Abstract
The survival of coral reefs largely depends among other factors on the recruitment of a new generation of coral individuals that are more adapted to a rapidly changing climate and other anthropogenic stressors (e.g., pollution, sedimentation). Therefore, a better understanding of the coral settlement process, the molecules involved as well as crucial environmental drivers that control settlement success are needed. In this study, we identified a novel settlement inducer for the brooding scleractinian coral Leptastrea purpurea and highlight the importance of light for the settlement process. Crude extract of the red-pigmented bacterium Pseudoalteromonas rubra reliably triggered attachment and metamorphosis in L. purpurea larvae in less than 24 h. Prodigiosin (II) and the two derivatives, cycloprodigiosin (I) and 2-methyl-3-hexyl prodiginine (III) were isolated and structurally elucidated from the crude extract of P. rubra. We demonstrated that the photosensitive pigment cycloprodigiosin (I) was the responsible compound for attachment and metamorphosis in L. purpurea larvae. Under the tested light regimes (i.e., darkness, constant light and a dark-light alternation), cycloprodigiosin (I) triggered approximately 90% settlement at a concentration of 0.2 μg mL–1 under a 12 h alternating dark-light cycle, mimicking the light-flooded coral reef environment. Our findings enable for the first time a mechanistic understanding of the light-dependent larva to polyp transformation by discovering the novel bacterial settlement cue cycloprodigiosin and its photosensitivity as a determining factor for coral settlement.
Highlights
The survival of coral reefs, which are severely threatened by global warming, depends among other factors on the recruitment of new generations of coral individuals
From 30 inductive bacterial monospecies biofilms isolated from H. reinboldii (Petersen et al, 2021) only one single red pigmented strain (#1783) produced a crude extract (CE) that induced full settlement in L. purpurea larvae at a high and significant level with a concentration of 10 μg mL−1 (77.8% ± 15.7, p = 0.034, see Figure 1B)
The liquid-liquid partitioning (LLP) resulted in four fractions, of which only the DCM fraction induced settlement at high and significant levels at concentrations of 5 μg mL−1 (88.9% ± 15.7, p = 0.034, see Figure 1B)
Summary
The survival of coral reefs, which are severely threatened by global warming, depends among other factors on the recruitment of new generations of coral individuals. Known cues include for example surface structure properties (Nozawa, 2008; Doropoulos et al, 2016) and audiological signals, as larvae of the Caribbean reef building coral Montastraea faveolata were reported to detect and respond to reef sound (Vermeij et al, 2010). Spectral cues, such as light intensity and its quality, have been reported to affect larval behavior and habitat selection for both brooding and spawning coral species (Mundy and Babcock, 1998; Gleason et al, 2006; Yusuf et al, 2019). Highly energetic light with shorter wavelengths such UVB (i.e., 280–320 nm) showed a negative effect on the settlement and survival of coral larvae from different scleractinian species (Gleason and Wellington, 1995; Kuffner, 2001; Wellington and Fitt, 2003)
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