Planktonic archaeal diversity and ammonia-oxidizer abundance change with depth in Lakes Malawi, Kivu and Superior

Abstract

Planktonic Archaea may play a key role in the nitrogen cycle by oxidizing ammonia, but little is known about these microbes in large lakes of the world. Differences in the abundance of total Archaea, marine group I Archaea (MG-1), and ammonia-oxidizing Archaea (AOA) measured in 2010, 2011, and 2012 were compared during stratified conditions in Lake Malawi, a tropical African great lake, to previous work completed in Lake Superior and Lake Kivu. Total Archaea, MG-1, and AOA abundances increased by more than two orders of magnitude in Lake Malawi from the warm epilimnion to the oxic upper hypolimnion during thermally stratified conditions from November to January, but remained abundant in the deeper anoxic hypolimnion. 16S rRNA clones related to the Thaumarchaeota, possible ammonia oxidizers, and archaeal clones from previous work in Lake Victoria were present in both Lake Malawi and Kivu, and euryarchaeal clones were common in the deeper anoxic waters. The distribution and diversity of planktonic Archaea in this tropical great lake was similar to that in Lake Superior, a temperate great lake of comparable trophic status.While more detailed seasonal work about archaeal abundance and community diversty has been completed in the western basin of Lake Superior, spatial patterns of archaeal distribution have not been evaluated across this lake. Here, I compared the abundance of these 3 archaeal gene markers in the epilimnion and hypolimnion from seven sites across Lake Superior during the stratified period in 2009 and 2010. The abundance of the total Archaea, MG-1, and AOA was consistently lower in the epilimnion, and at least an order of magnitude higher in the hypolimnion at all sites. Although my aim was not to elucidate the causes of such distribution in Lake Superior, this study does provide additional evidence that planktonic Archaea are more abundant in colder waters of the hypolimnion and their abundance is restricted in the surface waters of thermally stratified lakes.