which could limit comparability among studies. As such, we recommend the development of a standard methodology per subphylum or class of organism which will combine efficiency in digestion and recovery of microplastics with the use of the least toxic chemicals to preserve plastic polymers.

Particular attention should be given to harmonizing the way in which data are recorded (e.g., mass and number of isolated microplastics per mass of organism) to promote comparability. Prevention of overestimation of plastic contamination must be controlled by confirming the identity of a proportion of the recovered materials. This may be carried out either by chemical analysis, density separation, birefringent microscopy or other physical examination.

 
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