Active DNRA and denitrification in oxic hypereutrophic waters

Nutrient enrichment in coastal waters has led to eutrophication and algal blooms. Despite oxygenated conditions, some nitrogen cycling processes may still occur in low-oxygen microhabitats. Previous studies have not fully explored these processes in pelagic environments. It was already known that eutrophication alters microbial activity and nutrient fluxes. However, the extent of anaerobic nitrogen cycling in oxygen-rich waters remained unclear. This gap motivated an investigation into potential denitrification and DNRA in a hypereutrophic lagoon. The scarcity of data on pelagic nitrogen pathways in such systems highlighted the need for new methods. This study aimed to clarify the role of DNRA and denitrification in nutrient retention and ecosystem feedback.

This study aimed to investigate the activity of DNRA and denitrification in a hypereutrophic lagoon despite oxygenated bulk water. The specific problem was understanding how nitrogen cycling occurs in low-oxygen microzones within otherwise oxic environments. The motivation was to assess the role of these processes in nitrogen retention and algal growth. The study focused on a lagoon with high organic matter and frequent algal blooms. Researchers wanted to determine the contribution of DNRA and denitrification to nitrogen fluxes. The goal was to identify microbial taxa involved in these processes. The study also aimed to evaluate the significance of these pathways in sustaining eutrophication. Understanding these mechanisms could improve models of coastal nitrogen cycling.

The researchers used ¹⁵N isotope labeling to trace nitrogen transformations in the lagoon. They collected metagenomic data to identify microbial taxa involved in nitrogen cycling. RT-qPCR was employed to quantify gene expression related to denitrification and DNRA. Water and sediment samples were analyzed for oxygen and nitrate concentrations. Microscopy was used to observe cyanobacterial structures and their association with low-oxygen zones. The study focused on the bottom water and sediment layers of the lagoon. Filamentous cyanobacteria were examined for their role in creating microhabitats. The combination of isotopic, genomic, and microscopic methods allowed for a detailed analysis.

DNRA activity in the bottom water accounted for 83% of the total ecosystem DNRA. Denitrification was more active in the sediment than in the water column. Metagenomic data identified Bacteroidetes and Proteobacteria as key players in DNRA. Denitrification was primarily linked to Pseudomonas, Achromobacter, and Brucella. The ¹⁵N experiments confirmed active nitrogen transformation despite high oxygen levels. Microscopy showed cyanobacteria forming structures that create low-oxygen microzones. DNRA retained about 19% of the fixed nitrogen passing through the nitrate pool. These findings suggest a significant role for DNRA in sustaining algal growth.

The study found that DNRA and denitrification occur in hypereutrophic waters despite overall oxygenation. These processes are driven by specific microbial taxa in low-oxygen microhabitats. DNRA contributes significantly to nitrogen retention in the ecosystem. The results suggest that DNRA supports algal proliferation and eutrophication. The findings highlight the importance of microscale conditions in nitrogen cycling. The study confirms that DNRA is a major nitrogen recycling pathway in these systems. The results align with the hypothesis that microbial activity in microzones influences bulk water chemistry. These conclusions suggest a need to consider DNRA in models of coastal nutrient cycling.