Evaluation of secondary metabolites from algae and epibiotic bacteria as allelochemicals and/or inducers of coral larval settlement

Abstract

In the marine environment chemical signals play critical roles at every organisational level. One emerging area of interest in the field of Marine Chemical Ecology is the role of bacterially derived chemical signals, in particular for the induction of larval settlement of marine invertebrates. Although bacteria have long been recognised to induce larval settlement, to date no inducer of bacterial origin has been fully characterised. In Chapter 2, I describe the isolation of the bacterium Pseudoalteromonas J010 from the surface of crustose coralline algae, which trigger the larval settlement of acroporid corals. In a bioassay-guided fashion, I characterised the metamorphic inducer in this bacterium as tetrabromopyrrole (TBP). TBP rapidly induces larval metamorphosis, however the larvae fail to attach to the substratum when exposed to TBP. The toxic nature of this compound suggests that larval metamorphosis to TBP is a stress response, rather than an evolved response to a habitat specific cue. The production of TBP may provide Pseudoalteromonas J010 with an advantage to persist in the highly competitive biofilm environment. To further explore this hypothesis, I screened the allelochemical profile of this strain and characterised further bioactive metabolites, including novel korormicin derivatives and a polybrominated pyrrole with anti-larval, anti-bacterial, anti-fungal and anti-protozoal properties (Chapter 3). Because, TBP did not explain the inductive properties of CCA on coral larval settlement, I addressed the origin and characteristics of inductive cues from CCA (Chapter 4). This resulted in the isolation of purified fractions that readily induced complete larval settlement, including i) low molecular weight organic-soluble compounds identified as glycoglycerolipids and ii) high molecular weight polymeric aqueous-soluble cue(s). These common algal metabolites may explain the highly inductive properties of CCA on acroporid coral larval settlement. I further demonstrated that these settlement inducers can be immobilised, enabling controlled settlement of coral larvae on target surfaces. Targeted larval settlement on substrata is common practise in reef rehabilitation, particularly for techniques based on sexual reproduction of corals. To improve current reef rehabilitation techniques, I explored methods to enhance post settlement survival of settled coral spat (Chapter 5).