Environmental Engineering Reference
In-Depth Information
in addition to vertical transport, lateral processes are an additional source of
nutrients in the redox zone. Dissolved oxygen as energetically most favorable
electron acceptor in addition to a vertical diffusion mechanism is probably
transported laterally as has been demonstrated for microbial Mn(II) oxida-
tion. Mn(Fe) oxyhydroxides are other possible candidate electron acceptors
that cause increased oxidation rates of hydrogen sulfide in the interface. Their
vertical flux however is not sufficient to account for the major part of sulfide
oxidation (e.g., [27]). Future research should concentrate more on lateral mech-
anisms that influence composition and production of microbial communities in
the redox zone.
Existing data show a diverse population of bacteria involved in sulfur cy-
cling, particularly in sulfide and sulfur intermediates (elemental sulfur, sulfite
and thiosulfate) oxidation. They are represented by autotrophic
Thiobacillus
and
Thiomicrospira
and heterotrophic bacteria related to
Rhizobiaceae
. Some
evidence suggests that some of these bacteria can use Mn(III,IV) oxyhydrox-
ides as electron acceptor in addition to oxygen and by doing so provide a link
between sulfur and manganese cycling. Marine aggregates with encrusted Mn
oxyhydroxides provide high nutrient-rich niches for bacteria and may serve
as an active transport shuttle between the oxic and anoxic zones as has been
demonstrated in the redox zone of the Gotland Deep, Baltic Sea [32]. Sulfide
oxidation via anoxygenic photosynthesis mediated by green sulfur bacteria
Chlorobium
usually plays a minor role, but favorable light conditions occur-
ring in summer in the central Black Sea may cause their increased activity.
Recently discovered in the Black Sea chemocline 'anammox' bacteria oxidiz-
ing ammonium with nitrite/nitrate may play in important role in the total loss
of inorganic nitrogen from nutrient-rich anoxic waters. Their relationship with
nitrifying and denitrifying bacteria and specific role in the nitrogen cycling of
the redox zone deserves further studies. Maximum activity of sulfate reducing
bacteria is observed below the zone of active chemosynthesis. Sulfate reduc-
tion may be enhanced due to the supply of labile organic matter primarily of
chemosynthetic origin; however sulfate-reducers occur throughout the entire
water column and are the main source of hydrogen sulfide in the basin. The
most intriguing questions of the Black Sea microbial ecology are related to
bacteria of the methane cycling. Rate measurements of their activities using la-
beled compounds provided evidence that both processes of methane generation
and consumption occur simultaneously at the same depth in the upper anoxic
column and both are often measurable in the lower anoxic zone [41, Ivanov and
Lein, this volume]. Strictly aerobic methanotrophs isolated from the Black Sea
chemocline may be responsible for CH
4
oxidation rates measured at or several
meters below the upper anoxic boundary, where oxygen flux can be supported
by lateral diffusion/advection processes. Methane oxidation rates in the deeper
anoxic layers are probably related to anaerobic processes mediated by ANME-1
