Environmental Engineering Reference
In-Depth Information
80% loss of total S during the history of the lake [9]. Sulfate-depletion was most
likely a result of destabilization of meromixis, resulting in turnover of the water
column and subsequent venting of gaseous H
2
S, the product of bacterial sulfate
reduction, to the atmosphere [14]. There are several indications that these events
occurred in the ancient water column of Ace Lake: A diatom-inferred salinity
record showed that the salinity of the photic zone of Ace Lake was significantly
increased as a result of increased evaporation during deposition of sediments
between 10 and 30 cm [42]. Interestingly, in a parallel combined lipid and 18S
rDNA stratigraphy study on the Holocene sediments of Ace Lake, we found
that a haptophyte species initially recovered from sediment layers of Unit II,
re-colonized the photic zone during deposition of sediments between 15 and 21
cm (Unit I) [12]. Our carotenoid stratigraphy (Fig. 5d) provides evidence that
the increase in photic zone salinity caused at least one event of destabilization
and turn-over of the water column. For example, in the sediment layer between
17 and 19 cm, the chlorobactene concentration was 195 to 215 times lower
compared to adjacent sediment layers.
Biomarkers indicative of methanogens were also predominant within Unit
III, the oldest sediment layers studied. Two hydroxyarchaeol isomers were only
identified in Unit III (Fig. 5b; structures VIII, IX in Fig. 4). Both compounds
are known to occur only in methanogenic Archaea and, specifically, in the
orders Methanosarcinales and Methanococcales [28, 48]. In good agreement
with the observed shift in the archaeal community towards phylotypes with
99.5-100% sequence similarity with cultivated
Methanosarcina
species and
their predominance in Unit III [11], these Archaea most likely represented
methanogens, and were the sources of both hydroxyarchaeol isomers. Since
the hydroxyarchaeols and their sources were clearly restricted to the freshwater
sediment deposits of Unit III, it is likely that these phylotypes represented
remnants of archaeal species that thrived in the anoxic freshwater sediments at
the time of deposition [11]. Soon after sulfate-containing marine waters entered
Ace Lake, the hydroxyarchaeols as well as the sequences of methanogenic
Methanosarcina
species became undetectable and a shift towards archaeal
phylotypes related to uncultured Archaea from various marine settings occurred
[11]. Ace Lake was not likely to be sulfate-depleted during the marine phase.
Bacterial sulfate reduction to sulfide must have occurred soon after marine
waters entered Ace Lake since the occurrence of fossil carotenoids and a
sequence of the
Chlorobium
species indicated that at least part of the sulfide
was oxidized via anoxygenic photosynthesis.
5. CONCLUSIONS
The combined absence of carotenoids and 16S rDNA of Chlorobiaceae
and the presence of hydroxyarchaeols of Archaea related to methanogenic
