A marine bacterium produces three different factors that stimulate animal metamorphosis

Microbes are ubiquitous entities in the ocean and can have powerful influences on the evolution and development of animals. One example of this phenomenon is the induction of animal metamorphosis by bacteria, whereby bacteria coating submerged surfaces serve as a stimulant for the settlement and metamorphosis of invertebrate larvae like tubeworms and corals. While inductive microbes could be harnessed as tools for restoring degraded marine ecosystems, only a few bacterial factors and their effect on different animal species have been described to date.

To study the role of two bacterial factors on animal metamorphosis, we focused on the bacterium Pseudoalteromonas luteoviolacea, previously shown to be a strong inducer of tubeworm, coral, and urchin metamorphosis. Through genomic analyses, we found that P. luteoviolacea encodes two distinct factors, Metamorphosis-Associated Contractile Structures (MACs) and tetrabromopyrrole. Although the effects of these two inducers have been studied on tubeworms and corals, respectively, no study has yet tested their effect under the same conditions. To test the effect of MACs and tetrabromopyrrole on tubeworm and Hydractinia (cnidarian) larvae, we created mutants lacking one or both factors. We found that MACs induce tubeworm metamorphosis but kill Hydractinia, while endogenous tetrabromopyrrole was not sufficient to induce metamorphosis in either tubeworms or Hydractinia. Interestingly, a P. luteoviolacea strain lacking both MACs and tetrabromopyrrole still induced Hydractinia metamorphosis, suggesting that P. luteoviolacea produces an undiscovered third factor that induces metamorphosis. Our results indicate that P. luteoviolacea may be a “hub” for stimulating animal metamorphosis by producing three different inductive factors.