Environmental Engineering Reference
In-Depth Information
reaction [40]. At least one phylotype retrieved from a 217 m water depth in
the Black Sea water column was closely related to
Desulfobacterium
species
[58]. Autotrophic CO
2
fixation was reported at least in aerobic methanotrophs
possessing ribulose-biphosphate carboxylase (
Methylococcus
species as repre-
sentative) [23]. Type II aerobic methanotrophs that belong to α-Proteobacteria
employ the serine pathway for formaldehyde assimilation, in which CO
2
is
assimilated too. These bacteria can potentially use the external pool of CO
2
and contribute to the measured fixation rates. Finally, autotrophic methanogens
which activity was detected (see discussion below) and related phylotypes re-
trieved [58] may also be responsible for CO
2
fixation rates measured well below
the oxic/anoxic interface. The simultaneous rate measurements of the processes
mentioned above and dark CO
2
fixation rates at and below the interface suggest
that the latter represent a mixed signal.
The isotopic composition of POC in the redox zone indicates that organic
matter production there is mostly due to CO
2
fixation, or chemosynthetic pro-
duction. POC had the carbon isotopic composition of -28.5 to -26.5‰ in
the redox zone at σ
t
=16.15-16.50 (100-250 m), where the maximum dark
CO
2
fixation rates were observed (Fig. 3) [14, 18]. Bacterial chemosynthesis
generally results in greater utilization of the lighter isotope and as a result,
chemosynthetically produced carbon has a more negative isotopic composi-
tion than carbon of photosynthetic origin [11]. Depleted in
13
C POC at the
oxic/anoxic interface originates from chemosynthetic bacteria. Below the zone
t
of
particulate organic carbon is probably due to the preferential consumption of
chemosynthetically produced bacterial biomass by anaerobic heterotrophs,
primarily sulfate-reducing bacteria [14].
We have obtained integral chemosynthetic production rates in the range be-
tween 9.6 and 17.3 mmol C m
−
2
day
−
1
in spring in the open sea [39]. Sorokin
et al. [53] estimated that the integral microbial production in the Black Sea
water column during summer and fall was in the range of 25.0-66.7 mmol C
m
−
2
day
−
1
and accounted for 50-80% of primary production. Chemosynthesis
was responsible for 85-90% of the total microbial production in the chemocline
and for 20-40% of total microbial production in the water column. Other data
show that chemosynthesis in the Black Sea usually accounts for 20% and in
some cases up to 50% of the total primary production [39]. These values are
surprisingly high; they however resemble those obtained in the Cariaco Basin
redoxcline, where chemoautotrophic production accounted on average for 70%
of the overall primary production [Taylor et al., this volume]. Since about 10%
of the organic carbon originated in the oxic zone from primary producers reach
the chemocline [48], an additional carbon source is needed to fuel such high
chemosynthetic production rate. The data suggest that chemosynthetic produc-
tion is driven by the CO
2
sources other than those originated in the photic zone
of active chemosynthesis (σ =16.3-16.5) the
“
lighter
”
isotopic composition
