Mean, Green, Cleaning Machines: Using Algae to Help Keep Aquaculture Systems Clean

Aquaculture — also known as “aquatic farming” — has incredible potential to help meet the growing global demand for animal protein. The use of closed systems with water recirculating systems on land can prevent fish escapes and minimize the spread of disease to wild populations. There’s one catch though: keeping the water clean.

The top waste offenders in aquaculture are nitrogen and phosphorous, which are produced by uneaten fish food and fish feces. Recirculating systems can treat the water through a combination of physical and chemical processes to remove these compounds, but this can be a costly and unsustainable process.

This is where algae come in.

Algae are an incredibly diverse group of organisms with equally diverse metabolic capabilities. Not only can these microscopic workhorses use light for energy, but they also take up nitrogen and phosphorous — the nemeses of aquaculture.

So why not just hook up an algae-containing bioreactor to a recirculating water system and let them do their thing?

Not so fast, say a team of researchers in the Department of Integrative Biology.

“Algae are a very promising tool to remove nutrients from aquaculture wastewater,” says lead investigator Dr. Andreas Heyland. “But not all algae are created equal when it comes to bioremediation.”

Heyland and his team of graduate students, including PhD candidate Kathleen Nolan, who led the study, recently set out to find a way to screen different strains of algae for their ability to remove nitrogen from wastewater. Because algae can vary widely in their response to nitrogen levels and other environmental conditions, finding the most suitable candidates for use in aquaculture systems is critical.

The team’s goal was to establish a high-throughput miniaturized screening system that can assess algal growth rates under a range of nutrient conditions, using imaging to automate counts of algae.

“Despite the simplicity of the system, it is highly effective,” explains Heyland. “And its simplicity is what makes it practical.”

The team tested their screening system by determining how well two species of algae grew in different combinations and concentrations of waste nitrogen. High growth rates indicated which algae would be the most efficient at removing these compounds.

They also used their system to test another important screening criterion: the ability of algae to survive in the absence of nitrogen. Because nutrient levels can fluctuate in an aquaculture system, selected strains must also be able to withstand periods of nitrogen starvation. While both species were able to grow in the absence of nitrogen, one species exhibited a higher growth rate — demonstrating a greater level of tolerance to low nitrogen levels.

“Our system offers a scalable, efficient method for identifying algal strains with desirable traits for bioremediation of aquaculture wastewater,” says Heyland. “This is a key step towards enhancing the sustainability of aquatic food production systems.”

Incorporating algae into aquaculture offers other important benefits as well: the algae can be co-harvested as a food source or biofuel, or used for novel applications, such as pharmaceuticals.

“Algae have interesting metabolic pathways, so there is a lot of motivation to use biotechnology to expand the types of products that can be produced from these pathways,” Heyland explains.

While there is still a long way to go before we fully understand these mean, green, cleaning machines, it is clear that algae have a lot to offer towards sustainable food production.

“Algae are an incredibly diverse group of organisms,” says Heyland. “They all do cool stuff relevant to biological research, but are poorly understood. Our goal is to make our screening method available for other researchers, as there is a huge biodiversity and potential for novel applications that remains unexplored.”

Read the article in Current Research in Biotechnology.

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