In this study, we elucidated the desorption potency of polycyclic aromatic hydrocarbons (PAHs) sorbed on microplastics (MP; polyethylene) in the digestive tract of two fish species: gastric red seabream and agastric mummichog. In our in vitro assay system using the real gut sample of unexposed fish, the digestive tract was firstly removed from the fish and divided into three parts. Then, MP that had previously been sorbed with 16 PAHs were incubated with extracts of the gut contents or tissue with buffer or only a buffer. The desorption potency of PAHs was individually assessed for gut contents and tissue, which revealed that PAH desorption from MP was elevated in extracts of the gut contents compared with that in the buffer alone for both fish species. PAH desorption potency was the highest in the midgut for gastric red seabream and in the foregut for agastric mummichog, which indicates that PAH desorption from MP varies among different parts of the digestive tract and among fish with distinct gut morphology. In the midgut contents of red seabream and foregut contents of mummichog, the desorption fraction was 5.6% and 8.1% of the total PAHs sorbed on MP, respectively. The desorption fraction enhancement achieved by adding gut contents extracts tended to be greater with an increase in the n-octanol/water partition ratio, suggesting that enhancement of the desorption fraction in the digestive tract depends on the physicochemical properties of PAHs. Thus, morphological differences in digestive tracts and PAH properties should be considered when evaluating the effect of MP vector on pollutant exposure in fish.

Methane emitted by insects is considered to be an important source of atmospheric methane. Here we report the stimulation of methane emission in the cockroach Periplaneta americana and termite Coptotermes chaohuensis, insects with abundant methanogens, by neonicotinoids, insecticides widely used to control insect pests. Cycloxaprid (CYC) and imidacloprid (IMI) caused foregut expansion in P. americana, and increased the methane emission. Antibiotics mostly eliminated the effects. In P. americana guts, hydrogen levels increased and pH values decreased, which could be significantly explained by the gut bacterium community change. The proportion of several bacterium genera increased in guts following CYC treatment, and two genera from four could generate hydrogen. Hydrogen is a central intermediate in methanogenesis. All increased methanogens in both foregut and hindgut used hydrogen as electron donor to produce methane. Besides, the up-regulation of mcrA, encoding the enzyme for the final step of methanogenesis suggested the enhanced methane production ability in present methanogens. In the termite, hydrogen levels in gut and methane emission also significantly increased after neonicotinoid treatment, which was similar to the results in P. americana. In summary, neonicotinoids changed bacterium community in P. americana gut to generate more hydrogen, which then stimulated gut methanogens to produce and emit more methane. The finding raised a new concern over neonicotinoid applications, and might be a potential environmental risk associated with atmospheric methane.

In this study, we aimed to identify impacts of plastic ingestion by the ecologically important rocky shore crab Pachygrapsus transversus. We sampled individuals from August 2019 to January 2020 in a reef environment and determined their body condition and diet diversity. In order to test our hypothesis that plastic retention in the foregut is able to decrease the condition factor, we compared it between contaminated and non-contaminated individuals. A correlation test of number of ingested plastic fibres against trophic diversity was made to corroborate the hypothesis that plastic ingestion modifies the feeding patterns. Our results demonstrated that contaminated individuals had lower body condition. Also, we confirmed that debris ingestion can influence feeding patterns. These outcomes were probably linked to starvation and nutrient loss effects. We discussed that this crab is a potential sentinel specie for addressing impacts of solid pollution and a candidate for monitoring plastic contamination in reef environments.

The effects of microplastic exposure in aquatic organisms have been widely reported. Nonetheless, there is limited evidence of the effects of exposure in soil systems. Thus, the objective of this study was to evaluate the effects of microplastic exposure using as a bioindicator the species Lumbricus terrestris, exposed to different concentrations of microplastic (2.5%; 5%, and 7% w/w). Avoidance bioassays were carried out for 48 h in soil with and without microplastic; the gastrointestinal tract—crop/gizzard, foregut, and midgut—was dissected and acetylcholinesterase (AChE) was used as a biomarker of neurotoxicity stress. In parallel, bioassays of microplastic ingestion were carried out, and after 48 h of initiating the ingestion assay, using a stereo and fluorescence microscope, the microplastic distribution was observed in the different earthworm segments. The results obtained in the avoidance assay indicated a lack of preference for either soil type; however, upon moving, the earthworms lost surface mucus, resulting in burns and lesions on their bodies, which were reflected in the increase in AChE enzyme levels, which was not directly related to microplastic ingestion, but rather likely acts as an external physical stress agent. The results of the ingestion bioassay showed that microplastic was present in all the earthworm segments, with a higher number of particles in the hindgut. The Lumbricus terrestris did not distinguish microplastics from soil particles, and given the high exposure concentrations, microplastics produced physical lesions on the mucus membranes of earthworms. Lumbricus terrestris showed to be a suitable bioindicator for testing the exposure to microplastic contamination in soil.

Earthworms are commonly referred as environmental engineers and their guts are often compared with chemical reactors. However, modeling experiments to substantiate it are lacking. The aim of this study was to use established reactor models, particularly PFR, on the gut of the vermicomposting earthworm Eudrilus eugeniae to understand more on its digestion. To achieve the objective, a mathematical model based on first-order kinetics was framed and used to determine the pattern of digestion rates of nutrient indicators, namely total carbon (%), total nitrogen (%), C/N ratio, ¹³C (‰), and ¹⁵N (‰) at five intersections (pre-intestine, foregut, midgut A, midgut B, and hindgut) along the gut of E. eugeniae. The experimental results revealed that the concentrations of TC, TN, ¹³C, and ¹⁵N decreased during gut transit, whereas C/N ratio increased. The first-order model demonstrated that all the nutrients exhibit a linear pattern of digestion during gut transit, which supports the PFR model. On this basis, the present study concludes that the gut of E. eugeniae functions as PFR.

Vermi-composting is considered to be a feasible method for reducing tetracycline resistance genes (TRGs) in the sludge. Nevertheless, the way different gut passages of earthworm might affect the fates of TRGs and whether this process is affected by tetracycline (TC) concentrations need to be further investigated. In this study, we examined the effects of TC concentrations on changes in TRGs and bacterial communities in gut passages of earthworm were determined by using quantitative PCR and Illumina high-throughput sequencing. TRGs and intI1 were mainly reduced in the hindgut under the TC concentrations ranging from 0 to 25 mg/kg, while they were enriched under higher TC stress exposure. Consequently, we suggest the TC limitation of 25 mg/kg in the domestic sludge (DS) for vermi-composting. Although the predominant genera were TC sensitive under TC stress, many bacterial hosts harboring multiple TRGs (especially those in the hindgut) should be paid further attention to. In the foregut, five genera with abundant tetracycline-resistant bacteria (TRB) were specialized taxa. Among these genera, Unclassified_Solirubrobacterales and Pirellulaceae were probably related to the digestion processes. Other unclassified taxa related to the TRGs were probably derived from the DS. Five genera with abundant TRB were shared in the gut passages, and three specialized genera in the hindgut. These genera could spread TRGs and intI1 to the environment. These results suggest that vermi-composting is a feasible approach for TRG control in the DS containing TC concentration that does not exceed 25 mg/kg. Fates of TRGs and intI1 widely differ in the gut passages, showing inevitable connections with bacterial communities.