Dinitrogen fixation along the global freshwater ecospace

Bioconversion of dissolved dinitrogen (N2) into reactive forms is a central biochemical process in aquatic environments that often controls primary and secondary productivity, thus critically impacting geochemistry and “ecological health” along the spatiotemporal scales. Biological N2 fixation is catalyzed by a specific group of bacterial and archaeal prokaryotes termed diazotrophs. During the last decade, new studies have pointed out that even minimal N2 fixation rates can critically impact the eco-stoichiometry and microbial activity, due to the close ecological links between the freshwater and terrestrial ecospace. Yet, despite the potential importance of diazotrophs in freshwater ecosystems, there are still basic knowledge gaps; from the environmental factors that govern abundance, diversity and activity of freshwater diazotrophs to their lifestyle and N input from the streambed to the overlying water. Our recent research scope includes a global sampling plan. The sampling is done by research partners around the globe. The preliminary network includes 91 research groups from 68 countries and six continents. Sampling is done by a newly developed kit and handling protocol (available also at https://youtu.be/8vK6IF98Ab8) that includes items and procedures to analyze different freshwater environments for diazotrophs abundance, diversity and N2 fixation rates as well as nutrients, organic carbon and trace elements. Contact us if you would like to join our growing network.

Association of diazotrophic with aggregates

Recent studies report that heterotrophic N2 fixation is often supported by aggregates in various aquatic environments, especially those with adverse conditions for diazotrophy. It is suggested that relative to the surrounding water, aggregates support heterotrophic N2 fixation mainly by: (i) providing a micro-environment with a higher C:N ratio, therefore inducing “N-limited” conditions; (ii) containing carbon-rich molecules that can be hydrolyzed, thus supplying an available energy source; and (iii) minimizing the irreversible damage inflicted on the nitrogenase enzyme by dissolved oxygen. We have recently established a direct link between heterotrophic diazotrophs and aggregates in different aquatic environments. Altogether, we have found out that aggregates play a central role in the activity of freshwater and marine diazotrophs. And, yet much of the life cycle of these aggregate-associate diazotrophs is still missing.

Streambed diazotrophy: from dinitrogen fixation rates to lifestyle

Only a few reports on N2 fixation within streambeds are available, despite the close biochemical links between the subsurface and the overlying water. Overall, streams and rivers hold a disproportionate impact on biogeochemical cycles at the global scale given their limited surface coverage compared to other aquatic environments. The metabolic process and biochemical transformations of these fluvial environments occur at high rates within the hyporheic zone, forming hotspots of intense microbial activity. Currently, the lifestyle of these diazotrophs (i.e., biofilms or free-living) and activity are yet to be discovered, even though their potential contribution to freshwater productivity is extremely high. Our recent findings are that these diazotrophs can be found associated with biofilms and fix N2 at super-high rates (50-250 times higher) than the overlying stream-water.


Impact of desalination on the marine environment

Global seawater reverse osmosis (SWRO) desalination capacity is expected to triple during the 21st century. Alongside potable water production, desalination facilities produce hypersaline brine that also include various chemical additives. This dense brine can sink to the bottom and creep over the seabed for up to a few km from the discharge point. Our group focuses on the effects of SWRO brine discharge on marine benthic biogeochemistry as well as fauna and flora. And yet, the holistic and chronic impacts on the near and far field are still unresolved. Nevertheless, we are also moving forward beyond the impacts and delve into operational technologies (such as hybrid systems that include membrane distillation) to minimize the volumes of brine discharged (“zero liquid discharge approaches”), as well as switching to environmentally friendly additives.

Impact of the marine environment on desalination

Coastal water is often the feed source of large-scale desalination facilities, thus critically impacting freshwater production. And yet, coastal ecosystems are routinely exposed to anthropogenic pollution such as domestic and industrial wastewater runoffs as well as natural sources (often effected by climate change) such as jellyfish swarms and algal blooms. We found that the impact of sewage and jellyfish swarms could severely impair the pretreatment process and be detrimental to the RO systems. Therefore, new operational solutions or/and active barriers must be developed to ensure the continued production of freshwater during such events as numerous countries relay on desalination for freshwater production.

Membrane & spacer biofouling

Biofouling is currently the primary cause of reduction in RO membrane performance. The biofilm matrix is composed of different microbial communities that develop and proliferate on the RO membrane surface. To maintain required volumes of desalinated water and overcome the reduction in salt rejection due to the biofilm layer, pressure on the RO membrane must be increased with time. Our group is focusing on the impacts of biofouling on the performance of various membrane-based systems, while developing new applications to minimize their effects. Currently, our group is focusing mainly on the effects of biofouling on membrane distillation, forward osmosis (FO) and reverse osmosis (RO) that operate under various conditions. We found that the properties, structure and diversity of these biofilm are highly dynamic, thus require different approaches to minimize their development and impact on system performance.

Enhancing the operational efficiency of soil aquafer treatments

Over-withdrawal to accommodate the growing population and modern life-style highlight the urgent need for significant water reuse. Treated wastewater (WW) by infiltration of secondary effluent WW through recharge basins, namely soil aquifer treatment (SAT) is a robust procedure that produces high quality freshwater with near zero energy consumption. However, increasing volumes of domestic WW, high demand for land and escalating real-estate prices around urban areas highlights the urgent need in re-designing SAT to maximize WW recharge without sacrificing effluent quality. Our research aspires to significantly increase recharge-flux of WW into the aquifer without compromising water quality by enhancing biodegradation efficiency. Specifically, we will focus on organic matter removal including trace pharmaceuticals and hormones using lab-scale microcosms and large scale- on-site experiments. Nonetheless, there are still numerous basic knowledge gaps as well as constant operational challenges to be solved.


Over billions of years, microbial cell-to-cell and cell-to-surface interactions have evolved into a complex microbial entity, referred to as biofilm. A biofilm is defined as a sessile assemblage of complex microbial communities, which are permanently attached to a surface and held together within a matrix of predominantly self-produced, extracellular polymeric substances, EPS. Regardless to decades of research, impending biofilm formation on various type of membranes is still one of the greatest challenges for the water sector. Our group is studying the various parameters that control the initial stages of biofilm formation, focusing on the impact of water flow through spacers. These type of projects include inert particles as well as different types of bacteria.

The biomechanics of biofilms, namely the structure and viscoelastic properties of these complex structures are affected from various abiotic and biotic parameters. Our group is studying the changes in biofilms biomechanics by bacteriophage infections and by external hydraulic pressure.

Bacteriophages: We found that bacteriophages dramatically change the viscoelastic properties of single cells that comprise the initial stages of the biofilm. Concurrently, recent research indicate that the mechanical properties of the EPS matrix are also affected. However, the knowledge gaps related to the effects of bacteriophages on biofilm biomechanics are still immense.

Hydraulic pressure: Our group also discovered that hydraulic pressure has a significant impact on biofilm deformation. Specifically, we found that the pressure has a different impact on the deformation of the single microbes that comprise aquatic biofilms. In contrast to single cells, and although the subject is highly debatable, a detailed study over the effects of hydraulic pressure on the biological/cellular aspects that constitute biofilms has never been conducted. Our recent findings suggest that the impact of hydraulic pressure on biofilms that develop on membranes is dramatically different than those formed on opaque surfaces. However, it is not clear whether hydraulic pressure has only a mechanical impact over the biofilm structure or whether it inducts biological responses resulting in architectural and physiological changes in the biofilm consortium.