Effects of an auxin-producing symbiotic bacterium on cell growth of the microalga <i>Haematococcus pluvialis</i>: Elevation of cell density and prolongation of exponential stage

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<P><B>Abstract</B></P> <P>Recently, there has been a growing interest in bacterial diversity in the phycosphere of microalgae because plant growth promoting bacteria (PGPB) increase algal biomass. However, the phycosphere bacterial community associated with the unicellular microalgae, <I>Haematococcus</I> strains, has not been studied to date, despite the broad industrial applications of these microalgae. Accordingly, in this study, we analyzed the characteristics of phycosphere bacterial communities associated with microalgae. The phycosphere bacterial community structure was dramatically shifted with each growth stage of <I>Haematococcus lacustris</I>. A symbiotic bacterium, <I>Achromobacter</I> sp. CBA4603, was identified and isolated from the <I>Haematococcus</I> culture and was found to produce high levels of auxin compared with other PGPB strains. During co-cultivation of <I>Haematococcus pluvialis</I>, which has high commercial value, with the isolated strain CBA4603, the growth of the microalga was enhanced at all growth stages, and the exponential stage was prolonged. However, the accumulation of astaxanthin was not high, which could be explained by the previously reported characteristics of exogenous auxin. Taken together, our results suggested that this <I>Achromobacter</I> strain may be useful for industrial and biotechnological applications to enhance biomass and for generation of high value-added products of <I>H</I>. <I>pluvialis</I>.</P> <P><B>Highlights</B></P> <P> <UL> <LI> Our study first identified phycosphere bacterial community of a <I>Haematococcus</I> member. </LI> <LI> We isolated the plant growth promoting bacteria (PGPB), <I>Achromobacter</I> sp. CBA4603 from a <I>Haematococcus</I> strain. </LI> <LI> <I>Achromobacter</I> sp. CBA4603 produced indole-3-acetic acid (IAA) in quantities 20–390 times higher than other PGPB strains. </LI> <LI> <I>Achromobacter</I> sp. CBA4603 promoted cell growth of <I>Haematococcus</I> strain. </LI> </UL> </P>