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