Phytohormone-based metabolic regulation: From endogenous secretion discovery to Indole-3-butyric acid strategies for enhancing low-temperature denitrification

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Abstract
Low-temperature conditions severely inhibited microbial metabolism and disrupted quorum sensing (QS) processes, leading to downregulation of nitrogen metabolism genes and a subsequent decline in denitrification efficiency. Given the high cost and instability of traditional N-acyl homoserine lactones (AHLs), novel metabolic activators are urgently needed. This study demonstrated that activated sludge microorganisms possess the capacity to synthesize various phytohormones, although the endogenous pool declined linearly with decreasing temperature. Notably, supplementation with 1 μM IBA was identified as an optimal strategy, enhancing 5°C denitrification activity by 37.2%. Mechanistically, IBA triggered the indole-QS cascade to induce an auxin biosynthesis, resolving cold-induced hormonal deficiency. This signaling flux reinforces antioxidant defenses, resulting in a 37.6% reduction in malondialdehyde (MDA) levels. Furthermore, IBA improves membrane fluidity by elevating monounsaturated fatty acids (MUFA) by 59.1% and carotenoids by 39.2%. These physiological adaptations stabilize the physical environment for electron transfer, leading to a 45.2% increase in electron transport system activity (ETSA). Consequently, this coordinated response enriches a psychrotolerant consortium of Acidovorax, Simplicispira and Azonexus, ensuring stable nitrogen removal at 5°C. This research provided novel insights into stabilizing cold-region wastewater treatment through phytohormone-based metabolic regulation.

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