Start with the ecosystem, not nutrients, to stop algal blooms, scientists say

  • Human activity is warming the climate around the world and polluting the environment
  • Microcystin, the substance that makes algal blooms toxic
  • Krausfeldt’s study unveiled 30 kinds of cyanobacteria that had never before been detected

Summer brings hours of sunshine, lazing about with friends and enjoying the warmth of the season, but it also wakes up a sinister plague that suffocates waterways and kills wildlife. Algal blooms cause havoc (and a headache) for water users worldwide. Microcystis is the most well-studied – and most feared – cyanobacterium genus that triggers these toxic algal blooms. The summer’s warmth initiates an algal growth explosion, which is thought to be driven by increasing phosphorus, nitrogen, and other nutrients in a water system – primarily caused by nutrient runoff from agriculture and sewage. New research is now suggesting it is not only Microcystis causing the damage but other microbes too.

Powered by DNA sequencing advances, results indicate that reducing nutrients alone may not be enough to prevent these toxic masses. According to some scientists, ecological factors and slowing pollution must also be considered. One scientist suggests viruses target the main competitor of the Microcystis cyanobacterium, which may encourage algal blooms, whilst another suggests nitrogen fixation by other bacteria may provide an added boost.

“Interspecies biological interactions help determine blooms,” says Kevin Johnson, a marine scientist at the Florida Institute of Technology who was not involved in the work. “The more details we understand of bloom creation, the better our knowledge of how they might be prevented or controlled.”

Human activity is warming the climate around the world and polluting the environment, fuelling a rise in harmful algal bloom incidences. They are “a pretty wicked problem,” says Ariane Peralta, a microbial ecologist at East Carolina University.

Although lowering fertiliser runoff in some lakes seemed to stop blooms, they ultimately came back. Lake Erie, which is regularly choked with algal blooms, has a similar action plan in place that experts funded by the National Science Foundation and other US agencies warn could backfire. In 2014 an algal bloom on Lake Erie resulted in severe drinking water shortages in the local city of Toledo, Ohio. The United States and Canada agreed to reduce phosphorus from the lake by 40% as a result.

This may not be a simple fix, however. Scientists simulated the planned strategy alongside analysing more than 100 related scientific papers and concluded that while reducing phosphorus may dwindle Lake Erie blooms, there’s also a chance they could become more toxic. Any photosynthesis Microcystis left in the lake would receive more sunlight and have more nitrogen available – two conditions that are favourable for an increase in their production of microcystin, the substance that makes algal blooms toxic – as a result of lower overall microbial growth. The solution? Scientists say nitrogen levels within the lake should also be reduced.

That’s not all, however. The scientific simulation also implied that other microbes could indirectly influence the impact of Microcystis. Historically, scientists researching algal blooms have often overlooked the complex ecosystem of microbial inhabitants within a lake. From vast arrays of diatoms and other eukaryotes and viruses to various bacteria, there are many factors to consider. “Everyone glosses over them as not of managerial concern,” says Cody Sheik, a microbial ecologist at the University of Minnesota, Duluth.

Identifying what microbes are doing within an aquatic ecosystem has been a central part of the problem. But Lauren Krausfeldt, a microbiologist at Nova Southeastern University, recently turned to metagenomics, a strategy of sequencing all the DNA in water samples and other environments, to reconstruct the microbial ecosystem in Florida’s Lake Okeechobee – the largest lake in the US southeast. Annual summer blooms in Okeechobee have begun to make their way down rivers and spill into the Gulf of Mexico, and the Atlantic Ocean, spelling disaster for the Sunshine State as beaches are forced to close. From April to September 2019, the primary bloom season, Krausfeldt and her colleagues collected multiple water samples at 21 places across the lake. Using fragments of DNA isolated from the samples and sequences, they could begin to identify the whole genomes of specific species.

Krausfeldt’s study unveiled 30 kinds of cyanobacteria that had never before been detected in the lake, and in some cases new to science, including 13 that may cause blooms, she reported last month at Microbe 2022, the annual meeting of the American Society for Microbiology. “I was surprised at the diversity,” Krausfeldt says.

When no algal bloom was present, the most common organisms were the picocyanobacteria. However, as the bloom season continued, DNA belonging to bacterial viruses, known as phages, that infect the picocyanobacteria increased markedly. Soon after, the concentration of toxic Microcystis rose exponentially. Thanks to an analysis of its genome, there are some suggestions why: Microcystis contains several antiviral defences, such as the system that spawned the genome editor CRISPR, that picocyanobacterial lack. The bloom-forming cyanobacterium also possesses genes that allow it to store nitrogen. This essential nutrient may provide another competitive advantage over the many lake microbes that did not.

Krausfeldt expects the phages to lie dormant until some unknown environmental cue activates them. After the virus begins to attack more and more picocyanobacteria, newly available nitrogen, phosphorus, and more light fuel a Microcystis bloom, Krausfeldt proposes. She concludes that the phages’ destruction of its host cells could release even more nutrients, which is pivotal in enabling algal blooms.

Sheik, who says he had not considered phages as a factor in blooms but now wants to explore such viral dynamics, embraces Krausfeldt’s ecosystem mindset. “By taking a holistic approach, we can better understand how supporting organisms can help sustain blooms,” he says.

As well as gene activity assessments, Sheik and his colleagues have added metagenomics to their studies of several small lakes in Minnesota. These lakes contain not only Microcystis but also another bloom-forming cyanobacterium known as Dolichospermum. Between 202 and 2021, Sheik and his team witnessed Dolichospermum quickly become the most abundant microbe, only to have its population fall by July. Nitrogen levels in the lake skyrocketed and crashed in parallel with the microbe, implying it was fixing nitrogen and increasing its concentration in the water.

Nitrogen is typically less abundant in these relatively pristine lakes, despite the nutrient being essential for microcystin production. That could explain why Sheik and his colleagues observed levels of Microcystis and its toxin increase after the bloom in nitrogen-fixing Dolichospermum. Microcystis must depend on other species within the freshwater ecosystem to fix nitrogen or to recycle it by breaking down other life forms, Sheik says.

“I’m blown away” by the metagenomic work, says Benjamin Wolfe, a microbiologist at Tufts University, because it can illuminate the lake’s microbial interactions in great detail.

Evidence emerging from the Dolichospermum studies showcases how complex algal blooms can be. There is some good news, however. In the United States, Dolichospermum species in the United States lack the genes to make toxins, unlike in Europe —at least for now, says Sheik, who plans to keep watching for them in his metagenomic studies.

Microbial dynamics and how they drive algal bloom growth is an intricate topic, and many questions remain to be answered. The more we research, the fewer answers and the more questions we have. “We are grappling with understanding what parts of complex microbial communities are changing and what we can change to produce a different outcome,” Peralta says. But she’s optimistic that in time, “we can figure out what levers we can move.”