In Lake Kinneret, a strain of toxic cyanobacteria is rewriting the survival playbook for extreme heat. Researchers at the Israel Oceanographic and Limnological Research Institute—the Kinneret Limnological Institute—have discovered that these heat-tolerant cells don't simply hunker down and protect their photosynthetic machinery the way scientists long assumed. Instead, they perform a dramatic metabolic switcheroo: when photosynthesis falters under thermal stress, respiration kicks in to keep the cells alive.
The finding challenges decades of assumptions about how cyanobacteria cope with environmental stress, and it matters urgently. Microcystis aeruginosa, the toxic cyanobacterium at the center of this research, forms harmful algal blooms that degrade water quality, threaten fisheries, and contaminate drinking-water supplies across the globe. These blooms are expected to intensify as climate change drives rising temperatures, making it critical to understand exactly how these organisms survive—and thrive—under heat.
The study, published in Science Advances, compared two strains of Microcystis aeruginosa. One came from Lake Kinneret itself, where the local strain exhibits unusual behavior: it repeatedly blooms during late winter under relatively cool water temperatures, defying the typical warm-water pattern of its species worldwide. The second strain was PCC7806, a frequently studied laboratory strain. To understand their differences, the researchers induced an extreme temperature spike of 20 degrees Celsius (68 degrees Fahrenheit), then waited a full 48 hours to see how each strain responded.
The experimental approach was meticulous. Under light conditions, researchers used a pump-and-probe spectrophotometer to track electrons moving through the photosynthetic machinery cell by cell. Under dark conditions, a gas-exchange mass spectrometer measured oxygen consumption—the signature of respiration. This dual measurement revealed not just that photosynthesis broke down, but precisely where the breakdown occurred and whether respiration ramped up in response.
What emerged was striking. The heat-tolerant laboratory strain—the one that survived—did something counterintuitive: it actually decreased photosynthesis and increased respiration. "The local strain from the Kinneret used all its energy to keep photosynthesis working until exhaustion and cell-density loss," explained Dr. Oded Liran, lead author of the study. "It means that the local strain evolved and rewired all its abilities to maintain photosynthetic activity until it basically suffocated itself." The heat-tolerant model strain, by contrast, took a different approach: it eased off photosynthesis and leaned into respiration—essentially switching to a survival breathing mode.
Perhaps most surprising was what the researchers did not find. The heat-tolerant strain didn't survive because it maintained superior Photosystem II performance, the central photosynthetic component long considered the gold standard of stress resilience. Instead, survival was associated with enhanced respiratory activity that compensated for heat-related disruptions in photosynthetic electron transport. This suggests that respiration may play a far larger role in heat resilience than scientists have previously recognized.
As climate change intensifies thermal stress on freshwater ecosystems worldwide, these findings offer a critical window into cyanobacterial survival strategies. Understanding that heat tolerance involves flexible energy management—not just photosynthetic stubbornness—could reshape how researchers predict which strains will thrive in warming waters, and how to manage the toxic blooms that threaten the health of lakes and drinking-water systems everywhere.