16s rRNA diversity of Mirror Lake in Gilindire Cave (Turkey) shows abundant Nitrospira

We present the prokaryotic microbial diversity of Mirror Lake, located at the end of Gilindire Cave (Turkey), whose geomorphology shows development in multiple geologic periods and by multiple mechanisms. The lake comprises brackish water with both fresh and seawater inputs. In total, 5 liters of water was sampled from Mirror Lake and was filtered through a 0.22 µm membrane, and after the DNA isolation, 16S amplicon sequencing was conducted to get whole prokaryotic diversity. The bacterial community of this system is predominately composed of nitrite-oxidizing Nitrospira with a relative abundance of 28 %. We hypothesize that Nitrospira recovered in our samples mediates nitrification by reciprocal feeding with ammonia-oxidizing archaea (Nitrososphaeria). We found Nitrospira had a close association with Planctomycetes CL500-3 clade and Marinimicrobia (SAR406) in the cave habitat, with a relative abundance of 8.3 % and 5.7 %, respectively. To our knowledge, this is the first time that the presence of marine clade SAR324 has been reported from brackish cave waters.