Planktonic Archaeal Communities Linked to Ammonia Oxidation in Lake Superior

Molecular information about enzymes produced by planktonic microbes can aid our understanding of in-lake processes like nitrogen cycling. Archaeal community structure was examined in Lake Superior during 2007 and 2008. Two discrete clusters of archaeal assemblages were present under stratified conditions but only one cluster was evident during mixed conditions. The composition of the surface water assemblage changed, but the deeper assemblage remained unchanged in the hypolimnion. Analysis of 16S rDNA clones indicated many non-thermophilic crenarchaea were present and similar to Nitrosopumilus maritimus strain SCM1, a marine crenarchaeal ammonia oxidizer. After stratification, there were more copies of the archaeal 16S rDNA gene and a putative archaeal ammonia monooxygenase (amoA) gene below the deep chlorophyll maximum than in the epilimnion. The archaeal amoA gene became more abundant relative to the archaeal 16S rDNA gene after the water column stratified. The nitrification rate peaked in the hypolimnion below the deep chlorophyll maximum during stratified conditions, where more archaeal and bacterial amoA gene copies were found. Some archaeal members of these planktonic microbial communities may contribute to nitrification in the water column.