Anthropogenic-derived nutrition increased microbial structure and nitrogen fixation: implication from different of Manzala and Burullus Lagoons in Nile Delta

Coastal lagoons and estuaries are critical ecotones that deliver valuable ecosystem services. However, they are being increasingly threatened by human activities. The response of sediment microbes to these pressures, specifically their structural composition and the carbon and nitrogen fixation functions they mediate, has not yet been well defined. This study assessed two Mediterranean coastal lagoons in Egypt (Manzala Lagoon, ML and Burullus Lagoons, BL), and examined the bioenvironmental properties, microbial community structures, and carbon and nitrogen fixation functions of sediments, using metagenome sequencing. The population surrounding ML was 2.0 times larger than that of BL, resulting in significantly elevated environmental pressures: the total nitrogen content in ML was twice that of BL, the total organic carbon (TOC) was 1.2 times higher, and the levels of pollutant heavy metal were 1.3 times greater. In response, Proteobacteria (65.5 %) and Chloroflexi (9.1 %) emerged as the dominant microbes and differed significantly between the two lagoons (p < 0.05). Furthermore, they mediated higher levels of carbon and nitrogen fixation genes in ML than in BL, and the nitrogen fixation genes in ML were 1.4 times higher than that in BL. Redundancy discriminant analysis revealed that elevated TOC promoted the proliferation of Chloroflexi in ML, which was further linked to an increase in nitrogen fixation gene abundance. These findings demonstrate the significance of wastewater treatment and nutrient input management for the health of the Nile Delta lagoon, and provide a genomic basis for nutrient input control in the management of coastal lagoon ecosystems worldwide.