Pollutant-degrading pathways in the gut microbiome of a stranded killer whale

Killer whales (Orcinus orca) serve as valuable indicators of ocean health due to their sensitivity to environmental pollution. Because of strong biomagnification in marine food webs, they are exposed to the highest pollutant concentrations of any mammal, making it especially important to understand how their gut symbionts respond to such exposures, a question that remains unexplored. This study therefore aimed to detect pollutant-related genes in the orca gut microbiome, specifically those encoding enzymes involved in degradation pathways of organic pollutants. We analyzed the gut microbiome of a killer whale that stranded in De Panne, Belgium, in 2023 using shotgun sequencing, complemented by full-length 16S PacBio sequencing to examine both gene functions, as well as microbial composition along the intestinal tract. Our analyses revealed a gene set of dehalogenases and dioxygenases, catalyzing key steps in pollutant degradation, such as halogen removal and aromatic ring cleavage. Pathway reconstruction showed that lindane and atrazine degradation were nearly complete, with all major enzymatic steps represented. For DDT, we detected genes from both anaerobic and aerobic pathways, capturing enzymes responsible for sequential dehalogenation, hydration, and decarboxylation steps that ultimately lead toward mineralization. Structural comparisons of predicted enzymes with known references supported their functional conservation. Taken together, these findings reveal that the orca gut microbiome encodes an enzymatic toolkit for degrading pesticide pollutants. While further work is needed to study population-wide …