Lignocellulolytic and plastolytic potential of groundwater and sediment bacteria from the serpentinization-driven hyperalkaline springs

Four serpentinization-driven hyperalkaline springs (HAS) in the Jurassic ophiolites of western Serbia, with pH values between 11.1 and 11.7, were selected to assess the lignocellulose- and plastic-degrading potential of cultivable bacteria found in both the groundwaters and sediments of the zone of emergence of the investigated occurrences. Also, the physico-chemical properties of the groundwaters and petrological and mineralogical composition of sediments were examined. The HAS investigated are cold (temperature: 14.7–19.4 °C) and low in minerals (total dissolved solids: 104.1–450.4 mg/L) and belong to the Ca²⁺–OH⁻ and Ca²⁺, Na⁺–OH⁻ genetic type. Ca²⁺ was the most abundant cation (39.7–132.7 mg/L), followed by Na⁺ (2.0–82.5 mg/L) in three and Mg (6.6 mg/L) in one HAS, respectively. OH⁻ was the most abundant anion (24.6–123.2 mg/L), followed by CO₃²⁻ (18.0–36.2 mg/L) and Cl⁻ (12.4–71.0 mg/L) in all tested groundwaters. Binocular examination revealed that gravelly spring sediments consist predominantly of peridotite, serpentinite, carbonate and quartzite clasts, while the powder X-ray diffraction experiments identified calcite and lizardite as the main mineral phases in the silt fraction. In total, 210 groundwater and sediment isolates were screened on lignocellulose and plastic substrates, and 33.8% of all screened HAS isolates (9.1% from groundwaters and 38.4% from sediments) degraded carboxymethyl cellulose. Selected bacterial isolates were identified by partial 16S ribosomal DNA sequencing to belong to the genera Bacillus, Peribacillus, Paenibacillus and Lysinibacillus; these could have potential applications in various commercial sectors requiring cellulose degradation. All identified isolates demonstrated growth on the plastic substrates Impranil® DLN-SD (SD) and Impranil® DL 2077 (DL), while three isolates, belonging to genera Bacillus, Peribacillus and Paenibacillus, respectively, demonstrated growth on all four tested plastic substrates (SD, DL, polycaprolactone diol and bis(2-hydroxyethyl) terephthalate). These isolates should be further explored as potential candidates for bioremediation treatments of plastic-polluted groundwaters and sediments.