Alteration of Na+ and K+ ion composition of microbial consortium isolated from oil reservoir at high salinities

The microbes being injected into the oil layers for the purpose of Microbial Enhanced Oil Recovery (MEOR) undergo the influence of extreme environment of oil reservoir like high salinity, high temperature and high pressure which can suppress their viability and production of the desired by-products such as gases, acids, surfactants and the others. To plan MEOR projects for implementation in the oil reservoirs in the North Sea fields, it needs to take into consideration that the salinity of the formation waters is high, varies in a wide range and depends on the closeness to the salt domes. For the most of bacteria salinity above 50 g/l would have an inhibiting effect. The limits of the optimal conditions for microbial propagation can be overcome through the adaptation processes the bacteria-MEOR candidates go through during laboratory studying. Alteration of the ionic composition is known as playing role in the adaption to salinity [1-3].
The change in the concentration of Na+ and K+ cations in the microbial solution exposed to higher concentrations of sodium chloride was under investigation. The experiment was conducted with the consortium of microbes isolated from the oil-saturated core sample extracted from the depth of 2 km from Dan field at several NaCl concentrations varying from 40 to 70 g/l. The samples of microbial solutions were taken during 11 days. The concentration of Na+ and K+ cations in the solution were measured using ICP/MS spectrometry, Optima 3000 DV.
The ion concentrations versus time as a function of salinity were plotted for sodium and potassium separately. In general, for both sodium and potassium ions the lowest concentrations were recorded for salinity of 40 g/l, the highest for 70 g/l with 50 g/l and 60 g/l in between. For Na+ concentration that can be explained solely by salt concentration, but for K+ that effect can be just due to the processes that occurred within microbes. Moreover, the K+-concentration significantly increases and Na+-concentration decreases after 8th day of experiment showing that K+ move out of the cell and Na+ move in. Negative correlation between pH change and K+-concentration was also observed.
[1] J. Chr. Skou. (1998). The identification of sodium pump. Nobel Lecture. Bioscience Reports, Vol.18, No.4.
[2] Apte S.K., Reddy B.R., Thomas J.(1987). Relationship between Sodium Influx and Salt Tolerance of Nitrogen-Fixing Cyanobacteria. Appl. Environ. Microbiology. 53(8):1934-1939.
[3] Sheldon Adler and Donald S. Fraley. (1977). Potassium and intracellular pH. Kidney International, Vol.11, pp.433-442.