Genetic Structure and Community DNA Similarity of Picoplankton Communities from the Laurentian Great Lakes

The similarity of picoplankton assemblages in the epilimnia (5 m) and mid-hypolimnia (45 to 70 m) of several Laurentian Great Lakes was compared by quantitative 16S rRNA-based and community DNA hybridizations to determine if different picoplankton communities develop during the summer in these water masses. Total nucleic acids were extracted and purified from picoplankton collected in Lakes Erie, Ontario, Huron, Michigan, and Superior during July and August 1992. Based on 16S Rrna hybridizations, the majority of picoplanktonic nucleic acids (91% to 98%) were from bacteria. Microeucaryotes accounted for less than 3% of the total nucleic acids. Up to 2% of the picoplanktonic nucleic acids were contributed by archaea. The genetic similarity of picoplankton communities was estimated by pair wise hybridization of heterogeneous DNA samples. This similarity is an estimate of the fraction of total DNA shared in common between two communities. Total picoplanktonic DNA in the epilimnia of all lakes except Lake Superior was similar during August (≥ 70% similarity). At most sites in these lakes, however, the community DNA of epilimnetic and hypolimnetic picoplankton was different. A high abundance of cyanobacterial nucleic acids in Lake Ontario's epilimnion may explain why the community DNA of epilimnetic and hypolimnetic picoplankton was different in this lake. Together, the results verify that bacteria account for the majority of picoplankton in these great lakes but also indicate that different picoplankton communities can form in the epilimnion and hypolimnion during summer after these lakes become thermally stratified.