Planktonic anammox bacteria toward a better understanding of ecophysiological traits and harnessing the untapped potential as a bioresource.

Anaerobic ammonium-oxidizing (anammox) bacteria play a crucial role in biogeochemical nitrogen cycling and have been applied to wastewater treatment as a revolutionary nitrogen removal technology. Despite the successful application of anammox technology in engineering, our understanding of anammox bacteria in terms of their physiological and biochemical characteristics remains the tip of the iceberg, and challenges mainly arise from their slow growth rates and the absence of pure cultures. The development of enrichment cultures, particularly through membrane bioreactors, is important in addressing these challenges. In this review, we highlight the key factors that are vital for optimizing planktonic cell growth and preventing cell aggregation, i.e., calcium and magnesium concentration, oxygen level, and solids retention time, and propose the involved regulation strategies which help improve our understanding of the ecology of anammox bacteria and their competitive advantages, particularly in nitrogen-limited environments. Then, insights into the unique cellular structures of anammox bacteria (including anammoxosome and their distinct lipid membranes) and the complex metabolic pathways involving unique nitrogen intermediates were discussed, partially based on anammox planktonic cells. Finally, recent advances pertaining to non-traditional growth conditions and novel applications, such as ladderane lipid biosynthesis, extracellular polymeric substance production, and electro-anammox processes are discussed, underscoring their potential in innovative bioresource utilization beyond wastewater treatment. This review provides an in-depth understanding of planktonic cultivation techniques, growth dynamics, and biochemical characteristics of anammox bacteria, and highlights promising avenues for future research and application of valuable anammox bacteria resources, propelling their application in both ecological and engineered systems.