The Prut River is the second main sources of fresh water in the Republic of Moldova, following the Dniester River. Both rivers are part of the Black Sea Basin. Each of these transboundary rivers has its own critical points, which were taken into account when sampling. The study of the Prut River bacterioplankton, as part of the comprehensive scientific monitoring, was carried out during the vegetative season (spring-summer-autumn) 2018, 2019 and 2020 at the stations Costesti (dam), Braniste, Leuseni, Cahul, Cislita-Prut and Giurgiulesti. The petroleum hydrocarbon-degrading and phenol-degrading microorganisms (MOs) were cultured on the selective nutrient media at (20 – 22)ºC. Enumeration of colonies was performed in accordance with the national standard SM SR EN ISO 6222:2014 (direct method of colony counting). The numerical density of studied microorganisms was presented in colony-forming units (CFU/mL). The water quality based on microbiological indices was performed according to the methodological guide [1].
The microbial community of a water body is the most variable component of aquatic biota that responds quickly to environmental changes. Biodegradation of hydrocarbon and phenol compounds in aquatic and soil environment can be carried out by various organisms, including actinomycetes (Nocardia paraffinae), fungi (molds), yeasts (Candida lipolytica, Candida tropicali), anaerobic and aerobic bacteria (Pseudomonas fluorescens, Corynebacterium petrophillum, Mycobacterium paraffinicum). The listed MOs are not strictly specific for degradation of these organic substances only. Their detection on nutrient media containing petroleum hydrocarbon or phenol is not direct evidence of contamination of the water body with oil products or phenolic compounds. However, the presence of these MOs in the water sample indicates that in this station (or upstream) there are currently (or slightly earlier) organic substances that served as a substrate for the accumulation of these microorganisms.
The studied groups of microorganisms were detected in water samples at all stations of the Prut River. Their numerical density varied in the range of (22 – 1976) CFU/mL for phenol-degrading MOs and (25 – 680) CFU/mL for hydrocarbon-degrading MOs. Peak values were distributed differently in different years depending on the hydrological situation in the river basin. Seasonal dynamics was registered for bacteria that biodegrade phenolic compounds: the highest values were observed in the autumn season. Such dynamics characterizes natural seasonal processes in the aquatic ecosystem: when the biomass accumulated during the summer season dies, phenolic compounds accumulate in the water. Regarding spatial dynamics, the highest numbers of these MOs were recorded in autumn 2018 at Giurgiulesti and Cahul stations: 1156 and 1976 (CFU/mL) respectively. In the autumn of 2019, the largest number of this group of microorganisms was 486 UFC/ml at Giurgiulesti station. As for this last station, it is located at a distance of 940 m from the confluence of the Prut River with the Danube, therefore its characteristics directly depend on the hydrological situation on the Danube River.
No seasonal or spatial trend was observed in the dynamics of petroleum hydrocarbon-degrading MOs. The highest number was detected in autumn 2018 at Cahul station (680 CFU/mL), in spring 2019 ─ at Sculeni station (317 CFU/mL), in spring 2020 ─ at Giurgiulesti station (211 CFU/mL). Contamination of the Prut River with petroleum products is associated with human activity: the oil production unit is located in Lake Beleu, from where hydrocarbons can enter the Prut River during the spring or rain floods. One of the constant sources of oil product pollution is urban and industrial wastewater, the prescriptive treatment of which in the Republic of Moldova is not currently provided by any treatment facilities. The Prut River water quality based on number of phenol-degrading MOs varies from II (relatively polluted) to III (moderately polluted) class; based on number of petroleum hydrocarbon-degrading MOs varies from III to IV (polluted) class.

References:
1. Monitoring of the water quality and assessment of the ecological status of aquatic ecosystems. Chisinau, 2015.