Environmental Engineering Reference
In-Depth Information
direct evidence of anaerobic ammonium oxidation, as discussed above. As
yet no comparisons of incubation-based and -independent anammox rates are
available, but such should be an important component of future investigations.
2.2 Membrane Lipids
Specific lipid biomarkers, so-called 'ladderane' lipids, were used to trace
anammox bacteria in particulate organic matter collected from various depths
across the suboxic zone of the Black Sea [35]. Ladderane lipids [62] are the
main building blocks of a unique bacterial membrane that surrounds the 'anam-
moxosome', a special compartment of the anammox cell, where the anaerobic
oxidation of ammonium to N
2
takes place (Fig. 1). During the anammox process
the intermediates hydrazine and hydroxylamine are formed, which readily dif-
fuse through conventional biomembranes [78]. The energy loss associated with
the loss of 10% hydrazine from the anammox cells would lead to a 50% decrease
in biomass yield [62]. Therefore, the limitation of diffusion is extremely impor-
tant for these bacteria. The biomembrane surrounding the anammoxosome is
more resistant to diffusion than conventional biomembranes due to dense and
rigid ladderane lipids. Therefore, the ladderane based biomembrane is prob-
ably a unique adaptation to the anammox metabolism. This is supported by
the fact that ladderane lipids were not found in phylogenetically related planc-
tomycetes that do not have the capacity to anaerobically oxidize ammonium
[62]. In addition to their unique structure, ladderane lipids are characterized
by a specific isotopic signature that can be used to trace these biomarkers in
the environment. Compound specific stable isotope analysis show that lipids
derived from anammox bacteria are depleted in
13
Cbyupto50‰ compared
to dissolved CO
2
[59]. This large isotopic offset may indicate that anammox
bacteria use the acetyl-CoA pathway for carbon fixation [59].
Figure 1.
Morphology of the anammox cell and proposed model for the anammox process;
HH: hydrazine (N
2
H
4
) hydrolase; HZO: hydrazine oxidizing enzyme; NR: nitrite reducing
enzyme.
