Role of phycospheric bacteria: Promoting the growth of Isochrysis zhanjiangensis and ammonia nitrogen removal in rearing water

Isochrysis zhanjiangensis is crucial in the breeding of bivalves larval as an artificially cultivated feed microalga. To investigate their roles in algal–bacterial interactions, we isolated and identified two phycospheric bacterial strains, Halomonas piezotolerans and Alteromonas abrolhosensis, from I. zhanjiangensis and examined their effects on algal growth and ammonia nitrogen removal in aquaculture systems. The results indicated that both bacterial strains promoted the growth of I. zhanjiangensis; however, the effect of A. abrolhosensis was more pronounced. At a concentration of 108 cfu/mL, the A. abrolhosensis treatment group reached a cell density of 1.082 × 107 cells/mL on day 10, significantly higher than the control group by 16.26% (p < 0.05). The chlorophyll a content was 4.99 μg/L, representing a 43.8% increase compared to the control group (p < 0.05), and the total polysaccharide content was 8.366 g/L, significantly higher by 59.9% (p < 0.05). H. piezotolerans at a concentration of 106 cfu/mL increased the soluble protein content, with the co-culture group showing a 4.65% increase (p > 0.05). Furthermore, H. piezotolerans enhanced soluble protein synthesis at low concentrations. In ammonia nitrogen removal experiments, H. piezotolerans showed superior and stable activity, while A. abrolhosensis also promoted nitrogen reduction. The qPCR validation revealed upregulation of nirB and nosZ genes in H. piezotolerans, and enhanced expression of nirB, glnG, glnL, and glnA genes in A. abrolhosensis, confirming their ammonia nitrogen removal potential. This study demonstrates that phycospheric bacteria isolated from I. zhanjiangensis promote algal growth and facilitate ammonia nitrogen removal during bivalves larval rearing. These results provide new insights into bacteria–algae interactions and indicate practical implications for enhancing algal productivity and maintaining environmental stability in hatchery systems.