An intracellular pH gradient in the anammox bacterium Kuenenia stuttgartiensis as evaluated by 31P NMR

Abstract

The cytoplasm of anaerobic ammonium oxidizing (anammox) bacteria consists of three compartments separated by membranes. It has been suggested that a proton motive force may be generated over the membrane of the innermost compartment, the "anammoxosome". 31P nuclear magnetic resonance (NMR) spectroscopy was employed to investigate intracellular pH differences in the anammox bacterium Kuenenia stuttgartiensis. With in vivo NMR, spectra were recorded of active, highly concentrated suspensions of K. stuttgartiensis in a wide-bore NMR tube. At different external pH values, two stable and distinct phosphate peaks were apparent in the recorded spectra. These peaks were equivalent with pH values of 7.3 and 6.3 and suggested the presence of a proton motive force over an intracytoplasmic membrane in K. stuttgartiensis. This study provides for the second time--after discovery of acidocalcisome-like compartments in Agrobacterium tumefaciens--evidence for an intracytoplasmic pH gradient in a chemotrophic prokaryotic cell.

Fig. 1

Schematic representation of the different compartments in anammox bacteria (Fuerst 2005). The cytoplasm is divided into the paryphoplasm (the outer-most compartment), the riboplasm (where the ribosomes and chromosome are located), and anammoxosome (where most or all of the cytochromes c are present and where the catabolism is hypothesized to take place)

Fig. 2

The experimental setup for in vivo ³¹P NMR employing a high purity, well-mixed anammox culture consisting of fully suspended K. stuttgartiensis cells

Fig. 3

In vivo ³¹P spectra of resting K. stuttgartiensis cells under non-gassed conditions (I) and under dinitrogen gas sparging conditions (II–III). There is a shift in the main phosphate peak from 2.52 (pH 7.1 in I) to 3.1 ppm (pH 7.8 in II), while a shoulder appears at 2.84 ppm (pH 7.4 in II). A peak at 1.55 ppm (pH 6.4) is present throughout the experiment. Start of the active conversion of catabolic substrates ammonium and nitrite (IV–V) does not significantly change the pattern. Acquisition time = 1 h (6,700 spectra)

Fig. 4

pH values in the intracytoplasmic compartments of cells of K. stuttgartiensis as revealed by ³¹P NMR. Symbols represent the assigned pH for: diamonds extracellular medium, squares anammoxosome compartment, and triangles riboplasm compartment. At pH 6.9 and 7.4, the pH of the riboplasmic compartment was too close to the medium pH to a give a clearly independent peak/shoulder

Fig. 5

Full in vivo ³¹P spectra of K. stuttgartiensis converting ammonium nitrite. Acquisition time = 4 h (26,800 spectra), line width = 25 Hz. The three pH related peaks are 1.5 ppm (pH 6.3, assigned to the anammoxosome), 2.9 ppm (shoulder, pH 7.4, assigned to the riboplasm), and 3.1 ppm (pH 7.8, the extracellular pH). Four non-pH peaks are visible at −18.6, −10.8, −5.2, −0.8, 4.1, and 4.5 ppm, which can probably be assigned to β-ATP (−18.6), NAD/NADP (−10.8 ppm), γ-ATP (−5.2), phosphoenolpyruvate (PEP, −0.8 ppm). The peaks at 4.1 and 4.5 ppm are probably a phosphate diesther and a phosphate monoesther, respectively