Polyhydroxyalkanoates are a class of biodegradable homopolymers and copolymers whose material properties can rival those of conventional plastics. Microorganisms including bacteria, archaea and algae synthesize them as intracellular carbon and energy storage compounds, packaging the polymer into granules known as carbonosomes, which typically measure between 0.2 and 0.5 micrometers across. These inclusions are built from sugars, waste materials, lipids, or even greenhouse gases such as methane and carbon dioxide. The biosynthetic pathway hinges on three key enzymes: β-ketothiolase (PhaA), NADP-dependent acetoacetyl-CoA reductase (PhaB), and above all the PHA synthase PhaC, the central polymerization enzyme whose substrate specificity determines which R-hydroxyacyl-CoA monomers are threaded into the growing chain. Because managing plastic waste is becoming ever harder and the natural resources needed for conventional plastic production are being depleted, the search for organisms that can make PHAs efficiently, and cheaply, has taken on real urgency.

The ocean has long been viewed as a promising but underexplored hunting ground for such organisms. Marine microbes are exposed to dynamic swings in temperature, salinity and nutrient availability, including transient nutrient surges during algal blooms, conditions that favor the conversion of surplus carbon into storage compounds like PHA. Stress conditions in general tend to boost PHA accumulation, and halophiles isolated from marine settings have been reported to produce significant quantities of the polymers. Yet the logistical and technical challenges of sampling and cultivating ocean microorganisms have left much of this diversity hidden. Metagenomics sidesteps that problem entirely: by sequencing the collective genetic material of an entire environmental community, researchers can access the metabolic potential of organisms that have never been grown in a laboratory dish.

For this study, the team did not collect the water themselves. Instead, they retrieved a publicly available shotgun metagenomic dataset from the NCBI Sequence Read Archive under accession number SRR31715427, originally generated by the China University of Geosciences in Wuhan as part of a project on marine metagenome genome sequencing and assembly. The sample came from the deep chlorophyll maximum of the Western Pacific Ocean at coordinates 129.9996883°E, 13.9995375°N. The choice of the DCM layer was deliberate. Because this subsurface zone hosts elevated carbon sequestration and biogeochemical cycling alongside high biomass, the researchers reasoned that it should support specialized microbial communities, including organisms capable of synthesizing unusual biopolymers.

The computational pipeline behind the analysis was rigorous and multi-stage. Raw paired-end Illumina reads were first assessed with FastQC and trimmed using Trimmomatic, with only reads retaining Phred quality scores above 30, corresponding to 99.9 percent base-call accuracy, kept for assembly; reads shorter than 50 base pairs were discarded. The filtered reads were then assembled with MEGAHIT, a de Bruijn graph-based assembler optimized for complex metagenomic data, which adaptively selected k-mer sizes from 21 to 141 to maximize contiguity. From 57,552 assembled contigs, the team used MaxBin2, an expectation-maximization algorithm that clusters contigs into metagenome-assembled genomes, or MAGs, based on tetranucleotide frequencies, GC content and read coverage. Only bins meeting a completeness threshold of at least 70 percent and contamination below 10 percent, as assessed by CheckM, were carried forward, ultimately yielding 29 high-quality MAGs for detailed annotation.

Functional annotation proceeded through several complementary tools. Prokka and RASTtk identified coding sequences and assigned genes to metabolic pathways, while DRAM, the Distilled and Refined Annotation of Metabolism tool, performed in-depth metabolic reconstruction by drawing on databases including KEGG Orthology, Pfam, TIGRFAMs and UniRef90. PHA-related genes were verified using Hidden Markov Models against Pfam and TIGRFAMs, with only protein domain hits showing E-values below 1e-10 and alignment coverage greater than 70 percent counted as significant. Taxonomic identities were assigned with GTDB-Tk, which aligns conserved marker genes against the curated Genome Taxonomy Database, and a phylogenomic tree was built from 120 bacterial and 122 archaeal single-copy marker genes using FastTree, with bootstrap support values above 70 percent considered reliable.

The results were striking in both scale and breadth. Across the high-quality MAGs, the team identified a total of 34 putative PHA-related genes, spanning Class I through Class III PHA synthases, depolymerases encoded by phaZ, and accessory proteins such as phaP, the granule-associated phasin, and phaJ, an enoyl-CoA hydratase. Of the 58 assembled bins, 18 contained organisms with putative genes for PHA metabolism, and taxonomic classification showed that 83 percent of these were bacteria while 17 percent were archaea. Among the bacteria, the phylum Pseudomonadota dominated, accounting for roughly 60 percent of PHA gene contributors, with the Alphaproteobacteria subgroup alone representing 28 percent, followed by Gammaproteobacteria. Other notable bacterial phyla included Planctomycetota, Verrucomicrobiota, Chloroflexota, Acidobacteriota and Myxococcota.

The archaeal findings may be the most intriguing part of the study. Within the archaeal fraction, the phylum Thermoplasmatota accounted for about 67 percent of PHA gene-harboring bins, followed by Thermoproteota. These archaea harbored Class III-type phaC genes and accessory elements, suggesting a role in PHA granule formation under marine conditions. While PHA biosynthesis is well characterized in bacteria, its presence in archaea points to broader evolutionary conservation and a reservoir of underexplored enzymatic diversity. The observation complements experimental work on haloarchaea such as Haloferax mediterranei, where homologous overexpression of PHA synthase genes has been shown to significantly increase PHBV yield and 3-hydroxyvalerate content, underscoring the industrial potential of extremophilic production systems.

Equally significant was the functional picture that emerged. DRAM-based annotation confirmed complete PHA biosynthetic pathways in many MAGs, including the full complement of core enzymes needed to convert precursors such as acetyl-CoA and propionyl-CoA into polymerizable hydroxyalkanoate monomers. The Pseudomonadota MAGs encoded both Class I synthases, which polymerize short-chain-length substrates of 3 to 5 carbons, and Class II synthases, which handle medium-chain-length monomers of 6 to 14 carbons, alongside regulatory proteins hinting at dynamic control of synthesis in response to environmental conditions. Crucially, many MAGs also encoded phaZ depolymerases, and the co-occurrence of biosynthesis and degradation genes suggests these microbes can perform dynamic PHA metabolism, likely a survival strategy in nutrient-limited waters. The phylogenetic tree showed PHA-producer genomes dispersed across multiple lineages, some clustering near known producers such as Shewanella and others belonging to novel or uncultured clades, a pattern consistent with convergent evolution of PHA gene clusters.

The authors are careful to note the limits of the approach. The presence of PHA-associated genes is a strong indicator of metabolic potential, but it does not conclusively demonstrate active synthesis or degradation under natural conditions. Future work will need to isolate and cultivate these organisms, validate enzymatic activity experimentally, and characterize the enzymes structurally through crystallography or cryo-electron microscopy, while integrating meta-transcriptomic and proteomic data to reveal how PHA metabolism is regulated in situ. Even so, the implications are considerable. Marine PHA producers with innate stress tolerance could be harnessed for non-sterile, open-system fermentations, cutting process costs and improving economic feasibility, and the identified depolymerases, with their notable stability and substrate specificity, could support PHA circularity and biodegradation applications. The study, its authors conclude, expands the genomic landscape of PHA-producing microorganisms in oceanic environments and reinforces the marine microbiome as a valuable yet underutilized reservoir for the bio-based materials industry.

Subject of Research: Metagenomic discovery of polyhydroxyalkanoate biosynthetic genes in marine microorganisms from the Western Pacific Ocean

Article Title: Metagenomic mining of microorganisms for polyhydroxyalkanoate (PHA) biosynthetic genes from marine sample of the Western Pacific Ocean

Article References: Tiwari, K. N., Ezhava, S. R., & Rodge, S. P. (2026). Metagenomic mining of microorganisms for polyhydroxyalkanoate (PHA) biosynthetic genes from marine sample of the Western Pacific Ocean. Discover Oceans, 3(1), Article 13. https://doi.org/10.1007/s44289-026-00132-6

Image Credits: AI Generated

DOI: 10.1007/s44289-026-00132-6

Keywords: polyhydroxyalkanoates, metagenomics, marine microbiology, bioplastics, Western Pacific Ocean, metagenome-assembled genomes, PHA synthase, Thermoplasmatota, Pseudomonadota, biodegradable polymers, deep chlorophyll maximum, microbial biotechnology

Poseidoniales, the archaeal hosts of these viruses, are abundant in surface oceans and play a significant role in the natural functioning of marine ecosystems. These single-celled organisms contribute to organic-matter degradation and carbon cycling, processes that sit at the heart of how the ocean stores and transforms carbon. Because archaeal viruses can influence the metabolism, population dynamics, and evolutionary trajectory of their hosts, understanding the viruses that infect Poseidoniales is essential to building a complete picture of this ecologically significant group. Until now, however, Aoguangviridae had been represented by only a small number of genomes, leaving its true diversity, evolutionary structure, functional potential, and global distribution poorly characterized.

“Marine archaea of the order Poseidoniales are abundant in surface oceans and contribute to organic-matter processing, yet the viruses associated with them remain poorly characterized,” said Changyue Zhao, a researcher at the Ocean University of China in Qingdao, China, and lead author of the study. “Aoguangviridae had been represented by only a small number of genomes, leaving its true diversity, evolutionary structure, functional potential, and global distribution largely unresolved. We therefore integrated public marine metagenomic and metatranscriptomic datasets to build a broader genomic framework for this archaeal virus family and to examine where its members occur and are active.”

The methodological foundation of the study rested on metagenomics, the study of genetic material recovered directly from environmental samples rather than from organisms grown in the laboratory. This approach is particularly valuable for viruses, many of which cannot be cultured independently of their hosts. To construct their reference dataset, the researchers began with five high-confidence Aoguangviridae genomes that had served as seed genomes, representing the first identified and best-characterized members of the family. They then incorporated 54 additional Aoguangviridae-related genomes that had been assembled during previous metagenomic studies. After removing redundancies, this curated starting set of 59 genomes provided the benchmark against which the team could identify new members of the family hiding in publicly available ocean sequencing data.

With that framework in place, the researchers turned to comparative genomic analyses to understand how the family is organized evolutionarily. They identified eight single-copy core genes, genes present exactly once in each genome and inherited vertically, which provided a reliable basis for inferring evolutionary relationships within Aoguangviridae. Single-copy core genes are a standard tool in viral phylogenetics because their conserved presence across related genomes allows researchers to align sequences, build phylogenetic trees, and define taxonomic boundaries with confidence. Applying this framework to the expanded dataset revealed substantially greater diversity than previously recognized: the family now encompasses 22 subfamilies, 157 genera, and 167 species, a dramatic increase over what the original 59 genomes had suggested.

“Aoguangviridae is far more diverse and widespread than previously recognized,” Zhao said. “We expanded the non-redundant genome set from 59 to 227 and resolved extensive diversity at the subfamily, genus, and species levels. These viruses occur across the global ocean, are particularly abundant in epipelagic and Arctic waters, and show transcriptional activity in the deep chlorophyll maximum. Their genomes also encode auxiliary metabolic genes, suggesting the capacity to influence host metabolism during infection.” The deep chlorophyll maximum is the subsurface layer of the ocean where chlorophyll concentrations peak, often marking a zone of intense photosynthetic activity and nutrient gradients. Transcriptional activity there, detected through metatranscriptomic data, indicates that the viruses are not merely present but actively expressing their genes, a sign of ongoing infection and replication.

The global survey showed that Aoguangviridae is found in marine and coastal habitats worldwide, but the researchers also found that different lineages within the family occupy different environments. Some lineages were enriched in epipelagic waters, the sunlit upper layer of the ocean, and in polar waters, whereas others were more frequently detected at greater depths. These contrasting distribution patterns suggest ecological differentiation within the family, with different lineages occupying distinct niches across the global ocean. Such ecological specialization may also be associated with differences in how each lineage interacts with its Poseidoniales hosts and with the roles the viruses play in marine biogeochemical processes, the chemical cycles that move elements such as carbon and nitrogen through the ocean system.

One of the most intriguing findings concerns auxiliary metabolic genes, host-like metabolic genes carried in viral genomes that can supplement or redirect host metabolism during infection. In marine systems, such genes are known to modulate processes central to energy and carbon flow, and their presence in Aoguangviridae genomes suggests that these viruses may have the capacity to influence the metabolism of Poseidoniales during infection. Because Poseidoniales contribute to organic-matter processing and carbon cycling in the surface ocean, viruses capable of altering their hosts’ metabolic activity could, in principle, ripple outward to affect how organic carbon is degraded and transformed in marine waters. Establishing whether such effects are measurable in the real ocean is precisely what the team hopes to test next.

“We aim to recover additional complete genomes, improve host assignments, and combine time-series meta-omics with laboratory cultivation and infection experiments,” Zhao said. “Ultimately, we want to determine how Aoguangviridae interacts with Poseidoniales cells and whether these infections measurably alter organic-carbon processing and other biogeochemical functions in the ocean.” Time-series meta-omics, in which genetic and transcriptomic data are sampled repeatedly at the same location over time, can reveal seasonal patterns of viral activity, while laboratory infection experiments would provide the direct biological evidence needed to confirm what the genome-based predictions imply. Cultivating Poseidoniales and their viruses remains challenging, but pairing observational data with controlled experiments is the standard path from genomic inference to demonstrated ecological function.

The study, titled Global Distribution, Genomic Diversity, and Phylogeny of Aoguangviridae, was a collaborative effort involving Kaiyang Zhang, Haolin Jia, Hongbing Shao, Andrew McMinn, Min Wang, and Yantao Liang of the Ocean University of China, together with Yeong Yik Sung, Wen Jye Mok, and Li Lian Wong of the UMT-OUC Joint Center for Marine Studies. The research was supported by the Laoshan Laboratory, the Natural Science Foundation of China, a 2024 Graduate Self-Directed Research Project, and the Fundamental Research Funds for the Central Universities. By transforming a sparse genomic record into a rich, globally distributed catalogue, the work provides the reference framework that future studies of archaeal viruses in the ocean will build upon, and it underscores how much of the viral diversity shaping marine microbial life remains to be discovered in the sequence data already collected from the world’s oceans.

Subject of Research: Genomic diversity and global distribution of Aoguangviridae, viruses infecting Poseidoniales marine archaea

Article Title: Virus that infects some plankton more widespread than previously suspected

Article References: Virus that infects some plankton more widespread than previously suspected. (n.d.). Original publication

Image Credits: AI Generated

DOI: Not provided

Keywords: Aoguangviridae, marine viruses, Poseidoniales, marine archaea, metagenomics, metatranscriptomics, carbon cycling, auxiliary metabolic genes, ocean biogeochemistry, deep chlorophyll maximum, Arctic waters, viral diversity