Induction of Larval Metamorphosis of the Coral Acropora millepora by Tetrabromopyrrole Isolated from a Pseudoalteromonas Bacterium

Abstract

The induction of larval attachment and metamorphosis of benthic marine invertebrates is widely considered to rely on habitat specific cues. While microbial biofilms on marine hard substrates have received considerable attention as specific signals for a wide and phylogenetically diverse array of marine invertebrates, the presumed chemical settlement signals produced by the bacteria have to date not been characterized. Here we isolated and fully characterized the first chemical signal from bacteria that induced larval metamorphosis of acroporid coral larvae (Acropora millepora). The metamorphic cue was identified as tetrabromopyrrole (TBP) in four bacterial Pseudoalteromonas strains among a culture library of 225 isolates obtained from the crustose coralline algae Neogoniolithon fosliei and Hydrolithon onkodes. Coral planulae transformed into fully developed polyps within 6 h, but only a small proportion of these polyps attached to the substratum. The biofilm cell density of the four bacterial strains had no influence on the ratio of attached vs. non-attached polyps. Larval bioassays with ethanolic extracts of the bacterial isolates, as well as synthetic TBP resulted in consistent responses of coral planulae to various doses of TBP. The lowest bacterial density of one of the Pseudoalteromonas strains which induced metamorphosis was 7,000 cells mm−2 in laboratory assays, which is on the order of 0.1 –1% of the total numbers of bacteria typically found on such surfaces. These results, in which an actual cue from bacteria has been characterized for the first time, contribute significantly towards understanding the complex process of acroporid coral larval settlement mediated through epibiotic microbial biofilms on crustose coralline algae.

Crustose coralline algal species host distinct bacterial assemblages on their surfaces

“…What is known to date about the role of bacteria in the induction of coral larval settlement has focused on the role of individual bacterial strains. Several strains have been shown to induce settlement, suggesting that the presence of specific inductive species is more important than the overall diversity of the bacterial community (Negri et al, 2001;Tebben et al, 2011;Tran and Hadfield, 2011;Sneed et al, 2014). However, little work has been carried out to examine the importance of multispecies conglomerations of bacteria on larval settlement preferences.…”

“…On the basis of these preferences, it is clear that some coral larvae are capable of recognizing and discriminating among CCA species, however, the mechanism by which they are able to differentiate among CCA is unknown. There is growing evidence that bacteria associated with potential settlement substrata induce settlement activity in many invertebrates including corals (Webster et al, 2004;reviewed in Hadfield, 2011), and that in some cases, they respond to specific bacterial strains within the bacterial community (Negri et al, 2001;Tebben et al, 2011;Tran and Hadfield, 2011;Sneed et al, 2014). It is therefore likely that larvae may use bacteria to recognize appropriate settlement substrata.…”

Bacterial Biofilm Communities and Coral Larvae Settlement at Different Levels of Anthropogenic Impact in the Spermonde Archipelago, Indonesia

“…Differences in BCC were further analyzed with ALDEx2 (Fernandes et al, 2014) to identify differentially abundant OTUs (selected at a parametric significance threshold of 0.05) between conditions. Occurrence of bacteria classes containing potentially settlement inducing bacteria strains (PSIB, Huggett et al, 2006;Tebben et al, 2011;Tran and Hadfield, 2011;Sharp and Ritchie, 2012;Sneed et al, 2014) was recorded and correlation of most abundant bacteria and PSIB to environmental parameters determined using Pearson correlation and visualized in a heatmap.…”

“…One genus of bacteria mentioned frequently in the context of inducing larval settlement and metamorphosis is Pseudoalteromonas (Negri et al, 2001;Hadfield, 2011;Tran and Hadfield, 2011). Tetrabromopyrrole (TBP), a metabolite produced by strains of this genus, was identified as an inducer of metamorphosis, although in many cases no settlement occurred (Tebben et al, 2011;Sneed et al, 2014). While Tebben et al (2015) conclude from their findings that it is not bacteria but CCA that produce the essential settlement cues, coral larvae also settle on surfaces not covered by CCA, indicating the importance of bacterial biofilms (Webster et al, 2004).…”

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“…Crustose coralline algae (CCA; Rhodophyta, Corallinaceae) and associated communities have been shown to be one of the primary inducers of settlement and metamorphosis in coral larvae (Morse and Morse 1988;Morse et al 1996;Heyward and Negri 1999). While marine bacteria also influence settlement in coral larvae (Negri et al 2001;Tebben et al 2011;Tran and Hadfield 2011), recent work demonstrates that CCA species known to elicit the strongest settlement responses are also the most affected by the changes in ocean chemistry associated with climate change (Anthony et al 2008;Doropoulos et al 2012;Smith et al 2013), suggesting that changes in these CCA communities might be responsible for reduced coral recruitment.…”