The application of partial nitritation-anammox (PN/A) under mainstream conditions can enable substantial cost savings at wastewater treatment plants (WWTPs), but how process conditions and cell physiology affect anammox performance at psychrophilic temperatures below 15 C remains poorly understood. We tested 14 anammox communities, including 8 from globally-installed PN/A processes, for (i) specific activity at 10-30 C (batch assays), (ii) composition of membrane lipids (U-HPLC-HRMS/MS), and

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... i) microbial community structure (16S rRNA gene amplicon sequencing). Crucially, the key parameters impacting anammox activity were the membrane lipid composition and cultivation temperature. The size of ladderane lipids and the content of bacteriohopanoids were key physiological drivers of anammox performance at low temperatures. Higher contents of (i) short C18 [3]-ladderane alkyl and (ii) large phosphatidylcholine headgroup were determined in anammox more active at 15-30 C and 10-15 C, respectively. At below 15 C, the activation energies of most mesophilic cultures severely increased while those of the psychrophilic cultures remained stable; this indicates that the adaptation of mesophilic cultures to psychrophilic regime necessitates months, but in some cases can take up to 5 years. Interestingly, biomass enriched in the marine genus Candidatus Scalindua displayed exceptionally highest activity at 10-20 C (0.50 kg-N.kg-VSS-1.d-1 at 10 C, Ea10-30 C = 51+-16 kJ.mol-1), indicating outstanding potential for nitrogen removal from cold streams. Collectively, our comprehensive study provides essential knowledge of cold adaptation mechanism, will enable more accurate modelling and suggests highly promising target anammox genera for inoculation and set-up of anammox reactors, in particular for mainstream WWTPs.