Environmental Engineering Reference
In-Depth Information
colonized the ancient water column, their 16S rDNA must be well preserved
in the fossil sediments. During recent years, numerous reports have dealt with
the successful retrieval of ancient DNA sequences from remains sequestered
within aquatic sedimentary records including human skeletons [24], seagrass
[39] and resting eggs of
Daphnia
(e.g. [30, 40]). In addition, remnants of fossil
prokaryotes in ancient aquatic sediments were reported and the organisms were
identified based on 16S rDNA analysis [10, 26, 27]. Most likely, these organ-
isms were adapted to live within the sedimentary record for a substantial period
of time. Other reports have dealt with ancient rDNA within Holocene lake sed-
iments derived from Gram-negative obligate anoxygenic photosynthetic purple
sulfur bacteria [13] as well as oxygenic photosynthetic algae [12]. The latter
reports showed that ancient DNA derived from microorganisms from the photic
zone could also survive in the sediments provided that preservation conditions
were optimal. Optimal conditions for the preservation of DNA such as low
temperatures and anoxic, sulfidic conditions prevail in the bottom waters and
sediments of Ace Lake [12].
During the austral summer of 2000, we collected particulate organic matter
(POM) from various positions in the oxygenated water column as well as the
underlying anoxic, sulfidic waters. In addition, we obtained a 150-cm-long
sediment core from the anoxic depocentre of Ace Lake, spanning the final
10450 calendar years of deposition. The oldest sediment layers were deposited
during the freshwater lacustrine period, followed by the fjord period, and then
the subsequent isolation of Ace Lake.
Our main objectives were to study the impact of climate-induced changes
of the physical and chemical conditions of the ancient water column of Ace
Lake on the presence and diversity of species involved in anaerobic processes
during the Holocene. In this paper, the key-organisms involved in anoxygenic
photosynthesis [green sulfur bacteria (GSB) of the family Chlorobiaceae] were
identified based on a combined, high-resolution stratigraphic analysis of tradi-
tional lipid biomarkers (carotenoids) as well as 16S rDNA as a novel palaeo-
proxy. Since GSB of the family Chlorobiaceae are obligate anoxygenic pho-
tolithotrophic bacteria [36] they are restricted to the chemocline where both
light and sulfide are present. The brown-coloured species such as
Chlorobium
phaeovibrioides
and
Chlorobium phaeobacteroides
often have green coloured
counterparts such as
C. limicola
and
C. vibrioforme
that, apart from pigment
and carotenoid composition, are very similar in morphology and physiology
[52] and can be closely phylogenetically related (i.e. similarity values
>
90%)
[35]. Chlorobactene is the predominant carotenoid in the green-coloured speci-
es, whereas brown-coloured Chlorobiaceae often contain small amounts of
chlorobactene in addition to isorenieratene and β-isorenieratene [52].
Below the chemocline, light for photosynthesis is absent [6] and cells of
Chlorobiaceae below the chemocline were assumed to be decaying. The ratio
