Environmental Engineering Reference
In-Depth Information
interface between oxic and anoxic waters [10, 16, 39, 51, 53]. The presence
of the CO
2
fixation maximum in the chemocline is a typical feature of other
freshwater and marine euxinic environments such as the Cariaco Basin [Taylor
et al., this volume] and Framvaren Fjord [63]. In the Black Sea several peaks
of dark CO
2
-fixation rates were observed in the density range of σ
t
=16.20-
16.45. The upper peak usually corresponded to the depth of sulfide appearance,
while lower peaks occurred 10-50 m below (Fig. 2). The dark carbon dioxide
fixation rate is an ambiguous indicator for the autotrophic bacterial production.
Heterotrophic microorganisms are also able to fix up to 10% of CO
2
for the
biomass synthesis [44]. The problem of distinguishing between autotrophic
and heterotrophic fixation rates using existing radioisotopic method remains
obscure. Given unspecific reaction of commonly used inhibitors such as N-
serve, chlorate, picolinic acid, and aside their application in mixed microbial
communities provides unreliable results. They can not be used to distinguish
carbon dioxide fixation from different groups of microorganisms [34, 38].
Figure 2.
Combined depth profiles of dark
14
CO
2
fixation rates (CO
2
fix) and bacterial
cell numbers (DAPI) at two open-sea stations in May 1998 (panel A: Station 5; 43˚15.22'N,
33˚59.92'E, depth 2172 m; panel B - Station 6; 43˚59.91'N, 37˚30.06'E, depth 2123 m).
There is a range of microorganisms, which may potentially be responsi-
ble for observed peaks of CO
2
fixation rates below the oxic/anoxic interface.
Most probable candidates are sulfate-reducing bacteria, which can fix CO
2
dur-
ing autotrophic growth with hydrogen such as those related to
Desulfovibrio
,
Desulfobacterium
,
Desulfobacter
,
Desulfosarcina
species or those using exter-
nal CO
2
as an additional to organic carbon source by the pyruvate synthase
