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, two phycospheric bacterial strains, Halomonas piezotolerans and Alteromonas abrolhosensis , were isolated from I. zhanjiangensis and examined their effects on algal growth and ammonia nitrogen removal in aquaculture systems. The results indicated that both strains promoted the growth of I. zhanjiangensis ; with the A. abrolhosensis showing the stronger effect. At a concentration of 10⁸ cfu/mL, the A. abrolhosensis treatment reached a cell density of 1.082 × 10⁷ cells/mL on day 10, 16.22% higher than the control ( p < 0.05). Chlorophyll a content in this group was 4.985 μg/L (a 43.62% increase vs. control; p < 0.05), and the total polysaccharide content was 8.366 g/L (a 59.90% increase; p < 0.05). H. piezotolerans at a concentration of 10⁶ cfu/mL increased the soluble protein by 4.65% ( p > 0.05), and promoted soluble p rotein synthesis at lower inoculation levels. In ammonia nitrogen removal experiments, H. piezotolerans exhibited more stable and pronounced activity, while A. abrolhosensis also contributed to nitrogen reduction. Quantitative PCR (qRT-PCR) showed upregulation of nirB and nosZ genes in H. piezotolerans , and increased expression of nirB , glnG , glnL , and glnA in A. abrolhosensis , supporting their potential roles in ammonia nitrogen transformation. 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.