Study of Microbial Communities in the Soda Lake of Isabel Island: Identification of Polyhydroxybutyrate ( PHB ) Degrading Enzymes

ABSTRACT Crater Lake (Isabel Island, Mexico) is a meromictic, stratified, haloalkaline system. To identify and characterise PHB depolymerases across the vertical physicochemical gradients of the lake, we analysed seven metagenomes from the water column (0–23 m), one sediment metagenome, and the genomes of two organisms (HB105m and VN105m) isolated from 5 m. Taxonomic profiles revealed vertical stratification: Actinobacteriota and Cyanobacteriota dominated surface waters, while Pseudomonadota, Bacillota, and Bacteroidota prevailed in deeper layers and sediments. Alpha‐diversity indices peaked at 5 and 20 m and declined at 23 m. We identified 16 putative PHB depolymerases spanning a broader phylogenetic range than previously documented for haloalkaline ecosystems. These included homologues affiliated with Vreelandella , Thiomicrorhabdus , Chloroflexota, Candidatus Cloacimonadota, and Desulfobacterales. The structural variation observed in lipase‐box motifs and signal peptides suggests functional differentiation linked to redox and oxygen gradients across depths. Phylogenetic analysis of predicted and reference enzymes showed depth‐specific clustering, with extracellular depolymerases predominant in oxic layers and intracellular forms more common in microoxic–anoxic zones. Overall, our results expand the known diversity of PHB‐degrading lineages in extreme environments and highlight several candidate enzymes with potential biotechnological relevance for future experimental characterisation.