Ammonia oxidizers in the sea-surface microlayer of a coastal marine inlet

Planktonic archaea are thought to play an important role in ammonia oxidation in marine environments. Data on the distribution, abundance, and diversity of ammonia oxidizers in the coastal sea-surface microlayer (SML) are lacking, despite previous reports of high abundance of Thaumarchaeota in the SML of estuaries and freshwater lakes. Here, we failed to detect the presence of ammonia-oxidizing bacteria in any of our samples taken from a semi-enclosed marine inlet in Japan. Therefore, we shifted our focus to examine the archaeal community composition as well as the Thaumarchaeota marine group I (MG-I) and ammonia monooxygenase subunit A (amoA) gene copy numbers and composition in the SML and corresponding underlying water (UW, 20 cm). amoA gene copy numbers obtained by quantitative PCR were consistent with the typical values observed in the surface waters of oceanic and coastal environments where nitrification activity has been detected, but the copy numbers were two- to three-fold less than those reported from the surface layers and UW of high mountain lakes. Both amoA and MG-I 16S rRNA gene copy numbers were significantly negatively correlated with chlorophyll-a and transparent exopolymer particle concentrations in the SML. Communities of archaea and ammonia-oxidizing archaea in SML samples collected during low wind conditions (≤5 m s-1) differed the most from those in UW samples, whereas the communities in SML samples collected during high wind conditions were similar to the UW communities. In the SML, low ratios of amoA to MG-I 16S rRNA genes were observed, implying that most of the SML Thaumarchaeota lacked amoA. To our knowledge, our results provide the first comparison of ammonia-oxidizing communities in the coastal SML with those in the UW.