Utilization of algicidal bacteria as a biological agent have been receiving significant interest for controlling harmful algal blooms. While various algicidal bacterial strains have been identified, limited studies have explored the influence of bacterial culture conditions on its algicidal activity. Here, the effect of oxygen on the algicidal activity of a novel bacterium JK12, against a model diatom, Phaeodactylum tricornutum (P. tricornutum) was studied. Strain JK12 showed high algicidal activity against P. tricornutum and was identified as Pseudomonas chlororaphis (P. chlororaphis) by 16S ribosomal RNA gene analysis. JK12 culture supernatant exhibited strong algicidal activity while washed JK12 cells showed no obvious activity, indicating that JK12 indirectly attacks algae by secreting extracellular algicidal metabolites. Micro-aerobic culture condition dramatically enhanced the algicidal activity of JK12 by 50%, compared to that cultured under aerobic condition in 24 h. Extracellular metabolomic profiling of JK12 using gas chromatography–mass spectrometry and liquid chromatography–mass spectrometry analysis revealed significantly higher amounts of allantoic acid, urocanic acid, cytidine 2′,3′-cyclic phosphate, uridine 2′,3′-cyclic phosphate, and chlorinated tryptophan in the micro-aerobic culture. This is the first report to demonstrate the important role of oxygen on the algicidal activity of a non-pathogenic strain P. chlororaphis. In addition, the metabolomics analysis provided insights into the algicidal mechanism of P. chlororaphis.Electronic supplementary materialThe online version of this article (10.1186/s13568-018-0660-x) contains supplementary material, which is available to authorized users.

This work aims to produce a functional probiotic beverage using okara as the sole nutrient source. Hence, okara was fermented with Bacillus subtilis WX‐17 in submerged liquid fermentation and the supernatant was tested. Metabolomic analysis showed that the nutritional profile of the beverage was enhanced after fermentation. Essential amino acids as well as short‐chain fatty acids were significantly ( p < .05) upregulated. Total phenolic content and antioxidant content (in terms of DPPH radical scavenging activity) increased by 6.32 and 1.55 times, respectively. After 6 weeks, probiotic viability remains unchanged when stored at 4°C and the cell count is above the minimum dosage to confer health benefits. Antimicrobial activity was also detected against gram‐positive bacteria. The findings of this work showed the potential of submerged liquid fermentation of Bacillus subtilis WX‐17 using okara as sole substrate to produce a functional and low‐cost probiotic beverage.