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

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.