Bloom boom: Emerging human health risks from a changing aquatic climate

March 02, 2022

By Bree Iskandar. Mentored and edited by Sarah Lewin Frasier.

A large, green, smelly mass in Lake Erie contaminated Toledo’s municipal drinking water with toxins in 2014, leaving over 100 people ill and hundreds of thousands more without tap water for days. The culprit was a harmful algal bloom (HAB, for short), and such invaders are becoming more common as the climate warms. While many are familiar with climate change’s impact on marine ecosystems, new research is showing how phenomena like HABs bring profound risks to human health, as well.

HABs are overgrown colonies of algae, cyanobacteria and other microorganisms that produce toxins. Human exposure — either through swimming, breathing aerosolized droplets, or consuming contaminated seafood or drinking water — can cause acute symptoms such as nausea, vomiting, diarrhea, muscle weakness, and dizziness. HABs are becoming more frequent and widespread due to warming waters, increase in storm events, and greater rainfall.

Saurabh Chatterjee, an associate professor in environmental health sciences at the University of South Carolina, studies the effects of animal exposure to microcystin, one of the most common toxins produced by HABs. During a February 20th discussion panel at the American Association for the Advancement of Science annual meeting, he described how a growing body of research is implicating long-term HAB toxicity, especially microcystin toxicity, in liver damage, non-alcoholic fatty liver disease, and liver cancer.

Chaterjee’s lab has also shown that, rather inexplicably, microcystin can change our gut microbiomes to be more resistant to antibiotics. “In the genomic analysis of these host [gut] bacteria that have been exposed to microcystin…we are finding antibiotic resistance genes against common antibiotics like doxycycline, tetracycline, third generation cephalosporins, macrolides, and azithromycin,” he said. An important next step to solidify the link between microcystin and antibiotic resistance, he added, will be to profile existing antibiotic resistances (called the “resistome”) in patients with potential or known HAB exposures.

In recent years, HABs have occurred in formerly pristine bodies of water such as Lake Superior. George Bullerjahn, a Bowling Green State University biology professor who studies cyanobacteria in HABs, warned during the panel that the blooms typically thrive in warmer waters and are spreading outward from the tropics towards the poles due to climate change. This can have extreme consequences, as seen by the 2014 bloom event in Toledo.

Luckily, it’s not all bad news — there are actions communities can take to mitigate the frequency and spread of HABs. The blooms are largely dependent on nutrient runoff from agricultural practices and sewage, so minimizing nutrient availability would help minimize blooms.

“The main knob that we can tweak is controlling nutrients,” Bullerjahn said. HAB mitigation will require identifying nutrient sources and cutting them off, whether by adapting agricultural practices or fixing sewage treatment plants, and then monitoring these interventions in real time to make sure they are working.

A key strategy called synoptic monitoring, which tracks changes in the blooms’ genetic activity over time to identify the types and extent of toxins they are producing, has benefited greatly from “citizen scientist” community residents. Some charter boat captains near the Great Lakes, for example, collect samples while out on the water to bring back to academic labs like Bullerjahn’s for analysis. This not only facilitates bloom monitoring, but it also brings awareness to the emerging health risks HABs pose and engages the community with the water sources they depend on.

Educating medical professionals is also crucial to HAB mitigation. During the panel discussion Jennifer Pierce, a professor and physician at the University of South Alabama, stressed a need for training healthcare providers, especially EMTs and urgent care physicians, to recognize and diagnose symptoms of HAB toxicity. Medical HAB training, along with advertising national HAB reporting systems such as OHHABS to the public, can improve patient care and allow for faster identification, communication, and control of HAB events.

HAB toxicity’s long-term health effects in humans are not yet fully known, but climate change is guaranteed to increase the frequency of exposures in the coming years, the researchers said. But with strategies such as nutrient control, synoptic and real-time monitoring, citizen science, education of medical professionals, and open communication with government officials, they are hopeful we can start to mitigate HABs — and, as Pierce says, “get ahead of the bloom.”

Bree Iskandar is a pharmaceutical sciences Ph.D. candidate at the University of North Carolina at Chapel Hill. She writes for UNC’s graduate student science blog, The Pipette Pen, and is passionate about making and communicating scientific discoveries. Follow her on Twitter @briskybusiness_ or email her at

Image: Aerial shot of a harmful algal bloom in Western Lake Erie in July of 2019. Credit: NOAA Great Lakes Environmental Research Laboratory