Indigenous microbial communities in heavy oil show a threshold response to salinity

Abstract

Microbial degradation influences the quality of oil resources. The environmental factors that shape the composition of oil microbial communities are largely unknown because most samples from oil fields are impacted by anthropogenic oil production, perturbing the native ecosystem with exogenous fluids and microorganisms. We investigated the relationship between formation water geochemistry and microbial community composition in undisturbed oil samples. We isolated 43 microliter-sized water droplets naturally enclosed in the heavy oil of the Pitch Lake, Trinidad and Tobago. The water chemistry and microbial community composition within the same water droplet were determined by ion chromatography and 16S rRNA gene amplicon sequencing, respectively. The results revealed a high variability in ion concentrations and community composition between water droplets. Microbial community composition was mostly affected by chloride concentration, which ranged from freshwater to brackish-sea water. Remarkably, microbial communities did not respond gradually to increasing chloride concentration but showed a sudden change to less diverse and uneven communities when exceeding a chloride concentration of 57.3 mM. The results reveal a threshold-regulated response of microbial communities to salinity, offering new insights into the microbial ecology of oil reservoirs.

Imprints of ecological processes in the taxonomic core community: an analysis of naturally replicated microbial communities enclosed in oil

“…The fact that the relative abundances of the core taxa were variable may have resulted from variable selection due to small-scale environmental heterogeneity or due to biotic interactions. In a recent analysis of 43 water droplets from the Pitch Lake we found that 20.4% of the variability in community composition could be explained by local differences in the concentrations of chloride, sulfate, and potassium (Voskuhl et al 2021 ). The so far unexplained 79.6% of the variability may have been caused by local differences of other, unmeasured environmental variables.…”

“…The procedures for oil and water droplet sampling, DNA-extraction and 16S rRNA gene library preparation have been described previously (Pannekens et al 2020 , Voskuhl et al 2021 ). In short, oil was sampled from different locations on the Pitch Lake surface in Trinidad and Tobago in February 2017 and March 2018 and 2019 using sterile equipment, and shipped to Germany under nitrogen atmosphere.…”

“…The determination of absolute number of 16S rRNA gene abundances in the water droplet communities by quantitative PCR (qPCR) has been described by Voskuhl et al ( 2021 ), who applied the protocol of Takai and Horikoshi ( 2000 ) with minor modifications. In short, forward primer Uni340F (CCT ACG GGR BGC ASC AG), reverse primer Uni806R (GGA CTA CNN GGG TAT CTA AT) and the FAM-TAMRA-labelled Probe Uni516F (YCA GCM GCC GCG GTA AHA CVN RS) were used to amplify the V3-V4 region of the 16S rRNA genes of bacteria and archaea.…”

“…These water droplets are densely populated with metabolically active, anaerobic microorganisms and originate from the oil reservoir in the deep subsurface (Meckenstock et al. 2014 , Pannekens et al 2020 , 2021 , Voskuhl et al 2021 ). The microbial communities in the water droplets have exceptional ecological features, which make them interesting for addressing fundamental questions in ecology.…”

Natural and oil surface slicks as microbial habitats in marine systems: A mini review

“…However, at least according to older studies, gas exchange between water and atmosphere is not affected by oil slicks (Kinsey, 1973). Lowered salinity as caused by reduced evaporation might play an important role for microbial communities in oil slicks because microbial communities for instance in oil reservoirs have been observed to respond threshold-dependent to salinity, resulting in abrupt changes to less diverse and uneven communities (Shelton et al, 2016;Voskuhl et al, 2021).…”