Microbial Accumulation in the Ukrainian Brook Lamprey Gut—A Potential Adaptation to Contaminated Waters

The Ukrainian brook lamprey (UBL) (Eudontomyzon mariae) spreads in Poland and often replaces the European brook lamprey (EBL) (Lampetra planeri) in rivers contaminated by wastewater effluents. We aimed to investigate whether this phenomenon is associated with differences in the intestinal microbiota of the two lamprey species. We analysed seasonal changes in the midgut microbial content of the UBL ammocoetes collected from a section of the River Gać affected by wastewater treatment plant effluent. The numbers of cultivable psychrophilic (autochthonous) and mesophilic (allochthonous) microorganisms, along with Escherichia coli and faecal streptococci, were compared to those found in the EBL from the same site. Higher levels of the faecal indicator microorganisms were observed in the UBL gut content compared to the EBL across most seasons, particularly in the winter (all mesophiles), in contrast to the levels of psychrophilic bacteria. This may suggest a relatively greater accumulation of mesophilic, sewage-associated bacteria in the UBL gut. Varying intestinal bacterial counts in the UBL gut did not reflect trends observed for the microorganisms in surrounding water during the studied seasons. These findings support the hypothesis that the UBL potentially benefits from the uptake of faecal bacteria. Such adaptation may contribute to its dominance in contaminated freshwater ecosystems, where EBL populations decline.