Environmental Engineering Reference
In-Depth Information
and resulted in the identification of fossil biological sources of alkenones at the
species-level [12]. Alkenones are not as sensitive for diagenetic alteration as
carotenoids. Due to the diagenesis of chlorobactene, the stratigraphic profiles
of chlorobactene or DNA of AL-GSB 1 were not significantly correlated (Figs.
5d, 5e). The correlation would most likely improve if all diagenetic products
of chlorobactene could also be identified and quantified. Next to hydrogenated
forms, also sulfurized diagenetic products of chlorobactene are likely to occur
in the sediments of Ace Lake.
4.2 Anoxygenic Photosynthesis in Ace Lake During the
Holocene
The coinciding presence of carotenoids and 16S rDNA of a
Chlorobium
species in 9400-year-old and shallower sediments showed that Ace Lake be-
came stratified and sulfidic within ca. 100 years after marine waters first entered
Ace Lake. Moreover, an active cycling of sulfur between anaerobic sulfate re-
ducing bacteria and sulfide oxidizing GSB must have occurred during most of
the lake's history.
During deposition of Unit III when Ace Lake was a freshwater-filled melt
water lake, conditions for anoxygenic photolithotrophic growth such as an
anoxic, sulfidic chemocline were most likely absent. Carotenoids as well as
16S rDNA specific for Chlorobiaceae were indeed below the detection limit
in Unit III sediments. Phylotype AL-GSB 2 (Figs. 6, 7) which was detected
from Unit III by means of the GSB-specific PCR approach, was not affiliated
with anoxygenic photosynthetic Chlorobiaceae but to a cluster of 'unclassi-
fied GSB'. Sequences related to AL-GSB 2 were, for example, recovered from
nearby meromictic, sulfidic Pendant Lake [4] as well as the hypersaline Organic
Lake (Coolen, unpublished data) with anoxic, but non-sulfidic bottom waters
[16, 22]. The affiliation of AL-GSB 2 with the sequence of the non-sulfidic hy-
persaline Organic Lake points towards a physiology different from anoxygenic
photosynthesis.
4.3 Evolution of the Holocene Methane Cycle in Relation
to the Cycling of Sulfur
To date, two methanogenic Archaea have been isolated from the extant
monimolimnion of Ace Lake [17, 20] and low rates of methanogenesis us-
ing radiolabelled bicarbonate were measured below a water depth of 20 m
[18].
Methanococcoides burtonii
is a methylotrophic methanogen with an
optimum growth temperature of 23
◦
C [20]. Since this organism catabolizes
non-competitive substrates, its presence should not be restricted to sulfate-
depleted regions within the anoxic monimolimnion. However, methanogene-
sis does not occur in other non-sulfate-depleted, anoxic lake monimolimnia,
