AUTHOR=Navada Sharada , Laureni Michele , Vadstein Olav , van Loosdrecht Mark 
  
TITLE=Contrasting effects of osmolytes on nitrifying biofilms under salinity stress
  
JOURNAL=Frontiers in Environmental Engineering
  
VOLUME=Volume 5 - 2026
  
YEAR=2026
  
URL=https://www.frontiersin.org/journals/environmental-engineering/articles/10.3389/fenve.2026.1667285
  
DOI=10.3389/fenve.2026.1667285
  
ISSN=2813-5067
  
ABSTRACT=Several industries produce high or variable salinity effluents. This can be challenging for the microorganisms involved in the biological water treatment of these effluents, especially the nitrifying microorganisms. Some microorganisms can adapt to a salinity increase through the uptake of certain molecules called osmolytes (or osmoprotectants) from the environment. This salinity acclimation strategy has been effective over a range of microorganisms. Thus, osmolyte addition could be a sustainable strategy for osmoregulation, but it has never been investigated in nitrifying biofilms. In this study, we investigated the impact of adding an osmolyte cocktail (1 mM each of trehalose, sucrose, glycine betaine, proline, carnitine, and ectoine) on the functionality of nitrifying biofilms undergoing a salinity increase from freshwater to seawater. The experiment was conducted on moving bed biofilm reactors (MBBR) operated in a sequencing batch mode. The osmoprotectants did not improve the nitrification activity on the first day after seawater transfer. Moreover, after 2 days in seawater, the treatment with osmolytes showed a severe reduction in the nitrification activity. This was accompanied by the growth of heterotrophic microorganisms in the medium facilitated by the uptake of osmolytes as substrate. Thus, the reduction in nitrification activity was likely due to competition between the heterotrophs and nitrifiers for resources (such as oxygen) and/or osmolytes. This study highlights the complex effects of the addition of osmoprotectants on biofilms undergoing a salinity change. Future studies should investigate the impact of individual osmoprotectants, as their potential as growth substrate and as osmoregulators may vary.