Activity and diversity of ammonia-oxidizing prokaryotes in a small seasonally ice-covered lake. Ammonia-oxidizing prokaryotes in Lake Croche

Temporal variability of both processing and community structure were investigate over an annual cycle in the surface layer of a temperate lake. Ammonia oxidation (AO) rates were measured directly using 15N-labelled ammonium (NH4+) addition at in situ temperature and light conditions. AO occurred throughout the entire year with a peak of activity during winter. AO rates were similar to previous measurements in marine systems characterized by poor NH4+ concentrations, ranging from undetectable to 333 nmol L-1 d-1. There was minimal day-night variation in AO rates and these were most influenced by increase in ambient NH4+ concentrations induced by vertical mixing. Both ammonia-oxidizing archaea (AOA) and bacteria (AOB) were detected when targeting the ammonia monooxygenase (amoA) gene in the photic zone of the lake, with AOA being omnipresent over the year, while AOB were observed predominantly in winter. The sequencing of archaeal amoA genes reveals that most of the AOA are members of the Nitrosotalea cluster, which confirms the ecological relevance of this cluster in oligotrophic freshwaters. Our results indicate that winter may be a hot-moment for AO in temperate lakes and shows, like in oceans and large lakes, that it is an ongoing process in sunlit surface waters.