Recent years (2024) algea bloom in the Kinneret lake. Credit: Sharon Varulker
A new study challenges a long-standing assumption about how cyanobacteria survive environmental stress. The study, led by researchers at the Israel Oceanographic and Limnological Research (IOLR)—the Kinneret Limnological Institute (KLI), shows that survival under prolonged heat stress is not determined solely by the ability to protect photosynthesis. Instead, survival may depend on a remarkable shift in cellular energy balance, with dark respiration compensating when photosynthetic electron transport becomes impaired.
Microcystis aeruginosa is a toxic cyanobacterium that forms harmful algal blooms, affecting water quality, ecosystem health, recreation, fisheries and drinking-water resources worldwide. Globally, Microcystis blooms are usually associated with warm-water conditions and are expected to intensify as climate change drives rising temperatures. In Lake Kinneret (Sea of Galilee), however, the local strain of Microcystis shows an unusual seasonal pattern.
Published in Science Advances, the study compared two strains of Microcystis aeruginosa, a toxic cyanobacterium that forms harmful algal blooms in freshwater ecosystems worldwide. One strain originated from Lake Kinneret, where Microcystis displays unusual behavior: It repeatedly blooms during late winter, under relatively cool water temperatures. This local strain was compared with the frequently studied strain PCC7806.
To examine possible explanations for this phenomenon, the researchers used a unique experimental approach. First, they induced heat stress using an extreme temperature increase of 20 C (68 F), an approach commonly used in photosynthesis research but uncommon in ecological studies. Second, they waited 48 hours after inducing heat stress to assess how each strain responded and attempted to survive the heat shock.
Colony of Microcystis aeruginosa C-1004 isolated from Lake Kinneret. Credit: Dr. Alla Alster
In other words, they were examining how the cells regulated their energy budget—through photosynthesis and respiration—following severe heat stress.
Under light conditions, a pump-and-probe spectrophotometer tracked how electrons moved in the Microcystis cell through different parts of the photosynthetic machinery. Under dark conditions, a gas-exchange mass spectrometer measured how much oxygen the cells consumed through respiration.
Together, these measurements allowed the researchers to see not only that photosynthesis was disrupted, but where the disruption appeared in the electron flow, and whether respiration changed in response to heat stress.
"What we saw was remarkable," says Dr. Oded Liran, lead author of the study.
"The local strain from the Kinneret used all its energy to keep photosynthesis working until exhaustion and cell-density loss. It means that the local strain evolved and rewired all its abilities to maintain photosynthetic activity until it basically suffocated itself.
"On the other hand, the frequently studied model strain decreased its photosynthetic process and focused on respiration, which is breathing, and eventually survival. These findings suggest that cyanobacteria survive heat stress through a broader energy-management strategy than previously appreciated.
"Rather than simply protecting photosynthesis, the more heat-tolerant strain appears to survive by increasing respiration when photosynthesis is weakened. In these cells, photosynthesis and respiration are closely connected, so respiration may help keep the cell functioning when heat disrupts photosynthesis."
Dr. Oded Liran during sampling campaign in the Kinneret. Credit: Oded Liran
A surprising discovery
One of the study's most unexpected findings was that the more heat-tolerant strain did not survive by maintaining superior Photosystem II (PSII) performance, a central component of photosynthesis often used as an indicator of stress tolerance.
Instead, the researchers found that survival was associated with enhanced respiratory activity that helped compensate for heat-related disruptions in photosynthetic electron transport. This suggests that respiration may play a much larger role in heat resilience than previously recognized.
Looking beyond photosynthesis
The researchers found that the heat-tolerant strain did not survive by keeping photosynthesis highly active. Instead, photosynthesis slowed, while respiration increased. Because photosynthesis and respiration are closely connected in cyanobacteria, this increase in respiration may help support the cell's energy needs when heat weakens photosynthesis.
"The strength of this work is that we combined an extreme heat-shock experiment with an integrated view of photosynthesis and respiration," said Dr. Liran.
"Under milder conditions, this compensatory mechanism may have remained hidden. By pushing the cells to their physiological limit, we could see that the heat-tolerant strain was not simply protecting p…
Read the full article at Phys.org →
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