Lake Pavin History, geology, biogeochemistry, and sedimentology of a deep meromictic maar lake

250

cell counts (Herfort et al. 2009 ) and frequently retrieved in
lakes (Restrepo-Ortiz et al. 2014 ). Due to lack of cultivated
representatives, it is currently diffi cult to link the LDS clus-
ter with its ecological role and we do not know its metabolic
capacities. In the same way, DHVEG-6 was a largely
unknown archaeal group originally described as a hydrother-
mal vent lineage (Takai and Horikoshi 1999 ) but that was
also detected from relatively anoxic terrestrial soils (Takai
et al. 2001 ) to marine sediments (Teske and Sørensen 2008 )
and water column (Hugoni et al. 2013a , b ).
The approach using both 16S rRNA genes and 16S rRNA
also highlighted that groups that are not necessarily the most
abundant in freshwater ecosystems may have a potential
activity. In the oxycline abundant archaeal major groups
were similar from the epilimnion and affi liated with
Thaumarchaeota Marine Group I (56.4 % of the reads), fol-
lowed by Euryarchaeota LDS cluster, DHVEG-6 (26 and
7.4 % of the reads respectively). Nevertheless, in the oxy-

cline of Lake Pavin, the Miscellaneous Euryarchaeotic

Group (MEG) dominated the potentially active archaeal

assemblage throughout the entire year, suggesting that this

poorly-characterized group could play a key functional role

in this aquatic ecosystem (Fig. 15.2b ). This group was ini-

tially retrieved in terrestrial soil, marine sediments (Takai

et al. 2001 ) and in deep subsurfaces (Hirayama et al. 2007 )

but recent studies indicated its presence in some Spanish

lakes (Auguet et al. 2011 ) suggesting the existence of a

freshwater clade.

15.3.3 Thaumarchaeota Marine Group

I Versatility

In the oxycline, Thaumarchaeota Marine Group I repre-

sented 10.2 % of the potentially active archaeal fraction.

Overall, thaumarchaeal amoA transcripts abundance in both

16S rRNA

genes

Epilimnion Oxycline

16S rRNA

Epilimnion Oxycline

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

16S rRNA

genes

16S rRNA 16S rRNA

genes

16S rRNA

Thaum_Other groups

Thaum_MGI

Eury_Methanogenic lineages

Eury_RC-V

Eury_LDS Cluster

Eury_MEG

Eury_DHVEG-6

Eury_Other groups

Crenarchaeota

a

b

Fig. 15.2 Taxonomic
compostion of the archaeal
assemblage present in Lake
Pavin ( a ) and focus on the
different groups retrieved
among Crenarchaeota ,
Euryarchaeota (Eury) and
Thaumarchaeota (Thaum)
( b ). Thaum_Other groups
included SAGMCG-1 and
SCG, Eury_Other groups
included DSEG, Sm1K20,
MHVG, SAGMEG, TMEG,
MBGD and DHVEG-1,
Crenarchaeota included C3,
MBGB, MCG, Sulfolobus
and Terrestrial Group.
Pyrosequencing amplicons
were produced from the V4
to V5 region of the 16S
rRNA genes and 16S rRNA
using the archaeal primers
Arch519F (Herfort et al.
2009 ) and Arch915R
(Casamayor et al. 2002 ).
Pyrosequencing was
performed using a Roche
454 GS-FLX system with
titanium chemistry

G. Bronner et al.