Isolating Microplastics

In recent years, an increasing number of techniques have been developed to detect microplastics consumed by biota. Methods for extracting microplastics from biotic material include dissection, depuration, homogenization and digestion of tissues with chemicals or enzymes. Here, a range of optimized methods is consolidated, and their benefits, biases and areas of concern are evaluated.

Dissection

In a large proportion of studies, researchers target specific tissues, primarily the digestive tract (including the stomach and intestine). In larger animals, including squid [64], whales [97,98], turtles [95] and seabirds [96], dissection of the gastrointestinal tract and subsequent quantification of synthetic particles from the gut is the predominant method for assessing plastic consumption. In laboratory studies, it is more common for the whole organism (42% of studies) or the digestive tract (26% of studies) to be digested or analyzed (Figure 8.3a). In comparison, 69% of field studies targeted the digestive tract, and 27% looked at the whole organism (Figure 8.3b). Excision of the intestinal tract can also be used to ascertain consumption of microplastics by invertebrates and vertebrates including pelagic and demersal fish [19,34,41,65-67,69-80,83-93]. Investigation of stomachs and intestines is relevant for microplastics >0.5 mm in size. Microplastics larger than this do not readily pass through the gut wall without pre-existing damage, and the likelihood of translocation into tissues is too low to warrant regular investigation [131,132]. Localization of microplastics <0.5 mm can be determined by excising organs, such as the liver or gills [62,81,105] or, where the research question relates to risks of human consumption, edible tissues, for example, tail muscles of shrimp [38]. Microplastics present in dissected tissues can be isolated using saline washes, density flotation, visual inspection or digestion (see below).

Depuration

Should microplastic ingestion be the primary focus of the study, it is important that any externally adhered plastics are removed prior to treatment; typically, this is achieved by washing the study organism with water or saline water or using forceps [16,61]. A depuration step can be used to eliminate transient microplastics present in the intestinal tract. Depuration is facilitated by housing animals in microplastic-absent media (e.g., freshwater, seawater, sediment), with or without food, and leaving sufficient time for complete gut evacuation [54]; media should be refreshed regularly to prevent consumption of egested microplastics [23]. Depuration ensures only microplastics retained within tissues or entrapped in the intestinal tract are considered [23,37,51]. Depuration also provides opportunities for the collection of fecal matter, typically sampled via siphon, or pipette; feces can subsequently be digested, homogenized or directly visualized to assess and quantify egested microplastics. Fecal analysis has been used to determine microplastic consumption in a range of taxa, including sea cucumbers [28], copepods [13,17], isopods [42], amphipods [33], polychaetes [22] and mollusks [43-55].

 
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