Having defined the main specific features of MP and NP that need to be taken into account in laboratory studies, we identify the major limitations that could be avoided in such studies, and discuss some ways to improve how MP and NP are handled in laboratory experiments. Based on the literature, feedback from peer reviewers and knowledge gained from our experience, we make a review of the constraints and priorities for experimental exposures of marine organisms to MP and NP. In order to meet decision-makers' expectations, it is critical/essential to consider MP shape, interaction with organic matter, and the biological and chemical loads of microplastics, as well as using exposure doses as close as possible to environmentally realistic concentrations (Huvet et al., 2016 Karami, 2017).
As a consequence, the environmental relevance of laboratory exposure experiments has recently been debated and challenged (Lenz et al., 2016 Rochman, 2016). The difficulty of performing laboratory experiments to assess MP toxicity lies in the fact that MP consist of a complex, dynamic mixture of polymers and additives, to which organic material and contaminants can successively bind, along with microorganisms, influencing their density, surface charge, bioavailability and toxicity (Galloway et al., 2017). However, most previous experimental studies used unrealistic scenarios with mostly high doses of manufactured virgin microspheres that may not be representative of the variety of microplastics found in the environment. All environmental matrices appear contaminated: surface waters (Moore et al., 2001 Eriksen et al., 2013), the water column (Lattin et al., 2004 Ng and Obbard, 2006), sediments (Van Cauwenberghe et al., 2013 Vianello et al., 2013) and biota (Murray and Cowie, 2011 Fossi et al., 2012 Lusher et al., 2013 Devriese et al., 2015 Figure 1). Microplastics have been reported in the environment worldwide, from surface waters to deep-sea sediments (Eriksen et al., 2013 Vianello et al., 2013 Wright et al., 2013b Cózar et al., 2014 Lusher et al., 2015), even in areas far from human activities such as in polar waters (Lusher et al., 2015 Cózar et al., 2017 Munari et al., 2017 Obbard, 2018). MP can also be produced as such, mainly in the form of microbeads used in cosmetics, synthetic fibers discharged with washing waters, and industrial abrasives. (2018), respectively) whose presence in the Atlantic gyre has been recently suggested (Ter Halle et al., 2017). Once in the environment, plastic debris fragments into smaller particles such as microplastics (MP <5 mm, NOAA, 2008), and presumably nanoplastics (NP defined as particles <100 nm or <1,000 nm according to Galloway et al. As a direct consequence, plastic waste in the environment is a growing problem (e.g., Barnes et al., 2009) plastic waste entering the oceans was calculated for 2010 at 4-12 million tons per year and is predicted to increase by an order of magnitude by 2025 in the absence of waste management improvements (Jambeck et al., 2015).
Production of plastics is constantly increasing to sustain our broadening uses in daily life (Thompson et al., 2009).
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