Chronic polyaluminum chloride perturbation shaped community assembly patterns: An overlooked mechanism of nitrogen removal inhibition

Abstract

Prior research indicated that aluminum ions (Al³⁺) could disrupt iron homeostasis in cells in synchronous biological nutrient removal and aluminum-based chemical phosphorus removal (CPR). However, the underlying microbial community assembly mechanisms in response to chronic polyaluminum chloride (PAC) disturbance remained unclear. Herein, post-precipitation experiments were conducted with PAC at varying concentrations (0, 40, 80 mg/L) in continuous flow anaerobic-anoxic–oxic systems over a 240-day period. Phosphorus was mainly removed through chemical processes, with minimal changes in polyphosphate accumulating organisms (PAOs) abundance and metabolic genes for polyphosphate and poly-β-hydroxybutyrate. Chemical phosphorus precipitates accounted for 74.7 ∼ 79.3 % in the activated sludge, with aluminum- and iron-phosphorus compounds (AlPs and FePs) being the primary components. Under low-level PAC, the average total inorganic nitrogen (TIN) removal efficiency shifted from short-term enhancement (≈73 %) to long-term inhibition (<60 %), with the inhibition being more pronounced than the acute disturbance caused by high-level PAC (≈63.5 %). Nitrification was most vulnerable to damage compared to denitrification under PAC stress. Although Al³⁺ initially replaced cellular iron, the iron-induced autotrophic denitrifying bacteria (Azospira) or iron-reducing bacteria (Trichococcus) became dominant, thereby compensating for damage to the tricarboxylic acid cycle and electron transfer. Upon the accumulation of Al³⁺ reaching 100 mg/g, the ecosystem achieved a new state of stability. Acute PAC disturbance enhanced community cooperation and facilitated rapid network reconfiguration, whereas chronic disturbance resulted in the loss of species and a reduction in network complexity. PAC altered the dynamics of nitrogen conversion by modifying electron flow, with lower concentrations augmenting denitrification and higher concentrations detrimental to nitrification. The biosystem adapted to low PAC levels by rerouting electrons around complex IV, which enhanced electron flow efficiency; however, this adaptation did not preserve in chronic disturbance. This study elucidates microbial assembly mechanisms under varying PAC disturbances, providing insights into the ecological resilience and functional stability of wastewater treatment systems.