Comparative analysis of the composition and abundance of the gut bacterial community of the Japanese sea cucumber (trepang) Apostichopus japonicus and the soil from its habitat was carried out. In spite of the presence of strictly aerobic bacteria in the soil, gut microflora of this holothurian was shown to be formed by facultative anaerobes from the environment. Irrespective of the geographical location of the habitat, the sea cucmber gut community had a constant composition considered its normal microflora. Capacity of bactreia isolated from the Japanese sea cucumber for decomposition of starch, sodium alginate, chitin, chondroitin sulfate, Tweens, olive oil, casein, and gelatin was studied. Various degrees of enzymatic activity were shown for 33% of the studied bacterial strains, which indicates a considerable role of the trepang gut microflora in processing and assimilation of organic matter arriving with food.

A total of 137 strains of mesophilic and psychrotolerant hydrocarbon-oxidizing bacteria have been isolated from three regions of the Sea of Japan exposed to different levels of anthropogenic pollution. The taxonomic affiliations of the culturable bacteria from surface waters and bottom sediments that are involved in the biodegradation of petroleum hydrocarbons in these regions have been identified. The isolated microorganisms belong to the phyla Actinobacteria, Firmicutes, and Proteobacteria. The hydrocarbon-oxidizing ability of the bacteria Okibacterium sp., Lechevalieria flava, Patulibacter sp., P. minatonensis, Gracilibacillus massiliensis, Thalassobacillus sp., Virgibacillus dokdonensis, Chryseomicrobium amylolyticum, Jeotgalibacillus marinus, Moraxella osloensis, Idiomarina maritima, and I. piscisalsi has been recorded for the first time. The results we obtained extend our knowledge about the potential of biodegradation of pollutants by members of certain taxa in the marine environment.

—: Biodiversity of the bacterial communities in the digestive system of Crenomytilus grayanus inhabiting the coastal Sea of Japan waters with chronic anthropogenic pollution was investigated using metabarcoding. Apart from marine bacteria, the taxa typical under contamination with oil ( Rhodobacteraceae , Corynebacteriaceae ), heavy metals ( Asinibacterium ), and unprocessed municipal waste ( Cloacibacterium, Globicatella ) were revealed in the microbiota. A collection of 411 cultured heterotrophic bacterial strains isolated in the course of this study was characterized taxonomically. The intestinal microbiome of the studied mollusks was shown to have a unique composition, depending on their habitat. Ability of bacterial strains isolated from the C. grayanus digestive system to degrade various nutrient substrates (sugars, amino acids, and polysaccharides) and xenobiotics (oil hydrocarbons, bisphenol A, and atrazine) was studied. Most isolates degraded a broad range of organic substrates; 13% (54 strains) oxidized oil hydrocarbons; 1% (4 strains) oxidized bisphenol A; and 0.5% (2 strains) degraded atrazine. The possible role of the microbiome C. grayanus microbiome in symbiotic digestion and in detoxication of the mollusk is discussed.

Five strains of psychrotrophic hydrocarbon-oxidizing bacteria were isolated from the bottom sediments of Peter the Great Bay of the Sea of Japan. They were classified into the following species: Rhodococcus erythropolis, Rhodococcus sp., Sphingomonas sp., Pseudomonas sp., and Alcanivorax sp. All studied bacteria showed high oxidizing ability in relation to the decomposition of n-alkanes (C9–C27), phytane, pristane, and polycyclic aromatic hydrocarbons at 5 and 22°C. At the same time, the degradation of hydrocarbons was more intense at 5°C. Despite the different taxonomic affiliations of the obtained microorganisms, all strains primarily utilized short- (C9–C13) and long-chain (C21–27) alkanes, as well as polycyclic aromatic hydrocarbons. The highest hydrocarbon-oxidizing activity was shown by the strain Rhodococcus erythropolis AP_291. The latter utilized more than 50% of all hydrocarbons in the model mixture during the first week of the experiment at 5°C.

Ability of bacteria isolated from the southern coastal waters of the Sakhalin Island to degrade various hydrocarbons was studied. The population of marine microorganisms grown on oil was heterogeneous in terms of hydrocarbon degradation. The rate of bacterial degradation of oil hydrocarbons was shown to correlate with their growth rate on the model medium. The degradation rates were higher for aromatic hydrocarbons than for alkanes. Based on our data, the studied bacteria were conditionally assigned to three groups: active, intermediately active, and passive degraders. Ability to oxidize oil was previously not reported for members of the genus Cobetia.

Based on microbial indication results for 2023 and 2024, it is established that Avacha Gulf and the southwestern coast of Kamchatka are subject to significant oil pollution. The number of hydrocarbon-oxidizing microorganisms at most of the studied stations was 10³ CFU/mL (g) and higher. The results of molecular genetic analysis identifying marker bacterial genes for the oxidation of alkanes (ALK-1, ALK-2, and ALK-3) and PAHs (nah, phn, bphA1, xylE, GN-PAH, and GP-PAH) in the environment indicated the dominance of bacteria capable of oxidizing short- and medium-chain alkanes in the surface waters of Avacha Gulf in 2023 and bacteria that are potential destructors of medium- and long-chain alkanes in the waters of the southwestern coast of Kamchatka. Also, the waters of both areas were characterized by a significant presence of gram-negative bacteria that oxidize various types of PAHs. In 2024, the proportion of stations with bacteria that degrade phenanthrene and short-chain alkanes sharply decreased in Avacha Gulf, but the proportion of stations with microorganisms that oxidize long-chain alkanes increased. An increase in the proportion of water samples with bacterial biphenyl oxidation genes and PAH oxidation genes of gram-negative microbiota was noted for the southwestern coast of Kamchatka. In bottom sediment samples collected in 2023 from both study areas, the genes responsible for PAH oxidation by gram-positive microbiota and for the degradation of long-chain alkanes were most often detected. An analysis of bottom sediment samples collected in 2024 indicated a decrease in the proportion of stations with microorganisms capable of oxidizing various petroleum hydrocarbons (PHs) in both study areas.