Unraveling bacterial community structure and function and their links with natural salinity gradient in the Yellow River Delta.

Salinization can change the soil environment and affect microbial processes. In this study, soil samples were collected from Zone A (Phragmites australis wetlands), Zone B (P. australis and Suaeda salsa wetlands), and Zone C (Spartina alterniflora wetlands) in the Yellow River Delta. The microbial community and functional potential along the natural salinity gradient were investigated. Total nitrogen, ammonia nitrogen, and soil organic matter presented a downward trend, and salinity first increased and then decreased from Zone A to Zone C. Nitrospira and norank_f_Nitrosomonadaceae were widely distributed throughout the zones. Denitrifying bacteria Alcanivorax, Marinobacterter, and Marinobacterium were abundant in Zone B and preferred high salinity levels. However, denitrifying bacteria Azoarcus, Flavobacterium, and Pseudomonas were mainly distributed in low-salinity Zones A and C, suggesting their high sensitivity to salinity. Dissimilatory nitrate reduction to ammonia (DNRA) bacteria Aeromonas and Geobacter dominated Zone C, whereas Caldithrix performed DNRA in Zone B. Interestingly, DNRA with organic matter as the electron donor (C-DNRA) occurred in Zone A; DNRA coupled with sulfide oxidation (S-DNRA) was dominant in Zone B; and C-DNRA and DNRA with divalent iron as electron donor and S-DNRA occurred simultaneously in Zone C. Salinity was the key factor distinguishing low and high salinity zones, and total nitrogen and total phosphorus had important effects at the phylum and genus levels. The abundance of genes encoding cell growth and death was relatively stable, indicating that the microbial community had good environmental adaptability. The genes related to the biodegradation of xenobiotics and the metabolism of terpenoids and polyketides were abundant in Zone B, revealing high metabolic potential for exogenous refractory substances. The microorganisms under low-salinity Zones A and C were more sensitive to environmental changes than those under Zone B. These results suggest that salinity plays important roles in microbial processes and shapes specific functional zones in coastal wetlands.