Short-Term Exposure to Coal Combustion Waste Has Little Impact on the Skin Microbiome of Adult Spring Peepers (Pseudacris crucifer)

Abstract

Disruptions to the microbiome can impact host health as can exposure to environmental contaminants. However, few studies have addressed how environmental contaminants impact the microbiome. We explored this question for frogs that breed in wetlands contaminated with fly ash, a by-product of coal combustion that is enriched in trace elements. We found differences in the bacterial communities among a fly ash-contaminated site and several reference wetlands. We then experimentally assessed the impacts of fly ash on the skin microbiome of adult spring peepers (Pseudacris crucifer). Frogs were exposed to fly ash in the laboratory for 12 h, the duration of a typical breeding event, and the skin microbiome was assessed after 5 days (experiment 1) or after 5 and 15 days (experiment 2). We examined bacterial community structure using 16S rRNA gene amplicon sequencing and metabolite profiles using high-pressure liquid chromatography-mass spectrometry (HPLC-MS). We found little impact as the result of acute exposure to fly ash on the bacterial communities or metabolite profiles in either experiment, suggesting that the bacterial symbiont communities of adults may be relatively resistant to brief contaminant exposure. However, housing frogs in the laboratory altered bacterial community structure in the two experiments, which supports prior research suggesting that environmental source pools are important for maintaining the amphibian skin microbiome. Therefore, for contaminants like fly ash that may alter the potential source pool of symbionts, we think it may be important to explore how contaminants affect the initial assembly of the amphibian skin microbiome in larval amphibians that develop within contaminated sites. IMPORTANCEAnimals are hosts to many symbiotic microorganisms, collectively called the microbiome, that play critical roles in host health. Therefore, environmental contaminants that alter the microbiome may impact hosts. Some of the most widespread contaminants, produced worldwide, are derived from the mining, storage, and combustion of coal for energy. Fly ash, for example, is a by-product of coal combustion. It contains compounds such as arsenic, selenium, cadmium, and strontium and is a recognized source of ground and surface water contamination. Here, we experimentally investigated the impacts of short-term fly ash exposure on the skin microbiome of spring peepers, one of many species of amphibian that sometimes breed in open fly ash disposal ponds. This research provides a look into the potential impacts of fly ash on an animal's microbiome and suggests important future directions for research on the effects of environmental contaminants on the microbiome. Vertebrates host a wide array of symbiotic microorganisms, mainly in the gut, but there are unique microbial communities spread throughout the body from the nasal cavity to the lungs to the skin (1). While we have long understood that the microbiota residing in the gut are diverse and are important in helping to digest food, only recently have...

Environmental and Host Effects on Skin Bacterial Community Composition in Panamanian Frogs

“…The redundancy that we found in microbial functional groups, whereby similar functions may be attributed to more than one bacterial OTU, may corroborate with evidence of an amphibian skin functional core ( Hamady and Knight, 2009 ; Human Microbiome Project Consortium, 2012 ; Davis et al, 2017 ). From an evolutionary ecology perspective, such redundancy of microbial functional groups may be expected to be beneficial to the host in the face of potentially rapid microbial turnover ( Loudon et al, 2014 ; Hughey et al, 2016 ), as an essential metabolic function, if carried out by only a single OTU could easily be lost, to the detriment of the host’s fitness. Such redundancy of ecologically relevant microbial functional groups has also been observed in soil ( Wellington et al, 2003 ).…”

Culture Media and Individual Hosts Affect the Recovery of Culturable Bacterial Diversity from Amphibian Skin

“…The higher cutoff value 0.01% reduces the number of OTUs in the dataset, which could result in a higher overlap among cultured and culture-independent samples in terms of the OTU composition. We think a cutoff of 0.01%, which was determined in the present study as the cutoff level at which OTU richness leveled off in the dataset (per Bokulich et al, 2012 ), potentially represents a more realistic view of the actual bacterial species present in the community (Bokulich et al, 2012 ; Hughey et al, 2016 ).…”

“…Each OTU was represented by the most abundant sequence clustered within it, which was aligned to the Greengenes 13_8 reference database (DeSantis et al, 2006 ) using PyNAST (Caporaso et al, 2010a ) and assigned taxonomy using the RDP classifier (Wang et al, 2007 ). OTUs assigned to chloroplast or mitochondria were removed, and then OTUs with fewer than 0.01% (524 sequences) of the total reads were removed (Bokulich et al, 2012 ; Hughey et al, 2016 ). The sequencing depth per sample ranged from 3,991 to 164,519, including the two controls, which, as expected, had the lowest read counts at 3,991 and 8,299 reads.…”

Pesticides Could Alter Amphibian Skin Microbiomes and the Effects of Batrachochytrium dendrobatidis

“…Environmental pollutants can also directly impact soil and aquatic environmental microbial communities (Lupwayi et al, 2009 ; Muñoz-Leoz et al, 2013 ; Karimi et al, 2017 ), which changes the microbial species pool available to colonize amphibian skin microbiomes. For example, microbial community richness and phylogenetic diversity were lowest at a coal ash contaminated site compared to reference sites (Hughey et al, 2016 ). Although, the skin microbiomes of the frogs from these sites were not compared, it is known that the microbial species pool in the environment are important for maintaining a diverse skin bacterial community (Loudon et al, 2014 ).…”

“…It is possible that the coal ash-induced changes in the environmental microbial pool could alter the resident amphibian skin microbiome leaving them more susceptible to pathogens. However, a brief 12 h exposure of adult spring peepers ( Pseudacris crucifer ) to coal ash, which mimics a single night's breeding event, did not induce noticeable changes in skin microbiota (Hughey et al, 2016 ). The effects of chronic exposure to coal ash, or exposure at earlier life stages on the structure and function of the adult microbiome are still unknown.…”