Breath­ing poison: Mi­cro­bial life on nitric ox­ide res­pir­a­tion

The study star­ted off with a trip to Bremen’s wastewa­ter treat­ment plant. “We col­lec­ted sludge from their de­ni­tri­fy­ing tank”, Gar­rido Amador tells. “Back in our lab, we ad­ded the sludge to one of our biore­act­ors and we star­ted the in­cub­a­tion by feed­ing it with NO.” Biore­act­ors are de­signed and op­tim­ized to grow mi­croor­gan­isms un­der con­trolled con­di­tions, which closely mimic their nat­ural en­vir­on­ment. This biore­actor setup was very chal­len­ging, though, Gar­rido Amador re­ports, “Be­cause NO is toxic, we needed spe­cial equip­ment and had to take great care when hand­ling them for our own safety. Nev­er­the­less, we man­aged to keep the cul­tures grow­ing for more than four years now – and they are still happy and healthy!”

The liv­ing con­di­tions in the biore­actor thus favored mi­croor­gan­isms that could sur­vive and grow an­aer­obic­ally with NO. “Even­tu­ally, two pre­vi­ously un­known spe­cies turned out to dom­in­ate the cul­ture”, says Boran Kartal, group leader of the Mi­cro­bial Physiology Re­search Group the Max Planck In­sti­tute in Bre­men. “We named them Nitricoxidivorans perserverans and Nitricoxidireducens bremensis.” Gar­rido Amador adds, “From just two mi­croor­gan­isms grow­ing on NO, we gained valu­able in­sight into how non-model mi­croor­gan­isms, in par­tic­u­lar NO-re­du­cers grow. Some of our ob­ser­va­tions showed us that these mi­crobes did not con­form to how model or­gan­isms – or­gan­isms which eas­ily cul­tiv­ated and thus ex­tens­ively stud­ied – be­have, and show­cased the lim­it­a­tions of meta­bolic pre­dic­tions based solely on gen­ome ana­lyses.”

“Cur­rently we know little about the con­tri­bu­tion of mi­croor­gan­isms grow­ing on NO to ni­tro­gen cyc­ling in nat­ural and en­gin­eered en­vir­on­ments”, ex­plains Kartal. “Nev­er­the­less, we can spec­u­late that these mi­croor­gan­isms could po­ten­tially be feed­ing on NO and N₂O re­leased by other mi­croor­gan­isms while re­mov­ing ni­tros­at­ive stress and min­im­iz­ing the emis­sion of these cli­mate act­ive gases to the at­mo­sphere.”

The en­riched mi­croor­gan­isms con­ver­ted NO to dinitro­gen (N₂) very ef­fi­ciently. “There were vir­tu­ally no emis­sions of the green­house gas ni­trous ox­ide”, Kartal adds. The lat­ter – the sole pro­duc­tion of N₂ – is par­tic­u­larly rel­ev­ant for ap­plic­a­tion: Many other mi­croor­gan­isms con­vert NO to ni­trous ox­ide, which is a po­tent green­house gas. N₂, in con­trast, is harm­less. Thus, each mo­lecule of NO that is trans­formed into N₂ in­stead of ni­trous ox­ide is one less mo­lecule adding to cli­mate change.

In a next step, the Max Planck re­search­ers are cul­tiv­at­ing other NO-respir­ing mi­croor­gan­isms us­ing samples from nat­ural and en­gin­eered en­vir­on­ments. “Cul­tiv­a­tion and en­rich­ment of fur­ther NO-respir­ing mi­croor­gan­isms will help to elu­cid­ate the evol­u­tion of N-ox­ide re­duc­tion path­ways and the en­zymes in­volved. It will also al­low to de­cipher the role of NO in known and yet-un­known pro­cesses of the ni­tro­gen cycle and its im­port­ance in the nat­ural and en­gin­eered en­vir­on­ments where these pro­cesses take place.”, Gar­rido Amador con­cludes.

Pa­loma Gar­rido-Amador, Niek Storten­beker, Hans J.C.T. Wessels, Daan R. Speth, In­macu­lada Gar­cia-Here­dia, Boran Kartal (2023): En­rich­ment and char­ac­ter­iz­a­tion of a nitric ox­ide-re­du­cing mi­cro­bial com­munity in a con­tinu­ous biore­actor. Nature Mi­cro­bi­o­logy (2023). Pub­lished on­line July 10, 2023.

DOI: 10.1038/​s41564-023-01425-8

• Max Planck Institute for Marine Microbiology, Celsiusstraße 1, 28359, Bremen, Germany
• Translational Metabolic Laboratory, Department of Laboratory Medicine, Radboud University Medical Center, Geert Grootepleinzuid 10, 6525GA, Nijmegen, The Netherlands
• School of Science, Constructor University, Campus Ring 1, 28759, Bremen, Germany

• Mi­crobes can grow on nitric ox­ide (NO)
• Deciphering the microbial nitrogen cycle