The large amount of microbes colonizing the host and covering all the mucosal surfaces such as digestive, respiratory tissues, and urogenital tracts is known as the host-microbiome. The gut microbiome has drawn most attention because of its relationship with diet and its importance in many aspects of the host’s health and well-being (Bendtsen Bangsgaard et al. 2012, Bolnick et al. 2014b, Hanning and Diaz-Sanchez 2015, Zha et al. 2018). For example, most studies on wild animals have shown that diet in terms of prey species has a large effect on the microbial community composition (Wang et al. 2011, Bolnick et al. 2014a, 2014b, Smith et al. 2015). Other factors such as exposure to pollutants have been also shown to have an influence on host-microbiota (Merrifield et al. 2013, Ghanbari et al. 2015, Gaulke et al. 2016, Nasuti et al. 2016, Fang et al. 2018, Yuan et al. 2019). Despite knowledge on the effects of prey consumption on the consumer’s host-microbiota, we know little about the potential carry-over effects of pollutants across trophic levels in predator-prey interactions.
Microplastics (MPs), defined as plastic polymer particles smaller than 5mm, are pervasive emergent pollutants resulting from plastics that have been widely used in the last century, with a peak in production during the past decades (Geyer et al. 2017, Horton et al. 2017b, Jin et al. 2019, Menéndez-Pedriza and Jaumot 2020). MPs have become one of the largest wastes that are accumulated in the environment (Geyer et al. 2017, Liu et al. 2020). Plastic debris and MPs in marine ecosystems are recognized as a global threat to marine organisms (Eriksen et al. 2014, Revel et al. 2020). In recent years, a lack of studies on plastics and MPs in freshwater ecosystems has been identified as a matter of priority (Horton et al. 2017a). Indeed, studies quantifying MPs, assessing MP exposure and MP uptake in freshwater organisms have been performed (de Sá et al. 2018), demonstrating that MPs could have direct effects on the organisms, e.g., on life history-traits (Besseling et al. 2014, Au et al. 2015, de Sá et al. 2018). Moreover, the presence of ingested MPs in the gut imposes a threat as potential carriers of adsorbed hydrophobic organic chemicals or persistent organic pollutants that might be transferred to the organism (McCormick et al. 2014, Horton et al. 2018). This might result in additive or synergic activities between MPs and other environmental pollutants such as pesticides, and MPs and pesticides might therefore have physical and chemical effects on the host-microbiota of aquatic organisms after ingestion (Jeong et al. 2016, Liu et al. 2020). For example, Jin et al. (2019) showed that MPs caused changes in the microbiota of mice, and these microbiome changes were suggested to affect metabolic disorders in the host. Nasuti et al. (2016) showed that the pyrethroid permethrin reduced the abundance of several microbe groups in the guts of rats. However, few studies have focused on non-model organisms and on the combined effects of MPs and pesticides in the host and its microbiome.
Importantly, MPs and pesticides can have effects across trophic levels (Carbery et al. 2018, Athey et al. 2020, Costa et al. 2020, Elizalde-Velázquez et al. 2020). Changes in the nutrients and in the carbon source can modify the microbes in the environment (Townsend et al. 1998, Cabrerizo et al. 2019). It has been shown that the microbiome is highly affected by food availability as well as habitat disturbance (Roger et al. 2016, Foster et al. 2017), which potentially could result in bottom-up control of the microbes, i.e., affecting the microbiome of organisms higher up in the food chain (Fahnenstiel et al. 1998, Carrillo et al. 2008, de Vos 2017, Li et al. 2020). Pervasive pollutants such as MPs could be colonized and used as a carbon source by some microorganism which in turn could interact with other stressors (Zettler et al. 2013, Jacquin et al. 2019, Amaral-Zettler et al. 2020). Hence, MPs colonized by microorganisms could interact with pesticides affecting bottom-up food web dynamics, but few studies are available on such interactions.
In this work, we examined the effects of exposure to MPs, with and without an additional disturbance induced by sudden exposure to the pesticide deltamethrin (DMT). The pesticide DMT was chosen for examining effects on the microbiome. This pesticide is a pyrethroid that is extensively applied in agriculture, aquaculture, and forestry, as pest control (Mestres and Mestres 1992, Hong et al. 2020). DMT is known for its neurotoxic effects, acting mainly in the voltage-gated Na+ channels of the nervous system (Mestres and Mestres 1992, Hong et al. 2020), and in secondary targets involved in signal transduction (Toshio 1992, Toumi et al. 2015). DMT has been shown to have negative effects on a variety of organisms including mammals and birds, and it is also highly toxic to aquatic organisms such as fish and aquatic invertebrates (Dawood et al. 2020, Hong et al. 2020). Moreover, the effect of DMT in non-target organisms might be worsened due to the presence of other stressors, nutritional deficiencies, or other pollutants such as MPs (Menéndez-Pedriza and Jaumot 2020). Studies on the combined effects of MPs and DMT are rare (Horton et al. 2018, Felten et al. 2020), and do not take into account the host-microbiome.
To examine the effects of MPs, and DMT on trophic levels, we studied the changes in the diversity and abundance of the host-microbiome in a three-level food chain: planktonic crustaceans (daphnids), predatory damselfly larvae, and top predatory dragonfly larvae. Our manipulation of pollutants occurred at the first (MPs) and second (DMT) food chain levels. In addition, we estimated the survival of the damselfly larvae to the dragonfly top predator. We predicted effects on the diversity and abundance of the host-microbiome across all food chain levels. MPs would behave as substrates for the microbial community, leading to changes in microbial diversity and abundance. In the presence of pesticides, we predicted that the microbial diversity and abundance would decrease. For the combined exposure to MPs and pesticide, we predicted intermediate effects on the diversity and abundance of the microbiome compared to those observed in the exposures to MPs and pesticide alone: the MPs might sequester the pesticide by adsorption. We also predicted a higher predation rate when the damselflies were exposed to MPs or the pesticide alone, due respectively to a high accumulation of MPs in the body or intoxication of the pesticide. However, when the damselflies were exposed to both MPs and the pesticide, we hypothesized that the effect of the pesticide might be attenuated by the adsorption capacity of the MPs, resulting in lower predation rates than the exposure to MPs or pesticide alone.