作者：Tischer, J., Lehmann, M., Su, G., Lepori, F. & Zopfi, J.

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 N-15-labeled and unlabeled nitrate (NO3-) 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 (H2S) and methane (CH4), 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 CH4-dependent NO3- reducers, dominated by Sulfuritalea and Candidatus Methylomirabilis over several years of observation. Anoxic incubation experiments with H2S amendments revealed a strong stimulation of dissimilatory NO3- reduction to ammonium (NH4+) (DNRA), but not denitrification. High relative abundances of the archaeal NH4+ oxidizer Candidatus Nitrosopumilus and bacterial nitrifiers indicate intense NO3- 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 NO3- and ammonium pools predominates over N removal via complete denitrification.

有机营养反硝化作用是湖泊中重要的氮去除过程，但诸如无机营养型硫氧化反硝化等替代性氮还原过程，其重要性可能被严重低估。本文对瑞士卢加诺湖北盆地的部分混合水柱中的氧化还原过渡带进行了研究，以揭示与硫循环和碳循环相耦合的氮转化途径。利用氮-15标记和未标记硝酸盐进行的培养实验显示，反硝化速率很低，且普遍受到有机电子供体的限制。初级生产输出的生物量中最易被利用的部分，可能在沉降过程中于有氧水柱内已大量消耗，未能到达约100米深处的氧化还原过渡带。相反，沉积物中更难降解的有机碳组分经厌氧降解产生的主要终产物——硫化物和甲烷，为氧化还原过渡带提供了持续的能量来源，促成依赖硫和甲烷的硝酸盐还原菌群落的建立，多年观测显示该群落以 Sulfuritalea 和 Candidatus Methylomirabilis 为优势类群。添加硫化物的缺氧培养实验表明，异化硝酸盐还原为铵的过程受到强烈促进，而反硝化过程则没有。古菌氨氧化菌 Candidatus Nitrosopumilus 和细菌硝化菌的高相对丰度，指示在氧化还原过渡带上部存在强烈的由硝化作用驱动的硝酸盐再生过程。硝化作用与硫驱动的异化硝酸盐还原为铵之间的潜在相互作用尚不明确，但二者的共存表明，至少在碳限制条件下，硝酸盐和铵库之间的氮循环周转比通过完全反硝化进行的氮去除更为主要。

（来源：Limnology and Oceanography 2026 Issue 1  DOI: 10.1002/lno.70315）