Environmental Engineering Reference
In-Depth Information
the Black Sea [81]. An anammox rate of 2-20 fmol ammonium cell
−
1
day
−
1
(1 fmol = 10
−
15
mol) was found in laboratory bioreactors [65]. Assuming a
similar range of cell specific activity for the Black Sea, 300-3000 anammox
cells ml
−
1
would be needed to account for the observed ammonium oxidation
rates in the suboxic zone. Counts of cells stained with the newly designed FISH
probe (Amxbs820) gave an anammox cell density of
800) cells ml
−
1
∼
1900 (
±
(0.75% of all cells counted by DAPI) at the nitrite peak.
The rates of net ammonium and nitrate consumption calculated as a function
of depth indicate that nitrate reduction by denitrifiers coupled to anammox
accounts for a substantial loss of fixed inorganic nitrogen. Assuming that the
area (3 * 10
5
km
2
) below the shelf break (
<
200 m) [64] represents the total
surface area of the suboxic zone, 0.3 Tg fixed inorganic nitrogen per year may
be lost through nitrate reduction coupled to anammox.
3.3 Oxygen Minimum Zones
Nutrient measurements indicate that 30-50% of the total nitrogen loss in
the ocean occurs in the oxygen deficient waters of oxygen minimum zones
[9, 25]. The extremely low concentration of ammonium could indicate that
anammox bacteria also play an important role in the nitrogen removal from
oxygen minimum zone (OMZ) waters (e.g. [11, 14, 80]).
Recently, evidence was provided for extensive anammox activity in the OMZ
waters of the Benguela upwelling system [34]. Upwelling of nutrient-rich South
Atlantic mid-waters in the Benguela current system along the southwest African
continental margin sustains some of the highest primary production rates in the
ocean [7, 8]. Although the upwelling water is generally well oxygenated (
>
200 µMO
2
), bottom waters become severely oxygen depleted (
<
10 µMO
2
)
over large areas of the southwest African shelf. This is the result of oxygen
consumption associated with the decomposition of settling algal biomass [8].
A strong N-deficit (a decrease in the ratio of fixed inorganic N to P; [74])
in the bottom waters has been attributed to denitrification [8, 74]. Water in
the Benguela OMZ is exchanged rapidly and concentrations of oxygen vary
significantly [8]. These conditions at first glance seem to be unfavorable for
anammox bacteria, with their slow growth rate and anaerobic physiology, are
able to thrive under such highly dynamic conditions.
During an R/V
Meteor
cruise in March/April 2003 the role of denitrifica-
tion and anammox in Namibian shelf waters was investigated by combining
microbiological and biogeochemical techniques. As observed previously [8],
nitrate concentrations drop at the base of the oxic zone (
<
10 µMO
2
). This
decrease in nitrate has previously been attributed to the conversion of nitrate
to N
2
by denitrifying bacteria [8, 74]. The N:P ratios for dissolved inorganic
nutrients
<<
Redfield indicate extensive loss of nitrogen from the oxygen de-
