Reference: Petersen,K. L., A. Paytan, E. Rahav, O. Levy, J. Silverman, O. Barzel, D. Potts E. Bar-Zeev. 2018. Impact of brine and antiscalants on reef-building corals in the Gulf of Aqaba – Potential effects from desalination plants. Water Research, 144, 183-191. https://doi.org/10.1016/j.watres.2018.07.009
A Salty Situation
Clean, safe drinking water is a basic human need. For many areas around the globe, however, freshwater is becoming scarce. Coastal communities may look longingly upon the ocean and say:
“Water, water, every where, Nor any drop to drink.” – Samuel Taylor Coleridge, The Rime of the Ancient Mariner
What’s a parched seaside society to do?
Enter “desalination” – the process of removing salt from saltwater. In seawater Reverse Osmosis (SWRO), seawater is pumped through a membrane with very small pores that only lets water pass through, leaving salt and other molecules behind in a concentrated, salty brine solution. This super salty brine isn’t just a nuisance: it’s a waste product. Typically this waste is discharged back into the ocean, resulting in local salinity that is 1-10% higher than normal, sometimes up to 25% higher. Desalination plants also use antiscalants, which are chemicals that prevent crusty “scale” buildup on the reverse osmosis membranes. These chemicals also end up in the discharged waste. This begs the question: what is the impact of this waste on coastal ecosystems?
Desalination is a growing source of drinking water in the Middle East. Currently, there are plans for constructing several new, large-scale desalination facilities on the northern Gulf of Aqaba (off of the Red Sea near Israel). However, this area is home to very productive coral reefs, which provide many valuable ecosystem services and support great biodiversity. Seeing the potential threat to these precious reefs, Karen Lykkebo Petersen (University of California Santa Cruz) and colleagues sought to simulate what happens to coral reefs when they are exposed to desalination discharges.
Nubbins and Salinity and Antiscalants – Oh My!
The researchers conducted their study on coral fragments, or nubbins (Figure 1), collected from natural reefs located in the Gulf of Aqaba. They chose three reef-building species to observe their responses to brine discharge and antiscalants. The coral nubbins were placed in aquariums under controlled conditions. There were two treatments in the study: 1) seawater with elevated salinity, 10% above normal, and 2) seawater with both elevated salinity and antiscalant. These treatments mimicked the levels that corals would be exposed to in the ocean near desalination plants. As a control, the researchers also included seawater at ambient conditions (no increased salinity or antiscalant added).
To understand how these conditions may affect corals, it’s important to know a bit about these unusual creatures. You may be surprised to learn that they’re part of the animal kingdom (not a plant!) and that each reef is formed from innumerable, tiny polyps (Figure 2). These polyps work together as if they were a single organism and perform many vital functions, like grasping food, expelling waste, reproducing, and building the reef itself by secreting calcium carbonate, which forms a hard skeleton. The polyps are also home to a friendly type of algae, which in turn provides the coral with food through photosynthesis and creates its fantastic colors. Lastly, corals rely on many types of bacteria as well, which are known to help them adapt to their environment, including changes in temperature and water chemistry.
Dear Brine, You’re Stressing Me Out – Sincerely, Corals
At the end of the treatment period, all three coral species in both treatments showed signs of bleaching, tissue loss (Figure 3), and polyp retraction (i.e. “closing up” like in this video). Petersen and colleagues attributed these changes to osmotic stress, meaning that the rapid increase in salt concentration threw off the balance of water inside/outside of the coral’s cells. They also observed lower protein content in the corals and algae. This is a strong indication of declining coral health, especially when there is also tissue loss.
The authors found that calcification, or reef-building, rates decreased over time as well. Antiscalants exacerbated this effect, causing a drop of almost 50%. Since we know that the polyps retract due to stress and that they’re responsible for reef-building, this corroborates the findings above.
Respiration rates were also shown to decrease in response to the treatments. When bacteria and corals respire (or “breathe”) they consume oxygen and produce carbon dioxide just like we do. Therefore, lower respiration rates mean that there were fewer bacteria and coral cells after treatment. On the other hand, the authors also found that the antiscalants actually acted as a nutrient source for the bacteria! Even though there were fewer bacteria around, each bacterial cell was able to be more productive.
Lastly, the treatments lowered photosynthesis rates, which means that the algae inside the corals was negatively impacted. The coral species responded a bit differently from one another, but they all had lower abundance of algae from the treatments – up to a ~90% loss! The authors explained the species-specific responses could be due to different species of algae, different tissue thicknesses, and different types of polyps in each coral species. These traits could give one coral an advantage over another.
What Does It All Mean?
From this study, we learn that corals and their associated bacteria and algae can adapt to increased salinity, but a 10% increase may be pushing their limits. This was also the first known study to look at the effect of this type of antiscalant on corals. The scientists found that the antiscalant intensified the negative effects on the coral communities even though it could be used as a nutrient source.
Since desalination facilities often overlap areas where coral reefs exist, it’s important that the effects of the waste discharges are well-understood. This research indicates that regulators should take into account the multiple stressors involved (e.g. salinity and antiscalants) and create criteria based on the coral species found in their local reefs. The authors suggest using diffuser systems to dilute the brine, and also continuing this research to include effects of temperature and other water treatment chemicals.
Petersen,K. L., A. Paytan, E. Rahav, O. Levy, J. Silverman, O. Barzel, D. Potts E. Bar-Zeev. 2018. Impact of brine and antiscalants on reef-building corals in the Gulf of Aqaba – Potential effects from desalination plants. Water Research, 144, 183-191. https://doi.org/10.1016/j.watres.2018.07.009
Featured image: Acropora sp. in hobbyist’s aquarium (Photo credit: Logan Smith, 2010).