| Abstract: |
Oligotrophic Lake Superior and mesotrophic Lake Erie are trophic end members of the hydrologically connected Laurentian Great Lakes system, and as such exhibit different profiles of dissolved nitrogen species. Nitrification in Lake Superior has led to increasing nitrate concentrations over the past century, as opposed to Erie, where nitrate inventories have declined due to denitrification. In this study, we examined the abundance and diversity of nitrifying microbes involved in the oxidation of ammonia to nitrite, and nitrite to nitrate. By in situ hybridization methods, we enumerated the major planktonic ammonia oxidizing bacteria (AOB) and archaea (AOA) during a July 2011 cruise from Lake Superior to Lake Erie. In Lake Superior, AOA dominated compared to AOB, typically exceeding 5 × 103 mL− 1, whereas in Erie, AOB were more abundant than AOA. These data parallel prior work on Lake Superior and Lake Erie sediments, in which AOA are far more abundant in Superior, but AOB dominate in Erie. The lakes were sampled during stratification, and AOA and AOB were largely restricted to the hypolimnion, consistent with the observation that ammonia oxidizers are photoinhibited in surface waters. In Lakes Superior and Erie, we also detected nitrite oxidizing bacteria (NOB) in a pattern paralleling AOA/AOB abundance. Phylogenetic analysis of archaeal 16S rRNA revealed that the planktonic archaea of Lake Superior are members of the ammonia oxidizing Group I.1a Thaumarchaeota most closely related to Nitrosoarchaeum limnia. These AOA are distinct from the Group I.1a AOA in Lake Superior sediments. The major AOB of Lake Erie form a subcluster within the genus Nitrosospira.
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