Chemical and Metagenomic Studies of the Lethal Black Band Disease of Corals Reveal Two Broadly Distributed, Redox-Sensitive Mixed Polyketide/Peptide Macrocycles

Abstract

Black band disease (BBD), a lethal, polymicrobial disease consortium dominated by the cyanobacterium Roseofilum reptotaenium, kills many species of corals worldwide. To uncover chemical signals or cytotoxins that could be important in proliferation of Roseofilum and the BBD layer, we examined the secondary metabolites present in geographically diverse collections of BBD from Caribbean and Pacific coral reefs. Looekeyolide A (1), a 20-membered macrocyclic compound formed by a 16-carbon polyketide chain, 2-deamino-2-hydroxymethionine, and d-leucine, and its autoxidation product looekeyolide B (2) were extracted as major compounds (∼1 mg g–1 dry wt) from more than a dozen field-collected BBD samples. Looekeyolides A and B were also produced by a nonaxenic R. reptotaenium culture under laboratory conditions at similar concentrations. R. reptotaenium genomes that were constructed from four different metagenomic data sets contained a unique nonribosomal peptide/polyketide biosynthetic cluster that is likely responsible for the biosynthesis of the looekeyolides. Looekeyolide A, which readily oxidizes to looekeyolide B, may play a biological role in reducing H2O2 and other reactive oxygen species that could occur in the BBD layer as it overgrows and destroys coral tissue.

Bacterial Metabolic Potential and Micro-Eukaryotes Enriched in Stony Coral Tissue Loss Disease Lesions

“…This tool can be used to assemble bacterial genomes and genes from other members of the holobiont, as well as to characterize the functional capabilities of the microbial members, which may be imperative to explain how SCTLD manifests. For example, metagenomics has been used in coral biology to associate BBD with bacterial chemotaxis genes, motility genes (Tout et al, 2015), and gene clusters (Gunasekera et al, 2019). The study presented here uses shotgun metagenomics to characterize the taxonomy and potential function of bacteria associated with SCTLD identified from coral species sampled at outbreak reefs.…”

Bioactivity Potential of Marine Natural Products from Scleractinia-Associated Microbes and In Silico Anti-SARS-COV-2 Evaluation

“…Looekeyolide A ( 122 ) is easily autoxidized to give looekeyolide B ( 123 ), making it challenging to assess its natural functions. Thus, compound 123 was tested directly in various assays, where it did not show any cytotoxic or antibacterial activity [ 61 ]. Looekeyolide A ( 122 ) may play a role in reducing H 2 O 2 and other reactive oxygen species (ROS) that could occur in the BBD layer as it overgrows and causes bleaching of both inner and outer layers of coral tissue as well as destruction as previously occurred in the scleractinian coral Stylophora pistillata (family: Pocilloporidae) [ 62 ].…”

“…Over 20 μmol/L H 2 O 2 has been detected in the immediate coral diffusive boundary layer, which may aid corals in removing some of the internal H 2 O 2 produced by their endosymbiotic algae and possibly have a self-protective function. Looekeyolide A ( 122 ) may allow Roseofilum to cope with this coral-produced source of H 2 O 2 as it migrates across the coral colony, and reduces the H 2 O 2 concentration to nearly zero when being incubated with a 1 mM looekeyolide mixture [ 61 ].…”

Fungal Epithiodiketopiperazines Carrying α,β‐Polysulfide Bridges from Penicillium steckii YE, and Their Chemical Interconversion

“…However, the molecular bases of the formation of the α,βpolysulfide bridges and tailoring modifications remain mostly unclear. [2,4] In our characterization of microbial communities associated with the coral black band disease, [5] a fungal strain YE was isolated from an infected Pseudodiploria strigosa coral collected at Looe Key Reef, FL, USA. Its internal transcribed spacer region (GenBank ID: KY750301) shared a 99.5 % identity with Penicillium steckii KUC1681-1.…”

“…Its internal transcribed spacer region (GenBank ID: KY750301) shared a 99.5 % identity with Penicillium steckii KUC1681-1. [6] Chemical investigation of the fungal fermentation culture led to the identification of three new ETP analogues, named penigainamide A-C (2, 3, and 8, Figure 1), along with five known ones, adametizine A (N-methylpretrichodermamide B, 1), [3f,g] FA2097 (4), [3c] outovirin A (5) and C (6), [3h] and pretrichodermamide C (7), [3f] all of which carry an α,β-polysulfide bridge and modifications on C6 and C7.…”