How nutrient loading leads to alternative stable states in microbially mediated N-cycle pathways: A new insight into bioavailable nitrogen removal in urban rivers

Excessive nutrients have disrupted pathways of microbial-mediated nitrogen (N) cycle in urban rivers and caused bioavailable N to accumulate in sediments, while remedial actions sometimes fail to recover degraded river ecosystems even when environmental quality has been improved. It is not sufficient to revert the ecosystem to its original healthy state by restoring the pre-degradation environmental conditions, as explained by alternative stable states theory. Understanding the recovery of disrupted N-cycle pathways from the perspective of alternative stable states theory can benefit effective river remediation. Previous studies have found alternative microbiota states in rivers; however, the existence and implications of alternative stable states in microbial-mediated N-cycle pathway remain unclear. Here, high-throughput sequencing and N-related enzyme activities measurement were combined in the field investigation to provide empirical evidence for the bi-stability in microbially mediated N-cycle pathways. According to the behavior of bistable ecosystems, the existence of alternative stable states in microbial-mediated N-cycle pathway have been shown, and nutrient loading, mainly total nitrogen and total phosphorus, were identified as key driver of regime shifts. In addition, potential analysis revealed that reducing nutrient loading shifted the N-cycle pathway to a desirable state characterized by high ammonification and nitrification, probably avoiding the accumulation of ammonia and organic N. It should be noted that the improvement of microbiota status can facilitate the recovery of the desirable pathway state according to the relationship between microbiota states and N-cycle pathway states. Keystone species, including Rhizobiales and Sphingomonadales, were discerned by network analysis, and the increase in their relative abundance may facilitate the improvement of microbiota status. The obtained results suggested that the nutrient reduction should be combined with microbiota management to benefit the bioavailable N removal in urban rivers, therefore providing a new insight into alleviating adverse effects of the nutrient loading on urban rivers.

How environmental stress leads to alternative microbiota states in a river ecosystem: A new insight into river restoration

Catastrophic shifts in river ecosystems can abruptly degrade their structures and functions, often reducing the efficacy of traditional remediation targeting physicochemical properties. Alternative stable states theory can not only explain this phenomenon but also provide a new insight into river restoration; however, little is known about the existence and implications of alternative stable states in a river. Considering the important role of benthic microbiota in sustaining river ecosystem structures and functions, ecological theory and high-throughput sequencing were combined to firstly investigate multi-stability in microbial communities and its relationship with environmental factors in river sediments. The Nanjing reach of the Yangtze River was selected as the study area because of its huge spatial heterogeneity and varying degrees of pollution. Bimodal distributions combined with temporal variations of microbiota status provided direct evidence of bistability by showing the instability at the intermediate. In addition, environmental stress, particularly concentrations of NH₄⁺-N and NO₃^--N, was identified as an important driver of alternative microbiota states from the perspectives of the behavior of bistable ecosystems. Comparison of α-diversity indices and network properties between two alternative microbiota states revealed that the diversity and co-occurrence pattern of microbial communities will be high if they are settled in favorable environments (i.e., comprehensive sediment quality identification index > 3.7). Key taxa, including Clostridiales, Nitrospirales and Myxococcales, were discerned by combining LEfSe and network analysis, and their strong interspecies interactions were believed to be an important factor in triggering alternative microbiota states. This study suggests alternative stable states theory should be considered in river remediation to better understand the response of river ecosystems to environmental stress and the effect of hysteresis, benefiting the implementation of effective monitoring and restoration strategies in a river of urban area.