Total ammonia nitrogen removal and microbial community dynamics in an outdoor HDPE-lined shrimp pond with no water discharge

This study assessed the characteristics of a zero-discharge outdoor high-density polyethylene (HDPE)-lined demonstration aquaculture pond (400 m2) in terms of water quality variables, total ammonia nitrogen (TAN) removal and oxygen net primary production (NPP) indices, and photosynthetic microbiome over a 76-day crop cycle. The shrimp harvest yield was 4.1 t/ha/crop with an average daily gain (ADG) of 0.15 g/day and a feed conversion ratio (FCR) of 1.49. During shrimp cultivation with a semi-intensive stocking density (40 shrimp/m2), there were low levels of TAN (<1.01 mg-N/L), nitrite (<0.21 mg-N/L), and nitrate (<2.79 mg-N/L). Ammonia removal was mainly mediated by photoautotrophic uptake, as evidenced by the consistent estimated TAN uptake rate of 0.05 & PLUSMN; 0.02 g-N/m3 h, which was calculated based on photosynthesis. The overall TAN removal rate of 0.10 & PLUSMN; 0.08 g-N/m3 h was maintained by suspended microorganisms in water, which exceeded that of microorganisms attached to the HDPE liner surface. A positive oxygen NPP of 0.24-1.16 g-O2/m3 h during light availability indicated prevailing autotrophic conditions. The bacterial community in the water was dominated by Phyla Pseudomonadota (Proteobacteria; 16-32%), Bacteroidota (Bacteroidetes; 15-29%), Cyanobacteria (7-35%), and Actinomycetota (Actinobacteria; 3-39%). The chloroplast amplicon sequence variants (ASVs) indicated abundances of Bacillariophyta (27-97%) and Chlorophyta (0.4-67%). High centric diatom populations (Bacillariophyta) were detected throughout cultivation, while the proportion of green algae (Chlorophyta) was positively correlated with increased TAN availability when feeding was increased. The findings enabled estimations of TAN removal and oxygen NPP rates and provided an understanding of correlations between environmental variables and bacterial and phytoplankton populations in the outdoor lined shrimp pond.