Gidon Winters

Seawater desalination often discharges antiscalants with the brine into coastal environments, yet their effects on key benthic organisms remain poorly understood. This research focused on how brine with polymer or polyphosphonate-based antiscalants affected subsurface microbial biofilms and seagrass with their leaf and root microbial epiphytes. One-month long outdoor mesocosms were carried out using the Gulf of Aqaba coastal water, sediments, and the seagrass Halophila stipulacea. The impact of brine only (∼5 % above ambient) or with the addition of two different antiscalants on seagrass physiology as well as associated bacteria above and below the surface were compared to seawater. Compared to the ambient conditions, exposure to brine and polyphosphonate antiscalants for one month increased bacterial abundance in the sediments by 24 % but reduced bacterial counts by 22 % after exposer to polymer antiscalants. Clear changes in the bacterial community structure from sulfur cycling dominated bacteria to cyanobacteria and generalist species were observed with the addition of antiscalants. Correspondingly to their bacterial epiphytes, H. stipulacea leaves exposed to brine with polyphosphonate antiscalants led to an increase in new shoot density (11 %) and photosynthetic efficiency (34 %) relative to seawater. Differently, exposure of seagrass roots to brine and polymer antiscalants reduced the rhizome growth by 16 %. The findings of this research demonstrate that antiscalant chemical composition alters benthic microbial communities with downstream effects on seagrass physiology in varying degrees. Altogether, the impacts of antiscalants are highly dependent on the receiving environment, suggesting that discharge of polyphosphonate-based materials should be minimized under oligotrophic conditions, while prioritizing chemical additives based on polymers.