Effects of bacterial coexistence on the growth of a marine diatom Chaetoceros gracilis

The promotive and suppressive effects of coexistence of bacterial strains on the growth of a diatom Chaetoceros gracilis were investigated under laboratory conditions. Tested bacterial strains were isolated from the diatom mass culture tanks at a kuruma prawn farm. To evaluate the effect of each bacterial strain, the growth pattern was compared between cultures with and without bacterial medium addition. In diatom growth with the coexistence of Flavobacterium sp. in the ASP6 medium, the diatom cell yield during a 16-day culture period and specific growth rate were high compared to axenic culture as a control, whereas the duration of lag time and the variation among daily cell densities from four replicate cultures were as small as those in the control. These facts indicate that this bacterium greatly promotes diatom growth. However, four tested strains had no apparent effect and seven strains had a suppressive effect.