High archaeal richness in the water column of a freshwater sulfurous karstic lake along an interannual study.

We surveyed the archaeal assemblage in a stratified sulfurous lake (Lake Vilar, Banyoles, Spain) over 5 consecutive years to detect potential seasonal and interannual trends in the free-living planktonic Archaea composition. The combination of different primer pairs and nested PCR steps revealed an unexpectedly rich archaeal community. Overall, 140 samples were analyzed, yielding 169 different 16S rRNA gene sequences spread over 14 Crenarchaeota (109 sequences) and six Euryarchaeota phylogenetic clusters. Most of the Crenarchaeota (98% of the total crenarchaeotal sequences) affiliated within the Miscellaneous Crenarchaeota Group (MCG) and were related to both marine and freshwater phylotypes. Euryarchaeota mainly grouped within the Deep Hydrothermal Vent Euryarchaeota (DHVE) cluster (80% of the euryarchaeotal sequences) and the remaining 20% distributed into three less abundant taxa, most of them composed of soil and sediment clones. The largest fraction of phylotypes from the two archaeal kingdoms (79% of the Crenarchaeota and 54% of the Euryarchaeota) was retrieved from the anoxic hypolimnion, indicating that these cold and sulfide-rich waters constitute an unexplored source of archaeal richness. The taxon rank-frequency distribution showed two abundant taxa (MCG and DHVE) that persisted in the water column through seasons, plus several rare ones that were only detected occasionally. Differences in richness distribution and seasonality were observed, but no clear correlations were obtained when multivariate statistical analyses were carried out.