Response Of The Gut Microbiota Of Tree Sparrow Nestlings To Environmental Heavy Metal Pollution

Environmental heavy metals enter into the animal body through food chain and have an extensive impact on immune function,growth,metabolism,reproduction and other aspects.The gut microbiota plays an important role in regulating host health and adaptation,but as toxic target of heavy metals,which is inevitably affected by heavy metals.Laboratory studies have shown that heavy metals can lead to the imbalance in the gut microbiota,and their effects on the gut microbiota depend on the type of metals,dose and time of exposure,and there are gender differences.However,heavy metals in the environment are trace complexes of many heavy metal elements,and little is known about its effects on the gut microbiota of wildlife.The nestlings stage is a critical period for the colonization and assembly of the gut microbiota in birds,and the changes of the gut microbiota in this stage may have a long-term impact on the health of nestlings.In this study,the tree sparrow(Passer montanus),a resident songbird,was selected as the study subject.Two study sites were selected.One is Silong Town,Baiyin(BY)City,Gansu Province,which had been polluted by heavy metals(Cd,Pb,Cu,Zn)for a long time,the other is the relatively unpolluted Liujiaxia(LJX)Town,Linxia City,Gansu Province.The level of heavy metals,body condition and characteristics of the gut microbiota of tree sparrow nestlings at the age of 6,9 and 12 days in the two sites were compared and the relationship between nestling gut microbiota and environmental heavy metal was analyzed,so as to explore the response strategy of tree sparrow population to environmental heavy metal pollution from the perspective of the gut microbiota.The main results are as follows:1.Accumulation of environmental heavy metals in the organs of tree sparrow nestlings.The concentrations of Cd,Pb and Cu in duodenum of tree sparrow nestlings in BY were significantly higher than that in LJX(P < 0.05),while the concentration of Zn was significantly lower than that in LJX(P < 0.05).The concentrations of Cd,Pb,Cu and Zn in liver of tree sparrow nestlings in BY were significantly higher than that in LJX(P < 0.05).The concentration of Pb in liver and duodenum of 12-day-old tree sparrow nestlings in BY was significantly negatively correlated(P < 0.05),while the concentration of Pb in duodenum was positively correlated with liver Zn concentration(P < 0.05).The results showed that heavy metals could accumulate in the digestive glands and ducts of tree sparrow nestlings,and there were significant differences in the types and accumulation sites of heavy metals.2.Effects of environmental heavy metals on body condition of tree sparrow nestlings.The body condition of 6,9 and 12-day-old tree sparrow nestlings in BY was significantly lower than that in LJX(P < 0.05).There was a significant negative correlation between the tarsus length and the concentration of Cd and Cu in the liver of tree sparrow nestlings in BY(P < 0.05).The duodenal villi of tree sparrow nestlings in BY were shorter and much messier,and no inflammatory infiltration was found.The concentrations of interleukin-2(IL-2)and tumor necrosis factor-α(TNF-α)in the duodenum of 12-day-old tree sparrow nestlings in BY were significantly higher than that in LJX(P < 0.05),and the concentrations of interleukin-6(IL-6)and TNF-α in 6-day-old nestlings in BY were significantly higher than that in LJX(P < 0.05).The results showed that heavy metal pollution could significantly reduce the body condition,increase the level of intestinal pro-inflammatory factors and damage of intestinal structural integrity of tree sparrow nestlings.3.Response of the gut microbiota composition to environmental heavy metal pollution in tree sparrow nestlings.Firmicutes and Proteobacteria were the dominant in the gut microbiota of tree sparrow nestlings,but the relative abundance of firmicutes increased significantly and Proteobacteria decreased significantly in BY(P < 0.05).Lactobacillus,Campylobacter and Clostridium were the dominant in tree sparrow nestlings,but the relative abundance of Lactobacillus was increased significantly in BY(P < 0.05).The gut microbiota of 12-day-old tree sparrow nestlings in BY was significantly related to heavy metals concentration,mainly including Aeromonadaceae,Ruminococcaceae and Pseudomonadaceae.In addition,the relative abundance of Bifidobacteriaceae of 12-day-old nestlings was significantly positively correlated with body condition in BY(P < 0.05).The results showed that the gut microbiota composition of tree sparrow nestlings was significantly changed under environmental heavy metal pollution,the risk of pathogenic bacteria infection was increased and the abundance of bacterial groups related to the toxic repair of heavy metals was also significantly increased.4.Response of the gut microbiota diversity to environmental heavy metal pollution in tree sparrow nestlings.The diversity index of the gut microbiota of tree sparrow nestlings in BY were significantly higher than that in LJX(P < 0.05),but there was no significant difference among 6,9 and 12-days old tree sparrow nestlings in two sites.The gut microbiota of tree sparrow nestlings in two sites were not well clustered together,but community structure was significantly different and the dispersion was higher in BY(P < 0.05).The diversity index of the gut microbiota of tree sparrow nestlings in BY area were correlated with the accumulation of Cd and Cu.The results showed that the gut microbiota diversity decreased and of individual differences increased of tree sparrow nestlings under environmental heavy metal pollution.5.Response of the gut microbiota function to environmental heavy metal pollution in tree sparrow nestlings.The abundance of staphylococcus aureus infection,biosynthesis of vancomycin group antibiotics,glutathione metabolism,base excision repair,phosphotransferase system and lipid metabolism of gut microbiota in tree sparrow nestlings were significantly up-regulated in BY(P < 0.05),while the gene functions such as bacterial chemotaxis,protein digestion and absorption were significantly down-regulated(P < 0.05).The gut microbiota functions of 6-day-old tree sparrow nestlings were mainly concentrated in lipid metabolism,while that of synthesis and metabolism of amino acids in 12-day-old tree sparrow nestlings increased significantly in two sites(P < 0.05).The results showed that the gut microbiota of tree sparrow nestlings had enhanced functions related to energy metabolism,cell damage repair,antioxidant and immunity under environmental heavy metal pollution.6.Response of the gut microbiota to environmental heavy metal pollution in tree sparrow nestlings of different genders.Relative abundance of Enterococcus and Mycoplasma in the gut microbiota of tree sparrow male nestlings in LJX were significantly lower than that of female nestlings(P < 0.05),while Mycoplasma and Escherichia/Shigella of male nestlings in BY were significantly lower than that of female nestlings(P < 0.05)and Lactobacillus was higher than that of female nestlings.The pathways of glycolipid metabolism and glycolysis/gluconeogenesis in male nestlings were significantly up-regulated than that that in female nestlings(P < 0.05),while the pathways of bacterial secretory system and lipopolysaccharide synthesis were significantly down-regulated(P < 0.05).The results showed that the gender difference of the gut microbiota of tree sparrow nestlings increased under environmental heavy metal pollution,and male nestlings were more susceptible to heavy metals.In conclusion,heavy metals can accumulate significantly in digestive glands and ducts of tree sparrow nestlings under environmental heavy metal pollution,which lead to the decrease of body condition,damage of intestinal structural integrity and increase of pro-inflammatory factors.In addition,the gut microbiota of tree sparrow nestlings was susceptible to environmental heavy metals,which showed decreased diversity,increased dispersity,increased the risk of pathogen infection,decreased nutrient digestion and absorption ability,and increased gender differences.However,the gut microbiota of tree sparrow nestlings also showed that positive responses to adapt to environmental heavy metal pollution,which are mainly reflected in the following aspects: increased abundance of bacterial groups associated with heavy metal toxic repair and host health regulation,and up-regulated bacterial metabolic function related to resistance to heavy metal toxic damage.This study can provide basic data for exploring the adaptation mechanism of the gut microbiota of tree sparrow population to environmental heavy metal pollution,and for wildlife health monitoring and conservation.