Exploration

Salt Tolerance of Bacterial Carbon-Scrubbers Studied

September 21, 2018

The oceans are the world’s most valuable carbon sinks, with marine microorganisms absorbing and sequestering over 90% of Earth’s carbon dioxide (CO2). Being able to capitalize on the oceans’ ability to neutralize this harmful greenhouse gas could be of great value to the fight against global climate change, which is why a Masdar Institute student has studied the  genome of a local bacteria strain to find out how it responds to increasing salinity of the Arabian Gulf.

Cyanobacteria are photosynthetic bacteria that soak up to one ton of CO2 with each hectare of water they colonizes. While scientists have known about cyanobacteria’s contribution to Earth’s carbon cycle for a while, what is unknown is how an increase in the salt levels of water might impact this important microbe.

In the Arabian Gulf, low circulation to the open ocean coupled with significant desalination plant  discharges of salt make understanding how cyanobacteria react to increasing levels of salinity  a pivotal step in the country’s efforts toward environmental sustainability.

To find out how elevated salt levels affect this microbe, Masdar Institute Master’s student Sumaya Al-Hosani along with her supervisor, Dr. Lina F. Yousef, Assistant Professor of Chemical and Environmental Engineering, have studied a strain of the cyanobacteria that lives in the Arabian Gulf, called Prochlorococcus AS9601.

The study, which used ribonucleic acid (RNA) sequencing analysis is the first to look at how this bacterial strain copes with higher levels of salt. The study findings were recently published in the journal Microbiological Research.

“Desalination plants in the UAE produce millions of gallons of freshwater a day. The salt that’s taken out is primarily then dumped back into the ocean,” Al-Hosani explained. “Coupled with the extremely low addition of freshwater from rainfall and weak seawater movement, the Arabian Gulf is getting saltier. This higher level of salt will impact the local microflora, including cyanobacteria, which could lead to disturbances in the biosphere’s CO2 cycle.”

Prochlorococcus can live in high salt concentrations – the Arabian Gulf has higher salt levels than other oceans – but its tolerance to elevated seawater salinity is poorly understood. By examining the bacteria’s RNA responses to elevated salt levels, Dr. Yousef and Al-Hosani determined how the bacteria’s genome responds to higher salt concentrations. They found that the bacteria’s salt threshold is at roughly 5%, which is higher than the Arabian Sea’s salt content, which ranges from 3.8-4.8%. Above 5% salinity, the bacteria die.

Al-Hosani – who conducted this research as part of her Master’s thesis and is now a PhD student at Masdar Institute – hopes that this information will trigger policy makers and relevant stakeholders to develop plans and strategies to prevent the sea’s salinity from increasing further. Their findings highlight the issue of how best to dispose of the brine produced by desalination.

RNA SEQUENCING

This cyanobacteria study not only reveals important information about the effects of increasing salt levels in the ocean and how this could impact the marine and land environment, it is also the first time Masdar Institute researchers examined the response of an organism’s entire genome.

“RNA sequencing is emerging as a powerful tool for surveying gene expression, which has diverse applications in medicine, energy and many other areas,” said Dr. Yousef. “By pioneering this RNA-seq research on a regional species, we are developing the human capital needed to transform the UAE’s knowledge economy.”

According to Dr. Yousef, the Prochlorococcus’ simple genetic makeup makes it an attractive model organism for RNA transcriptome sequencing. Masdar Institute’s pioneering researchers performed high-throughput RNA sequencing methods to analyze the bacteria’s gene expression at elevated salt concentrations.

While the DNA of a cell holds all of the cell’s genetic information, the RNA produced from DNA carries the the information about which proteins the cell must make at any given time (in a process known as transcription) in order for the cell to carry out a particular action. Whatever genes are being transcribed in the cell at a given time is referred to as gene expression.

To identify which genes were expressed in Prochlorococcus when exposed to higher salt levels, Dr. Yousef and Al-Hosani, with help from Masdar Institute Assistant Professor in Computing and Information Science Dr. Andreas Henschel, used a technology called RNA sequencing, or RNA-seq. RNA-seq reveals a snapshot of RNA presence and quantity from a genome at a given moment in time.

“We captured snapshots of the expressed genes in the cell in response to higher levels of salt,” said Dr. Yousef. “Hundreds of genes were identified to be impacted by salt, which represent about one-third of the genome.”

Normally, the bacteria’s population takes two days to double. When induced with salt, the bacteria’s growth rate significantly slowed. The bacteria’s salt threshold was determined when the bacteria stopped growing – which was at 5% salinity. Most oceans’ salinity levels are at around 3.8%, making cyanobacteria a particularly salt-loving organism.


SALT-TOLERANCE IS ATTRACTIVE FOR BIOENERGY

Sequencing Prochlorococcus’ RNA produced pertinent information that gives much needed insight into the salt threshold of regional strain of bacteria. The research may also prove useful for another reason: It might help scientists understand how to improve the salt tolerance of sensitive microorganisms, which could then lead to an increase in the number of salt-loving organisms that could be used for bioenergy production.

“Alleviating the need for fresh water use in bioenergy production is an important step towards making the process more sustainable,” Al-Hosani explained. She believes her research can be used to help make other, less salt-tolerant organisms, more tolerant so they can be irrigated with seawater.

“We think scientists could apply cyanobacterial stress genes to other organisms, which would improve their salt tolerance, making them a good source for biofuel.”

Bioenergy is a key source of renewable energy and so research that establishes the conditions under which sources of bioenergy thrive is essential. Research into saltwater crops that can be used for bioenergy is therefore worth pursuing, believes Dr. Yousef. She thinks other researchers will be able to use this study’s findings to help in the design of algae and photosynthetic bacteria for bioenergy under high salt conditions.

Erica Solomon
News and Features Writer
20 September 2015