Nature-based practice for mariculture tailwater treatment: Microbial trajectory in three ponds and two dams

Intensive pond farming produces large amounts of protein for the global food supply, but leftover feed and waste from breeding ponds generate nitrogen-rich tailwater. Simple and low-cost treatment systems, especially the three ponds and two dams (3P-2D) approach that combines sedimentation, aeration, and biological ponds, are widely used to treat freshwater aquaculture tailwater. However, little is known about how well this system works for real mariculture tailwater and how microbial communities respond. Here, we built a 3P-2D system to treat saline tailwater and investigated the dynamics of microbial communities in each pond during the entire 10-week shrimp (Litopenaeus vannamei) farming cycle. We found that the system effectively removed total nitrogen (73.3%), ammonia (76.6%), nitrate (81.3%), nitrite (81.6%), and dissolved total organic carbon (56.1%). Diversity analyses showed that time duration, rather than pond type, critically shaped the community structure. Notably, we rarely observed enrichment of typical nitrifying microbes; Instead, nitrogen removal was largely driven by assimilation through complex microbial interactions. Besides, the microbial communities became more stable over time to sustain assimilation and reduce the abundance of antibiotic resistance genes (ARGs), especially blaB and strB associated with the genera Marivita, Ruegeria, and Winogradskyella. Our results suggest that the energy-intensive aeration pond may be redundant, and simply extending the treatment time could be a better strategy for treating mariculture tailwater outdoors.