Responses of microbial complexes to freezing/thawing in the surface and groundwater interaction zone

Abstract

This paper presents the research results of the activity of microbial communities (MCs) in relation to humic substances after cyclic freeze-thaw. Groundwater from different aquifer depths from wells at different distances from the riverbank filtration (RBF) zone and river water were used as inoculum. Cyclic freeze-thaw (CFT) was carried out in two stages: the first freezing lasted 30 days at -18°C; then 5 cycles of freezing and thawing were carried out alternately after 7 days. Two types of thawing conditions were created: slow thawing from -18°C to +4°C and fast thawing from -18°C to +23°C. Growth activity on peptone, an easily available substrate, confirmed the survival of the MCs from groundwater and river water after CFT. The maximum activity after CFT with sodium humate (HNa) at a thawing temperature of 4°C was shown by MCs from a depth of 41 m from wells 1,500 m off the bank. It was comparable to the MC activity in river water during the observation period. At a thawing temperature of 23°C, microorganisms in river and groundwater from wells close to the bank were highly active, regardless of the carbon source composition during the CFT period. The growth activity of MCs was affected by the thawing temperature of 23°C in distant wells following CFT, depending on the water sampling depth. Thus, in MCs from depths of 21 m and 41 m, the activity increased with the distribution of easy-to-access co-substrate. The thawing temperature had an impact on the change in the spectral characteristics of HNa after CFT. The distance of the wells from the bank affected the aromatic compound contribution to the composition of the HNa transformation products (λ =275 nm). At a temperature of 23°C, the aromatic compound values were higher in the MCs of river water and wells located in the RBF zone compared to those at 4°C. Slow thawing at 4°C had a positive effect on the transformation of humic substances by microorganisms from distant wells due to their natural adaptation potential.