Planktonic archaeal communities change seasonally in Lake Superior.

Abstract

Archaeal abundance and community structure were examined in Lake Superior during 2007 and 2008. When the lake water column was mixed there were approximately 1x106 archaeal 16S rRNA gene copies L-1. After stratification, archaeal 16S rDNA was more abundant below the chlorophyll maximum in the mid- and deep hypolimnion near the sediment but much less abundant in the epilimnion. T-RFLP fingerprints of archaeal communities from various depths were compared during both stratified and mixed lake conditions and with sediment samples collected in September 2007. Planktonic archaeal communities were very similar throughout the water column when the lake was mixed. Two discrete planktonic archaeal assemblages, however, were always present after the lake became thermally stratified. One assemblage was associated with warmer water above the thermocline and deep chlorophyll maximum, while the other assemblage was characteristic of the deep hypolimnion. The greatest difference in T-RFLP fingerprints was seen when sediment and planktonic archaeal communities were compared. Archaeal 16S rRNA gene sequences were obtained from both these assemblages. The composition of archaeal sequences shifted from a predominance of Crenarchaeota in the upper (30m) and mid (145m) water column to Euryarchaeota in the surface sediment. Though past studies have shown Archaea are present in lake ecosystems, many of these studies focused on sediment or were conducted in meromictic lakes where Archaea would be expected in the anoxic zone. My study focused on Archaea in the holomictic freshwater Lake Superior, where planktonic Archaea were present throughout the year but formed discrete communities after the water column became thermally stratified.