Abstract: We compared the density, pH, dissolved oxygen and colony structure of Nitzschia closterium f. minutissima bred in negative pressure photobioreactor and open bucket, respectively. It is shown that the algae grew fast in negative pressure photobioreactor with the highest density of 1.33×10⁷ cells·mL^–1, significantly higher than that in open bucket (8.36×10⁶ cells·mL^–1). The pH in algal solution increased with increase of algal density, and there was a significant positive correlation between them (P<0.01). The highest pH values in negative pressure photobioreactor and open bucket were 10.3 and 9.3, respectively. Oppositely, in negative pressure photobioreactor, the dissolve oxygen of algea decreased with increase of algal density, which finally stabilized at about 6.5 mg·L^–1. The decline of dissolved oxygen might be related to the fact that Roseobacter had become the dominant bacteria. Results of 16S rDNA gene high-throughput sequencing show that the bacterial diversity decreased significantly (P<0.05). Proteobacteria and Bacteroidetes were dominant bacteria at early stage of breeding. However, the dominant bacteria were Cyanobacteria and Roseobacter at late stage in negative pressure photobioreactor, which was obviously different from that in open bucket.