Environmental Engineering Reference
In-Depth Information
ANME-2 groups of
Archaea
in association with δ-Proteobacteria are possible
candidates mediating anaerobic methane oxidation in the water column. How-
ever neither consortia present in marine sediments nor direct rates of anaerobic
methane oxidation in the water column are known. Literature data suggest a
presence of an abundant community of protozoa, mostly ciliates, in the Black
Sea redox zone. Many of these microorganisms harbor bacteria as ecto- or en-
dosymbionts. A protozoa-bacteria food web can play an important role in the
organic matter transfer between the oxic and anoxic waters and can also have a
significant impact on functional characteristics of bacterial communities.
Keywords:
Black Sea, oxic/anoxic interface, water column, chemosynthesis, microbial com-
munity, microbial activity
1. INTRODUCTION
The Black Sea has been known as a meromictic basin for more than a century
since the first Andrusov's expedition [2]. In the central part of the sea, the water
column is aerobic from the surface to the depth of 90-100 m; at the continental
slope, dissolved oxygen disappears deeper, at depths of 130-180 m. Hydrogen
sulfide appears below the oxic zone and its concentration gradually increases
to the bottom and approaches 370 µM at 2200 m water depth [48].
The contact zone between oxygen- and sulfide-containing waters in the
Black Sea is of special biogeochemical interest. This zone is characterized
by heterogeneous distributions of hydrochemical parameters. The zonation
is regulated by a change of the redox potential over depth. For a long time
dissolved oxygen was believed to be the main oxidant for hydrogen sulfide and
other reduced compounds diffusing up from the anaerobic water. The zone,
where oxygen and sulfide co-exist (so called, redox-zone or C-layer) in the
Black Sea was considered to be 10-30 m thick [3, 51, 53]. During a US-Turkish
expedition in 1988, the co-existence of oxygen and sulfide in the interface
between oxic and anoxic waters was challenged. Measurements demonstrated
that oxygen concentrations are in the range 5-7 µM and sulfide is below the
detection limit in this zone [16, 29]. These data were later confirmed with a
modified Winkler method [62] and with oxygen sensor measurements [54].
The results were of crucial importance for understanding the redox processes
at the oxic/anoxic interface and have initiated a number of studies to investigate
alternative electron acceptors to dissolved oxygen involved in sulfide oxidation.
Many chemists now believe that Mn(III,IV) oxyhydroxides and less important
Fe(III) oxyhydroxides are responsible for hydrogen sulfide oxidation [22, 45].
A model for cycling of suspended and dissolved Mn and Fe in the Black Sea
redox zone has been developed and attempts have been made to relate cycling of
these elements to cycling of other elements undergoing redox transformations
at the interface such as nitrogen (NH
4
+
,NO
2
−
,NO
3
−
) and sulfur (H
2
S, S
0
,
