Discovery of an Antarctic Ascidian-Associated Uncultivated Verrucomicrobia with Antimelanoma Palmerolide Biosynthetic Potential

Actomonas aquatica gen. nov., sp. nov., a marine lineage in the phylum Verrucomicrobiota with genetic potential for carbon and nitrogen fixation

A novel bacterial strain, designated WL0086T, was isolated from a marine sediment sample collected in Lianyungang city, Jiangsu province, PR China. This strain showed the highest 16S rRNA gene sequence similarity to Geminisphaera colitermitum TAV2T (92.7 %) of the family Opitutaceae, and all the unclassified cultured and uncultured isolates with similarities >95 % were from marine environments. Cells were Gram-stain-negative, aerobic, non-motile cocci with a size of 0.6-0.8 µm in diameter. Strain WL0086T was positive for both oxidase and catalase, and grew at 20-37 °C (optimum, 28 °C), with 1.5-11.0 % NaCl (w/v; optimum, 2.5-4.0 %) and at pH 5.0-9.0 (optimum, pH 7.0). The major polar lipid profile of strain WL0086T consisted of phosphatidylethanolamine, phosphatidylglycerol, diphosphatidylglycerol, and phosphatidylcholine. The major isoprenoid quinone was menaquinone-7 and the predominant fatty acids were iso-C14 : 0, anteiso-C15 : 0, C16 : 0 and C16 : 1  ω9c. The complete genome consisted of a chromosome with 6 109 182 bp. The G+C content of genomic DNA was 64.0%. Results of phylogenomic analysis based on the 16S rRNA gene sequence and the whole genome suggested that strain WL0086T formed a distinct clade closely neighbouring the members of the family Opitutaceae. On the basis of phylogenetic, phenotypic, and chemotaxonomic evidences, strain WL0086T should represent a novel genus of the family Opitutaceae, for which the name Actomonas aquatica gen. nov., sp. nov. is proposed. The type strain is WL0086T (=MCCC 1K05844T=JCM 34677T=GDMCC 1.2411T).

Comprehensive insights on environmental adaptation strategies in Antarctic bacteria and biotechnological applications of cold adapted molecules

Climate change and the induced environmental disturbances is one of the major threats that have a strong impact on bacterial communities in the Antarctic environment. To cope with the persistent extreme environment and inhospitable conditions, psychrophilic bacteria are thriving and displaying striking adaptive characteristics towards severe external factors including freezing temperature, sea ice, high radiation and salinity which indicates their potential in regulating climate change's environmental impacts. The review illustrates the different adaptation strategies of Antarctic microbes to changing climate factors at the structural, physiological and molecular level. Moreover, we discuss the recent developments in "omics" approaches to reveal polar "blackbox" of psychrophiles in order to gain a comprehensive picture of bacterial communities. The psychrophilic bacteria synthesize distinctive cold-adapted enzymes and molecules that have many more industrial applications than mesophilic ones in biotechnological industries. Hence, the review also emphasizes on the biotechnological potential of psychrophilic enzymes in different sectors and suggests the machine learning approach to study cold-adapted bacteria and engineering the industrially important enzymes for sustainable bioeconomy.

Marine natural products

Covering: January to December 2021This review covers the literature published in 2021 for marine natural products (MNPs), with 736 citations (724 for the period January to December 2021) referring to compounds isolated from marine microorganisms and phytoplankton, green, brown and red algae, sponges, cnidarians, bryozoans, molluscs, tunicates, echinoderms, mangroves and other intertidal plants and microorganisms. The emphasis is on new compounds (1425 in 416 papers for 2021), together with the relevant biological activities, source organisms and country of origin. Pertinent reviews, biosynthetic studies, first syntheses, and syntheses that led to the revision of structures or stereochemistries, have been included. An analysis of the number of authors, their affiliations, domestic and international collection locations, focus of MNP studies, citation metrics and journal choices is discussed.

Uncovering Lasonolide A Biosynthesis Using Genome-Resolved Metagenomics

Invertebrates, particularly sponges, have been a dominant source of new marine natural products. For example, lasonolide A (LSA) is a potential anticancer molecule isolated from the marine sponge Forcepia sp., with nanomolar growth inhibitory activity and a unique cytotoxicity profile against the National Cancer Institute 60-cell-line screen. Here, we identified the putative biosynthetic pathway for LSA. Genomic binning of the Forcepia sponge metagenome revealed a Gram-negative bacterium belonging to the phylum Verrucomicrobia as the candidate producer of LSA. Phylogenetic analysis showed that this bacterium, here named "Candidatus Thermopylae lasonolidus," only has 88.78% 16S rRNA identity with the closest relative, Pedosphaera parvula Ellin514, indicating that it represents a new genus. The lasonolide A (las) biosynthetic gene cluster (BGC) was identified as a trans-acyltransferase (AT) polyketide synthase (PKS) pathway. Compared with its host genome, the las BGC exhibits a significantly different GC content and pentanucleotide frequency, suggesting a potential horizontal acquisition of the gene cluster. Furthermore, three copies of the putative las pathway were identified in the candidate producer genome. Differences between the three las repeats were observed, including the presence of three insertions, two single-nucleotide polymorphisms, and the absence of a stand-alone acyl carrier protein in one of the repeats. Even though the verrucomicrobial producer shows signs of genome reduction, its genome size is still fairly large (about 5 Mbp), and, compared to its closest free-living relative, it contains most of the primary metabolic pathways, suggesting that it is in the early stages of reduction. IMPORTANCE While sponges are valuable sources of bioactive natural products, a majority of these compounds are produced in small quantities by uncultured symbionts, hampering the study and clinical development of these unique compounds. Lasonolide A (LSA), isolated from marine sponge Forcepia sp., is a cytotoxic molecule active at nanomolar concentrations, which causes premature chromosome condensation, blebbing, cell contraction, and loss of cell adhesion, indicating a novel mechanism of action and making it a potential anticancer drug lead. However, its limited supply hampers progression to clinical trials. We investigated the microbiome of Forcepia sp. using culture-independent DNA sequencing, identified genes likely responsible for LSA synthesis in an uncultured bacterium, and assembled the symbiont's genome. These insights provide future opportunities for heterologous expression and cultivation efforts that may minimize LSA's supply problem.

Bacteria Associated with Benthic Invertebrates from Extreme Marine Environments: Promising but Underexplored Sources of Biotechnologically Relevant Molecules

Microbe–invertebrate associations, commonly occurring in nature, play a fundamental role in the life of symbionts, even in hostile habitats, assuming a key importance for both ecological and evolutionary studies and relevance in biotechnology. Extreme environments have emerged as a new frontier in natural product chemistry in the search for novel chemotypes of microbial origin with significant biological activities. However, to date, the main focus has been microbes from sediment and seawater, whereas those associated with biota have received significantly less attention. This review has been therefore conceived to summarize the main information on invertebrate–bacteria associations that are established in extreme marine environments. After a brief overview of currently known extreme marine environments and their main characteristics, a report on the associations between extremophilic microorganisms and macrobenthic organisms in such hostile habitats is provided. The second part of the review deals with biotechnologically relevant bioactive molecules involved in establishing and maintaining symbiotic associations.

Integrated Metabolomic-Genomic Workflows Accelerate Microbial Natural Product Discovery

The pairing of analytical chemistry with genomic techniques represents a new wave in natural product chemistry. With an increase in the availability of sequencing and assembly of microbial genomes, interrogation into the biosynthetic capability of producers with valuable secondary metabolites is possible. However, without the development of robust, accessible, and medium to high throughput tools, the bottleneck in pairing metabolic potential and compound isolation will continue. Several innovative approaches have proven useful in the nascent stages of microbial genome-informed drug discovery. Here, we consider a number of these approaches which have led to prioritization of strain targets and have mitigated rediscovery rates. Likewise, we discuss integration of principles of comparative evolutionary studies and retrobiosynthetic predictions to better understand biosynthetic mechanistic details and link genome sequence to structure. Lastly, we discuss advances in engineering, chemistry, and molecular networking and other computational approaches that are accelerating progress in the field of omic-informed natural product drug discovery. Together, these strategies enhance the synergy between cutting edge omics, chemical characterization, and computational technologies that pitch the discovery of natural products with pharmaceutical and other potential applications to the crest of the wave where progress is ripe for rapid advances.

Bioinformatic and Mechanistic Analysis of the Palmerolide PKS-NRPS Biosynthetic Pathway From the Microbiome of an Antarctic Ascidian

Complex interactions exist between microbiomes and their hosts. Increasingly, defensive metabolites that have been attributed to host biosynthetic capability are now being recognized as products of host-associated microbes. These unique metabolites often have bioactivity targets in human disease and can be purposed as pharmaceuticals. Polyketides are a complex family of natural products that often serve as defensive metabolites for competitive or pro-survival purposes for the producing organism, while demonstrating bioactivity in human diseases as cholesterol lowering agents, anti-infectives, and anti-tumor agents. Marine invertebrates and microbes are a rich source of polyketides. Palmerolide A, a polyketide isolated from the Antarctic ascidian Synoicum adareanum, is a vacuolar-ATPase inhibitor with potent bioactivity against melanoma cell lines. The biosynthetic gene clusters (BGCs) responsible for production of secondary metabolites are encoded in the genomes of the producers as discrete genomic elements. A candidate palmerolide BGC was identified from a S. adareanum microbiome-metagenome based on a high degree of congruence with a chemical structure-based retrobiosynthetic prediction. Protein family homology analysis, conserved domain searches, active site and motif identification were used to identify and propose the function of the ∼75 kbp trans-acyltransferase (AT) polyketide synthase-non-ribosomal synthase (PKS-NRPS) domains responsible for the stepwise synthesis of palmerolide A. Though PKS systems often act in a predictable co-linear sequence, this BGC includes multiple trans-acting enzymatic domains, a non-canonical condensation termination domain, a bacterial luciferase-like monooxygenase (LLM), and is found in multiple copies within the metagenome-assembled genome (MAG). Detailed inspection of the five highly similar pal BGC copies suggests the potential for biosynthesis of other members of the palmerolide chemical family. This is the first delineation of a biosynthetic gene cluster from an Antarctic microbial species, recently proposed as Candidatus Synoicihabitans palmerolidicus. These findings have relevance for fundamental knowledge of PKS combinatorial biosynthesis and could enhance drug development efforts of palmerolide A through heterologous gene expression.

Discovery of an Antarctic Ascidian-Associated Uncultivated Verrucomicrobia with Antimelanoma Palmerolide Biosynthetic Potential

The Antarctic marine ecosystem harbors a wealth of biological and chemical innovation that has risen in concert over millennia since the isolation of the continent and formation of the Antarctic circumpolar current. Scientific inquiry into the novelty of marine natural products produced by Antarctic benthic invertebrates led to the discovery of a bioactive macrolide, palmerolide A, that has specific activity against melanoma and holds considerable promise as an anticancer therapeutic. While this compound was isolated from the Antarctic ascidian Synoicum adareanum, its biosynthesis has since been hypothesized to be microbially mediated, given structural similarities to microbially produced hybrid nonribosomal peptide-polyketide macrolides. Here, we describe a metagenome-enabled investigation aimed at identifying the biosynthetic gene cluster (BGC) and palmerolide A-producing organism. A 74-kbp candidate BGC encoding the multimodular enzymatic machinery (hybrid type I-trans-AT polyketide synthase-nonribosomal peptide synthetase and tailoring functional domains) was identified and found to harbor key features predicted as necessary for palmerolide A biosynthesis. Surveys of ascidian microbiome samples targeting the candidate BGC revealed a high correlation between palmerolide gene targets and a single 16S rRNA gene variant (R = 0.83 to 0.99). Through repeated rounds of metagenome sequencing followed by binning contigs into metagenome-assembled genomes, we were able to retrieve a nearly complete genome (10 contigs) of the BGC-producing organism, a novel verrucomicrobium within the Opitutaceae family that we propose here as “Candidatus Synoicihabitans palmerolidicus.” The refined genome assembly harbors five highly similar BGC copies, along with structural and functional features that shed light on the host-associated nature of this unique bacterium.

IMPORTANCE Palmerolide A has potential as a chemotherapeutic agent to target melanoma. We interrogated the microbiome of the Antarctic ascidian, Synoicum adareanum, using a cultivation-independent high-throughput sequencing and bioinformatic strategy. The metagenome-encoded biosynthetic machinery predicted to produce palmerolide A was found to be associated with the genome of a member of the S. adareanum core microbiome. Phylogenomic analysis suggests the organism represents a new deeply branching genus, “Candidatus Synoicihabitans palmerolidicus,” in the Opitutaceae family of the Verrucomicrobia phylum. The Ca. Synoicihabitans palmerolidicus 4.29-Mb genome encodes a repertoire of carbohydrate-utilizing and transport pathways, a chemotaxis system, flagellar biosynthetic capacity, and other regulatory elements enabling its ascidian-associated lifestyle. The palmerolide producer’s genome also contains five distinct copies of the large palmerolide biosynthetic gene cluster that may provide structural complexity of palmerolide variants.