A Novel Freshwater Anammox Species of Candidatus Loosdrechtia Thriving Under Dual Salinity and Sulfate Stresses

Loostrechtia thiotolerans (HSAMX1), a novel nonmarine anammox species that became dominant under combined high salinity (3% by weight) and high sulfate concentrations (∼86 mM). Through integrated metagenomic and metatranscriptomic analyses, we reveal the physiological and molecular strategies enabling HSAMX1 to thrive under dual-stress conditions. In response to osmotic stress, HSAMX1 activated ion export systems and subsequently synthesized organic osmoprotectant solutes to maintain cellular homeostasis. It also encoded and strongly expressed the sulfide:quinone oxidoreductase (SQR) gene, which accounted for over 90% of the total community SQR transcription. Intriguingly, HSAMX1 did not emerge under either salinity or sulfate stress alone, suggesting a previously unrecognized niche shaped by the interactions of these two stressors. These findings expand our understanding of nonmarine anammox diversity and identify a promising candidate for nitrogen removal in sulfate-laden, saline wastewater.