Toxicity comparison of nano-sized and micron-sized microplastics to Goldfish Carassius auratus Larvae

Highlights

• MPs could induce oxidative stress, destroy intestine, liver and gill, increase heart rate, inhibits growth and movement of goldfish larvae.

• nMPs could enter into the muscle tissue, destroy nerve fibers, inhibit AchE activity, and show great adverse effects on larval movement.

• Both nMPs and mMPs at higher concentrations could cause damage to fish larvae and nMPs are potentially more hazardous.

Abstract

Plastic pollution is one of the most serious environmental issues worldwide. The negative influence of plastics on aquatic organisms has increasingly concerned, especially the influence of microplastic (MPs). In the present study, the toxicology of nano-sized MPs (nMPs) and micron-sized MPs (mMPs) were comparatively studied. Goldfish larvae were exposed to 10, 100 and 1000 μg/L nMPs and mMPs for 1, 3 and 7 days. The enrichment of MPs, body length, heart rate, motor ability, microscopic and ultrastructure of intestine, liver, gill and muscle tissue, as well as the oxidative stress were analyzed. Results showed that both 70 nm and 50 μm MPs were accumulated in the digestive tract of larvae. MPs at high concentrations could induce oxidative stress, destroy intestine, liver and gill tissues, increase heart rate, and inhibit growth and swimming speed of the larvae. The most important finding was that nMPs could enter into the muscle tissue through the epidermis of the larvae. It could cause damage to muscle tissue, destroy nerve fibers, inhibit acetylcholinase (AchE) activity, and show great adverse effects on larval movement than mMPs. In conclusion, both nMPs and mMPs at higher concentrations can cause damage to fish larvae and nMPs are potentially more hazardous.

Introduction

Plastic products are ubiquitous and bring great convenience in our daily lives. Global plastic production has increased exponentially since the 1960s, with annual production more than 300 million tonnes currently (Eerkesmedrano et al., 2015). However, a considerable portion of plastic products are not recycled and eventually enter the natural environment. Most plastic in environment is difficult to be degraded and remains as waste for a long time, with approximately 10 % ending up in the oceans (Thompson, 2015). Only a small part of these marine plastic wastes enters the oceans directly, and about 80 % originates from land-based sources, suggesting freshwater systems are important transport routes of the plastics to sea (Thompson, 2015). Plastic has been detected in rivers and lakes around the world, such as the Superior lake, the Huron lake and the Erie lake in the USA, the Nakdong river in South Korea, the Garda lake in Italy, the Rhine river and the Main River in Germany, and the Yangtze estuary in China (Eriksen et al., 2013; Lee et al., 2013a; Imhof et al., 2013; Sascha et al., 2015; Zhao et al., 2014). Plastic pollution is one of the most serious environmental issues worldwide.

The negative influence of plastics on aquatic organisms has increasingly concerned by researchers, especially the influence of microplastic (MPs). MPs can be further divided into nano-sized MPs (nMPs) and micron-sized MPs (mMPs). Generally speaking, mMPs are plastics with a size less than 5 mm (Andrady, 2011), and nMPs are not unambiguously defined and different studies set the upper size limit at 1 μm or 100 nm (Costa et al., 2016; Mattsson et al., 2015). mMPS and nMPs generally come in two sources: (1) intentionally manufactured in small sizes for personal care and cleaning products, and pre-production pellets for other plastic goods; (2) breakdown of larger plastic pieces due to UV-radiation and mechanical forces (Blair et al., 2017). Studies show that once accumulated in organisms, mMPs and nMPs have the potential to cause a lot of adverse effects, such as influence behavior, reduced feeding activity, inhibited growth and development, interacting with the immune system, decreased reproductive capacity, increased oxidative stress, disturbed lipid metabolism and even genotoxicity (Karin et al., 2015; Watts et al., 2015; Cole and Galloway, 2015; Ellen et al., 2013; Lard, 2012).

Several studies compared the toxicity of mMPs and nMPs, mostly using plankton as a research model. Toxicity test on marine copepod Tigriopus japonicus showed that 5 nm and 50 nm nMPs caused more mortality than that of 6 μm mMPs, and 50 nm nMPs and 6 μm mMPs beads caused a more significant decrease effect in fecundity than 5 nm nMPs (Lee et al., 2013b). Exposed monogonont rotifer Brachionus koreanus with 50 nm, 500 nm and 6 μm MPs led to significant size-dependent effects, and smaller microbeads were more toxic, including reduced growth rate, reduced fecundity, decreased lifespan, longer reproduction time, stronger responsiveness in antioxidant systems and MAPK signaling pathways (Jeong et al., 2016). The toxic effect of plastic particles (50 nm, 500 nm and 6 μm) on photosynthetic capacity and growth of green algae Dunaliella tertiolecta was also increased with the decreased particles size (Sjollema et al., 2016). Rist et al. showed that compared to 2 μm particles, 100 nm particles caused lower egestion and decreased feeding rates in crustaceans Daphnia magna (Rist et al., 2017). These studies all confirmed that the toxicity of nMPs and mMPs to plankton is different, and the toxicity increased with the decreased particles size in most cases. Similar phenomenons were also found in the study of vertebrate, zebrafish larvae. mMPs exhibited no significant effects except for the upregulated zebrafish visual gene expression; whereas nMPs inhibited the larval locomotion, significantly reduced larvae body length, inhibited the acetylcholinesterase (AChE) activity, and upregulated nervous system related genes (Chen et al., 2017). But study on adult zebrafish showed that 5 μm mMPs induced higher oxidative stress level and more metabolites than 70 nm nMPs in liver (Lu et al., 2016). However, to date, there are only a few studies focused on the comparison toxicity of nMPs and mMPs to fish. In addition, there is no study on the distribution of nMPs in fish tissues. Further studies are needed on the toxicity differences between the two plastics. Compared with adult fish, larvae are more sensitive to the external environment and easier to observe for their transparent body. In the present study, goldfish goldfish Carassius auratus larvae were used as test organism, and two different sizes of commercially polystyrene MPs were used as model nMPs (70 nm) and mMPs (5 μm). Distribution and damage of both nMPs and mMPs on the larvae were analyzed through microscopic observation, histological examination, enzyme activity measurement, etc. The aim of the present study was to increase understanding of the differences in fish toxicity between nMPs and mMPs.