Abstract
Organotrophic denitrification is an important nitrogen (N) removal process in lakes, but alternative N reduction processes such as lithotrophic sulfur (S)‐oxidizing denitrification may be greatly underappreciated. We studied the redox transition zone (RTZ) in the meromictic water column of the North Basin of Lake Lugano (Switzerland) to characterize N transformation pathways coupled to the S and carbon (C) cycles. Incubations with 15 N‐labeled and unlabeled nitrate (NO 3 − ) revealed low denitrification rates and a general limitation of organic electron donors. The most accessible fractions of exported primary production biomass may have been largely consumed in the oxic water column during sedimentation and did not reach the RTZ at ~ 100 m depth. Conversely, sulfide (H 2 S) and methane (CH 4 ), major end products of anaerobic degradation of the more recalcitrant organic carbon fractions in the sediment, represent a continuous source of energy to the RTZ, fostering the establishment of a community of S‐ and CH 4 ‐dependent NO 3 − reducers, dominated by Sulfuritalea and Candidatus Methylomirabilis over several years of observation. Anoxic incubation experiments with H 2 S amendments revealed a strong stimulation of dissimilatory NO 3 − reduction to ammonium (NH 4 + ) (DNRA), but not denitrification. High relative abundances of the archaeal NH 4 + oxidizer Candidatus Nitrosopumilus and bacterial nitrifiers indicate intense NO 3 − regeneration by nitrification in the upper RTZ. The potential interaction between nitrification and S‐driven DNRA is unclear. However, their co‐occurrence suggests that, at least under conditions of carbon limitation, N recycling between the NO 3 − and ammonium pools predominates over N removal via complete denitrification.