Microbial Degradation Rates of Natural Bitumen

Abstract

Microorganisms are present in nearly every oil or bitumen sample originating from temperate reservoirs. Nevertheless, it is very difficult to obtain reliable estimates about microbial processes taking place in deep reservoirs, since metabolic rates are rather low and differ strongly during artificially cultivation. Here, we demonstrate the importance and impact of microorganisms entrapped in microscale water droplets for the overall biodegradation process in bitumen. To this end, we measured degradation rates of heavily biodegraded bitumen from the Pitch Lake (Trinidad and Tobago) using the novel technique of reverse stable isotope labeling, allowing precise measurements of comparatively low mineralization rates in the ng range in microcosms under close to natural conditions. Freshly taken bitumen samples were overlain with artificial brackish water and incubated for 945 days. Additionally, three-dimensional distribution of water droplets in bitumen was studied with computed tomography, revealing a water bitumen interface of 1134 cm 2 per liter bitumen, resulting in an average mineralization rate of 9.4−38.6 mmol CO 2 per liter bitumen and year. Furthermore, a stable and biofilm-forming microbial community established on the bitumen itself, mainly composed of fermenting and sulfate-reducing bacteria. Our results suggest that small water inclusions inside the bitumen substantially increase the bitumen−water interface and might have a major impact on the overall oil degradation process.

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

“…For the inferences to be valid, it is necessary to have an approximate idea about the age of the water droplets in terms of number of microbial generations. Pannekens et al ( 2021 ) used anaerobic microcosms filled with equal amounts of Pitch Lake oil and artificial medium mimicking the chemical composition of the droplet water to measure growth and respiration rates over three years. In these experiments, microorganisms from the water droplets were able to enter the water phase from the oil because the water droplets, which are lighter than the heavy oil, rose to the surface of the oil phase where they came in contact with the overlaying water.…”

“…where Δρ is the difference between oil density, ρ o , and water density, ρ w , g is the gravitational acceleration, d is the initial diameter of the droplet, and µ o is the viscosity of the oil. Two different cases were studied, namely (i) with uniform initial droplet sizes, and (ii), with distributed initial droplet sizes, whereby an exponential size distribution was assumed based on the radiographic measurements of droplet sizes (Pannekens et al 2021 ). Simulations were carried out for a total water content of 0.1% (v/v), which was experimentally determined.…”

“…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.…”

Assessing anaerobic microbial degradation rates of crude light oil with reverse stable isotope labelling and community analysis

“…Hence, microbial activities and rates resulting in hydrocarbon degradation in the subsurface remain still unknown. So far, there have been only a few studies that aimed at modeling in situ oil degradation rates at the oil water transition zone (Head et al, 2003;Larter et al, 2003) or directly determining the rate within a heavily degraded bitumen reservoir (Golby et al, 2012;Pannekens et al, 2021). Oil degradation rates certainly depend on several factors such as the reservoir temperature, oil column height, oil gravity (API) and chemical composition, diffusion coefficients in the porous media and maybe most importantly the oil-water distribution and interface area (Larter et al, 2006;Pannekens et al, 2019).…”

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

“… 2014 ; Pannekens et al . 2020 , 2021 ). To investigate the relationship between environmental conditions and microbial community composition we analysed 43 water droplets that we extracted from oil that we sampled at six different sites on the Pitch Lake.…”