Environmental Engineering Reference
In-Depth Information
trophic bacteria [36] and their ecological niche is therefore restricted to just
below the oxic-anoxic interface where both light and sulfide are present. Since
no light penetrates the bacterial plate in the chemocline of Ace Lake [6],
their biomarkers (carotenoids and 16S rDNA) in the sedimentary record are
therefore expected to be of fossil origin and serve as indicators for past anoxic
conditions.
The phylogenetic affiliation of phylotype AL-GSB1 to a brown species (
Chl-
orobium phaeovibrioides
) (Fig. 7) would seem to imply that it was the fossil
source of isorenieratene and β-isorenieratene and not the source of chloro-
bactene. However, the comparable concentration profiles of the predominant
carotenoid chlorobactene with isorenieratene plus β-isorenieratene in both the
water column and in the sediment (Figs. 3a and 5d) implies the same GSB-
source for the three carotenoids. Within the upper 25 cm of Ace Lake sediments,
these carotenoids were found to be 10
15
enriched in
13
C compared to phy-
toplankton biomarkers, indicating that GSB are their biological source [46].
Although we identified only AL-GSB 1 using the sensitive and selective PCR
approach, two green-coloured species of Chlorobiaceae,
C. vibrioforme
and
C. limicola
(for which no 16S rDNA sequences are available) were enriched
from the chemocline of Ace Lake in 1988 [7]. In order to see whether addi-
tional GSB-phylotypes were present in the extant chemocline and Holocene
sediments, we identified the predominant bacterial 16S rDNA by PCR/DGGE
using non-selective primers for the domain Bacteria (data not shown). Also
using this non-selective PCR/DGGE approach, only one phylotype with
C.
phaeovibrioides
DSMZ 269T as its closest relative was detected from the wa-
ter column and the Holocene sediment layers. This indicated that if additional
phylotypes of GSB would have been present, they comprised less than 1% of
the total bacterial community [33]. Therefore, AL-GSB 1 was the predomi-
nant
Chlorobium
phylotype during the history of Ace Lake and the most likely
source of the predominant carotenoid chlorobactene as well as the less abundant
carotenoids isorenieratene and β-isorenieratene.
The highest concentration of DNA of GSB (600 ng.L
−
1
filtered water) was
measured at the chemocline (Fig. 3b), as were the highest concentration of
chlorobactene. Whereas chlorobactene concentrations varied only by a factor
of two with increasing water depth (Fig. 3a), the concentration of GSB DNA
declined significantly with depth. As a result, the ratio between DNA and the
carotenoid chlorobactene within the cells of GSB at a depth of 18 m, where light
for anoxygenic photosynthesis was absent, was 2 orders of magnitude lower
compared to the cells thriving within the photic, sulfidic chemocline (Fig. 3c).
This showed that a substantial part of the DNA was degraded in dead cells set-
tling towards the sediment. The DNA:chlorobactene ratio measured from the
deeper part of the water column (10
−
5
) did not further decline in the Holocene
sediments (Fig. 5f). This indicated that the DNA which became buried within
−
