Bacteria on coralline algae and their role as sea urchin settlement cues

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Copyright: Nielsen, Shaun
Most benthic marine invertebrates have a biphasic life cycle, in which a planktonic larval stage alternates with a benthic adult stage. The transition between the larval and adult stage is typically guided by habitat-derived settlement cues and thus understanding the nature and distribution of settlement cues is a central theme in larval ecology. Both coralline algae and their epiphytic bacterial biofilms can be important settlement cues for marine invertebrate larvae, but the relationship between settlement and specific communities of bacteria is largely unknown. I investigated bacterial mediated settlement for larvae of the Australian sea urchins Heliocidaris erythrogramma and Holopneustes purpurascens and compared this to the community ecology of bacteria on coralline algae. I conducted a meta-analysis of putative larval cues from macroalgae to test the importance of coralline algae as settlement cues for invertebrate larvae generally and sea urchin larvae specifically. The meta-analysis revealed that coralline algae were the most inductive macroalgae for a variety of larval groups, but epiphytic bacteria only enhanced larval settlement for a few larval groups including sea urchins. Using larvae of H. erythrogramma and H. purpurascens, I next showed in larval settlement assays that bacteria on coralline algae enhanced settlement for both species but only larvae of H. erythrogramma responded to specific variation in the bacterial community composition. This specificity of response was then tested by isolating bacteria from the surface of coralline algae, and testing these against settlement by both sea urchins. One bacterium, Pseudoalteromonas luteoviolacea, was isolated from different species of corallines and induced larval settlement of both sea urchins, suggesting a common settlement cue across coralline algae. 16S rRNA tag sequencing surveys of the relative abundance of bacteria on these algae in the field indicated a high abundance of bacterial groups not examined in larval assays and an extremely low number of the genus Pseudoalteromonas, suggesting a very low abundance of P. luteoviolacea in natural communities. In summary, this thesis presents a systematic overview of larval settlement studies using meta-analysis, a statistical framework for correlating larval settlement and bacterial communities and provides an understanding of dominant bacterial members that have yet to be examined in larval settlement studies.
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Nielsen, Shaun
Steinberg, Peter
Harder, Tilmann
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PhD Doctorate
UNSW Faculty
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